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|Photo ©John M. Randall/The Nature Conservancy||Photo ©Barry A. Rice/The Nature Conservancy|
Ochsmann  also cites evidence of hybridization between spotted and diffuse knapweed
(Centaurea diffusa) in at least 7 U.S. states. The hybrid is named Centaurea × psammogena Gayer.
FEDERAL LEGAL STATUS:
No special status
Spotted knapweed has been declared a noxious or restricted weed in at least 15 states in the U.S. and 4 Canadian provinces . See the Invaders or Plants databases for current information.
Spotted knapweed is native to eastern Europe, though it now occurs in western and central Europe. It was introduced to North America, probably as a contaminant in alfalfa (Medicago sativa) seed and/or ship's ballast, in the late 1800s [113,135,159]. In 1920, the distribution of spotted knapweed in North America was limited to the San Juan Islands, Washington. By 1980 it had spread to 48 counties in the Pacific Northwest. Between 1980 and 1998, the known range of spotted knapweed included 326 counties in the western United States, including every county in Washington, Idaho, Montana, and Wyoming . Although it is reported to occur in 45 of the 50 states [82,209], spotted knapweed is found primarily in the northwestern states and southwestern Canada. The Plants database provides a map of spotted knapweed's distribution in the United States.
|Estimates of spotted knapweed acreage as reported by state or province in 1988 and again in 2000 (adapted from ).|
|New Mexico||not reported||500|
|British Columbia||not reported||50,000|
The decrease in acreage reported in Montana is attributed to improved inventory methods during the past decade. Although inventories are more common and more accurate, 50% of these states reported only 50% accuracy, while 31% reported 51 to 75% accuracy, and 2 states reported 75 to 100% accuracy . Watson and Renney  reported that spotted knapweed was abundant in British Columbia, common in Ontario, Quebec and the Maritimes, and observed in southern Alberta in 1974.
Information on the distribution of spotted knapweed is limited for most North American states and provinces in which it occurs. It is commonly listed as occurring on roadsides and other disturbed areas in the Adirondacks , New England , the Northeast , Michigan , Illinois , Nebraska , the Great Plains , the Blue Ridge region of North Carolina, Tennessee, and Virginia , West Virginia , the Carolinas , and Florida [5,232].
Specific information on the plant communities in which spotted knapweed occurs is also limited outside its primary area of occurrence. The following lists reflect ecosystems and cover types in which spotted knapweed is commonly found, although the lists are not exhaustive.ECOSYSTEMS :
BLM PHYSIOGRAPHIC REGIONS :
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands
KUCHLER  PLANT ASSOCIATIONS:
K005 Mixed conifer forest
K008 Lodgepole pine-subalpine forest
K010 Ponderosa shrub forest
K011 Western ponderosa forest
K012 Douglas-fir forest
K013 Cedar-hemlock-pine forest
K014 Grand fir-Douglas-fir forest
K015 Western spruce-fir forest
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K019 Arizona pine forest
K022 Great Basin pine forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K038 Great Basin sagebrush
K048 California steppe
K055 Sagebrush steppe
K056 Wheatgrass-needlegrass shrubsteppe
K063 Foothills prairie
K068 Wheatgrass-grama-buffalo grass
K074 Bluestem prairie
K075 Nebraska Sandhills prairie
K081 Oak savanna
K082 Mosaic of K074 and K100
K095 Great Lakes pine forest
K100 Oak-hickory forest
K104 Appalachian oak forest
K106 Northern hardwoods
K109 Transition between K104 and K106
SAF COVER TYPES :
1 Jack pine
14 Northern pin oak
15 Red pine
20 White pine-northern red oak-red maple
21 Eastern white pine
42 Bur oak
43 Bear oak
44 Chestnut oak
50 Black locust
51 White pine-chestnut oak
52 White oak-black oak-northern red oak
53 White oak
55 Northern red oak
206 Engelmann spruce-subalpine fir
210 Interior Douglas-fir
211 White fir
212 Western larch
213 Grand fir
215 Western white pine
218 Lodgepole pine
220 Rocky Mountain juniper
222 Black cottonwood-willow
224 Western hemlock
227 Western redcedar-western hemlock
228 Western redcedar
229 Pacific Douglas-fir
233 Oregon white oak
236 Bur oak
237 Interior ponderosa pine
238 Western juniper
243 Sierra Nevada mixed conifer
244 Pacific ponderosa pine-Douglas-fir
245 Pacific ponderosa pine
249 Canyon live oak
250 Blue oak-foothills pine
SRM (RANGELAND) COVER TYPES :
101 Bluebunch wheatgrass
102 Idaho fescue
104 Antelope bitterbrush-bluebunch wheatgrass
105 Antelope bitterbrush-Idaho fescue
106 Bluegrass scabland
107 Western juniper/big sagebrush/bluebunch wheatgrass
109 Ponderosa pine shrubland
110 Ponderosa pine-grassland
215 Valley grassland
216 Montane meadows
301 Bluebunch wheatgrass-blue grama
302 Bluebunch wheatgrass-Sandberg bluegrass
303 Bluebunch wheatgrass-western wheatgrass
304 Idaho fescue-bluebunch wheatgrass
305 Idaho fescue-Richardson needlegrass
306 Idaho fescue-slender wheatgrass
307 Idaho fescue-threadleaf sedge
308 Idaho fescue-tufted hairgrass
309 Idaho fescue-western wheatgrass
310 Needle-and-thread-blue grama
311 Rough fescue-bluebunch wheatgrass
312 Rough fescue-Idaho fescue
314 Big sagebrush-bluebunch wheatgrass
315 Big sagebrush-Idaho fescue
316 Big sagebrush-rough fescue
317 Bitterbrush-bluebunch wheatgrass
318 Bitterbrush-Idaho fescue
319 Bitterbrush-rough fescue
320 Black sagebrush-bluebunch wheatgrass
321 Black sagebrush-Idaho fescue
322 Curlleaf mountain-mahogany-bluebunch wheatgrass
323 Shrubby cinquefoil-rough fescue
324 Threetip sagebrush-Idaho fescue
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
404 Threetip sagebrush
405 Black sagebrush
406 Low sagebrush
407 Stiff sagebrush
408 Other sagebrush types
409 Tall forb
411 Aspen woodland
412 Juniper-pinyon woodland
413 Gambel oak
504 Juniper-pinyon pine woodland
601 Bluestem prairie
602 Bluestem-prairie sandreed
603 Prairie sandreed-needlegrass
613 Fescue grassland
614 Crested wheatgrass
HABITAT TYPES AND PLANT COMMUNITIES:
Montana is the center of distribution for spotted knapweed in the United States. There it tends to favor ponderosa pine (Pinus ponderosa)/Douglas-fir (Pseudotsuga menziesii), and adjacent foothill prairie habitats (previously dominated by bluebunch wheatgrass (Pseudoroegneria spicata), Idaho fescue (Festuca idahoensis), rough fescue (F. altaica), and needle-and-thread grass (Hesperostipa comata)) [24,25,125,159,160], although it is said to grow in nearly every habitat type west of the Continental Divide in Montana . On undisturbed sites (e.g., Glacier National Park) additional associates may include slender wheatgrass (Elymus trachycaulus), prairie junegrass (Koeleria macrantha), timber oatgrass (Danthonia intermedia), Richardson needlegrass (Achnatherum richardsonii), western yarrow (Achillea millefolium), northern bedstraw (Galium boreale), field chickweed (Cerastium arvense), silky lupine (Lupinus sericeus), and cryptogams . On disturbed sites, old hayfields, and pastures, common associates are Kentucky bluegrass (Poa pratense), smooth brome (Bromus inermis), timothy (Phleum pratense), and cheatgrass (B. tectorum) [157,175]. Spotted knapweed is found in the Idaho fescue/bluebunch wheatgrass and bluebunch wheatgrass/blue grama (Bouteloua gracilis) habitat types, and the antelope bitterbrush (Purshia tridentata) series in Montana, as described by Mueggler and Stewart . Spotted knapweed occurs along roadsides in Montana and North Dakota in shortgrass prairie types with blue grama; in mixed-grass with wheatgrass (Triticacea), needlegrass (Achnatherum, Hesperostipa, and Nassella spp.) and little bluestem (Schizachyrium scoparium); in foothill grassland with bluebunch wheatgrass, Idaho fescue, and rough fescue; and in ponderosa pine, Douglas-fir, and subalpine fir (Abies lasiocarpa) forest types. Spotted knapweed tends to escape the roadside disturbance area and penetrate less disturbed communities in the shortgrass, mixed-grass, and foothill grassland environmental types as described by Meier and Weaver . Other Montana associates may include common snowberry (Symphoricarpos albus), Oregon-grape (Mahonia repens), basin wildrye (Leymus cinereus), green needlegrass (N. viridula), whitetop (Cardaria draba), long-leaved aster (Aster chilensis), spreading dogbane (Apocynum androsaemifolium) , pinegrass (Calamagrostis rubescens), elk sedge (Carex geyeri), thistles (Cirsium and Carduus spp.), and kinnikinnick (Arctostaphylos uva-ursi) . Spotted knapweed is also found in pinyon-juniper (Pinus-Juniperus spp.) communities in the Intermountain West .
Spotted knapweed is listed as a "dominance type" in Montana riparian areas and is found primarily on upper terraces of major river courses, relatively dry, disturbed sites, and gravel bars. Associated species are affected by the degree of past disturbance. On relatively undisturbed sites it may be found with bluebunch wheatgrass, redtop (Agrostis spp.), silver sagebrush (Artemisia cana), Idaho fescue and Wood's rose (Rosa woodsii). On more disturbed sites likely associates include Canada bluegrass (Poa compressa), Kentucky bluegrass, bulbous bluegrass (P. bulbosa), slender cinquefoil (Potentilla gracilis), crested wheatgrass (Agropyron cristatum), and Canada thistle (Cirsium arvense) . It may occur with other species associated with frequent disturbance such as common tansy (Tanacetum vulgare), creeping bentgrass (Agrostis stolonifera), and widely scattered seedlings of sandbar willow (Salix exigua) and black cottonwood (Populus balsamifera ssp. trichocarpa) . Hansen and others  list spotted knapweed occurring in 24 riparian habitat types in Montana. Dominant associates include red-osier dogwood (Cornus sericea), black cottonwood, grand fir (Abies grandis), ladyfern (Athyrium filix-femina), spruce (Picea spp.), Douglas-fir, Rocky Mountain juniper (Juniperus scopulorum), ponderosa pine, quaking aspen (Populus tremuloides), narrowleaf cottonwood (P. angustifolia), Geyer willow (Salix geyeriana), sandbar willow, Wood's rose, thinleaf alder (Alnus incana ssp. tenuifolia), water birch (Betula occidentalis), shrubby cinquefoil (Potentilla fruticosa), chokecherry (Prunus virginiana), western snowberry (Symphoricarpos occidentalis), water sedge (Carex aquatilis), Baltic rush (Juncus balticus), and reed canarygrass (Phalaris arundinacea).
In Washington state, spotted knapweed is found in openings in ponderosa pine/bunchgrass or Douglas-fir/shrub forests, especially on coarse, gravelly glacial soils . Spotted knapweed is common in disturbed forests in northeaster Washington, including ponderosa pine with bunchgrass, snowberry (Symphoricarpos spp.), or ninebark (Physocarpus malvaceus); or Douglas-fir with snowberry or ninebark .
Plants associated with spotted knapweed in British Columbia are characteristic of dryland range and pioneer sites including bluebunch wheatgrass, rough and Idaho fescue, bluegrass (Poa spp.), and many others . Bluebunch wheatgrass communities in British Columbia are especially susceptible to spotted knapweed infestation . In west-central British Columbia, an infestation of spotted knapweed occurred along the railroad with Kentucky bluegrass, red fescue (Festuca rubra), timothy, smooth brome, and clover (Trifolium spp.) . In open forests of British Columbia, spotted knapweed may be found with ponderosa pine, Douglas-fir, ninebark, Saskatoon serviceberry (Amelanchier alnifolia), mock orange (Philadelphus lewisii), toadflax (Linaria dalmatica), pinegrass, and Idaho fescue ; or with lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), subalpine fir, huckleberry (Vaccinium spp.) and fireweed (Epilobium angustifolium) .
Information on plant communities invaded by spotted knapweed in other parts of the country
is very limited. In North Dakota spotted knapweed is found primarily along roads and
sometimes in adjacent grasslands . In Michigan oldfields it is found growing with blackberry
(Rubus alleghaniensis), poverty oatgrass (Danthonia spicata), and broomsedge
(Andropogon virginicus) . In Shenandoah National Park, Virginia, spotted knapweed was
found growing along a trail in the bigfruit hawthorn (Crataegus macrosperma)/shrub forest
with black raspberry (R. occidentalis) and dwarf cinquefoil (Potentilla canadensis)
. Spotted knapweed is said to be widespread in California, occurring in disturbed areas up
to 6,600 feet (2,012 m) [67,183]. In Yellowstone National Park it is found in a campground in a
big sagebrush (Artemisia tridentata)/bluebunch wheatgrass habitat type .
|Photo ©John M. Randall/The Nature Conservancy|
GENERAL BOTANICAL CHARACTERISTICS:
Spotted knapweed is a nonnative, perennial forb that can live at least 9 years. Plants more than 7 years old may have high incidence of root rot, indicating senescence . Once established, spotted knapweed can form monotypic stands because its age class hierarchy allows it to occupy all available niches .
Spotted knapweed has a deep, stout taproot and basal rosette leaves that grow up to 8 inches (20 cm) long and 2 inches (5 cm) wide. Beginning usually the second year, each spotted knapweed plant produces 1 to 6 flowering stems (15 or more on older plants) that stand 8 inches to 4 feet (0.2 - 1.2 m) tall. Leaves are alternate and grow smaller near the tops of the stems. Stems branch in the upper half and bear terminal flowerheads that are 0.25 inch (6 mm) wide and 0.5 inch (12 mm) long. Flowerheads are solitary or borne in clusters of 2 or 3, with 25 to 30 flowers per head, and up to 60 flowering heads per plant . Achenes are oval, 1/8 inch (3 mm) long, bearing a pappus of simple bristles which are less than the length of the seed (~1-2 mm) and persistent . Most stems remain erect after drying, with leaves and flowerhead bracts attached .
Early and deep fine root development, and the colonization of spotted knapweed
roots by arbuscular mycorrhizal fungi, may contribute to its invasiveness in
native grasslands [116,117] by allowing for greater resource acquisition. Chemical
allelopathy of spotted and diffuse knapweeds has been suggested as a mechanism of
interference with the growth of other herbaceous species
RAUNKIAER  LIFE FORM:
Spotted knapweed reproduces almost entirely from seed. Plants are also able to extend lateral shoots below the soil surface that form rosettes adjacent to the parent plant, and multiple rosettes on a single spotted knapweed root crown are common [174,220].
Flowering and pollination: Spotted knapweed plants may remain in the rosette stage for 1 to 4 years, producing flowering stems the second year  or later [16,205]. Flowering during the year of seedling emergence is rare . Boggs and Story  observed the percentage of flowering plants increasing with age up to 5 to 7 years, with little or no flowering in the first and second years, in Montana. Flowers are pollinated by insects , and spotted knapweed is heavily visited by several species of bees [35,64,220]. Large pollen counts in late July and early August in the Missoula Valley, Montana, suggest that spotted knapweed is also wind pollinated . Fertilization in spotted knapweed requires cross-pollination between flowers on different plants (obligately xenogamous). This can limit the reproductive success of isolated individuals, but it also promotes genetic diversity, and may thereby improve competitive ability .
Seed production: The number of seeds produced by an individual knapweed plant or a population of plants is highly variable among plants, sites, and years. Reported average seed production per plant ranges from about 65 seeds per plant in Montana , 400 to 900 seeds per plant under range conditions in British Columbia , and about 2,000 seeds per plant, averaged across diverse sites in Washington and Idaho . A population of plants may produce about 5,000 to 40,000 seeds per m˛ per year [174,205]. Schirman  reported an average of 23.8 to 61.1 flowerstems per m˛, 11.2 to 16.8 seedheads per stem, and 24.3 to 33 seeds per head, producing 11,300 to 29,600 seeds per m˛ in Washington and Idaho. Similar stem and seed densities were observed in Glacier National Park, Montana . Only about 0.1% of the seed produced under these conditions would be needed to maintain the size of the stands observed .
|Spotted knapweed seed production and viability on Idaho fescue-bluebunch wheatgrass habitat types in Montana |
|Adults/m˛||Seeds/plant||Viability (%)||Total # seeds/m˛||Viable seeds/m˛|
|Site 1 (1994)||99||21||60||2,062||1,237|
|Site 1 (1995)||10||140||71||1,405||9,98|
|Site 1 (1996)||54||61||79||3,312||2,617|
|Site 2 (1994)||177||62||71||11,007||7,815|
|Site 2 (1995)||151||55||82||8,519||6,986|
|Site 2 (1996)||89||51||78||4,524||3,529|
Spotted knapweed seed production may vary with site conditions (available moisture, nutrient availability and competition), herbivory and seed predation, and age of plants. Site conditions and precipitation during the growing season probably have the greatest effect on the number of seeds produced each year, with more seeds produced during wet years [166,174] and on wetter sites . Schirman  reported a reduction in the number of viable seeds per flowerhead in dry years and an increase in the number of flowerheads per stem in wet years. On an irrigated site, spotted knapweed produced an average of 25,260 seeds per plant, compared to about 680 seeds per plant under range conditions in British Columbia . Adult plant density also appears to be sensitive to spring precipitation . Maternal treatment (control, herbivory, herbivory + nutrient shortage, and herbivory +nutrient shortage + grass competition) affected physiology, morphology, growth, and size of maternal plants and the numbers of seeds produced, but did not affect the mass or quality of seeds and offspring produced . In Glacier National Park, seed production is 3 to 4 times higher in plants immediately adjacent to the road than in plants on adjacent grassland .
Seed predation may be an important factor in seed production. The seed production numbers reported by Schirman  in 1973 through 1976 were measured before the introduction of Urophora affinis and U. quadrifasciata. Jacobs and Sheley  reported in 1998 that 90% of the seedheads collected in their study were infested with larva of these insects. Shirman  consistently found 2 to 10 times the seed production in his study, compared with data from Jacobs and Sheley . Differences in seed production may be attributable to differences in precipitation and/or seed predation between sites and years . Harris  reports similar findings and notes that prior to the release of biocontrol agents in British Columbia, Watson and Renney  reported an average of 26.6 seeds per head, whereas in 1986 Harris  observed an average of 15.4 seeds/head.
Seed dispersal: As soon as bracts open, any movement of the stem (e.g., by wind or passing animals) expels the loosely held seeds from the head with a flicking action. The seeds usually land within 3 to 4 feet (0.9-1.2 m) of the parent plant [130,173,220]. In this way, spotted knapweed populations spread outward and downwind from the perimeter of existing stands [159,174,220]. Dispersal of achenes over long distances is facilitated by animals and birds. Wallander and others  show that both domestic sheep and mule deer excrete viable seeds of spotted knapweed in their feces for 7 to 10 days after consumption, respectively. Seeds mixed with soil and mud may be carried by vehicles or other equipment that, in turn, create an ideal seedbed for spotted knapweed establishment [159,174,220]. Spread of seeds on logging trucks, off-road vehicles, and trail bikes has contributed greatly to the spread of knapweed into new areas in British Columbia . Spotted knapweed seeds can also be transported in rivers and other watercourses, and in crop seed and hay .
Seed banking: Spotted knapweed seeds are known for their longevity and durability. They have a thick, durable pericarp that protects the seeds but does not restrict water imbibition or water loss . Nolan and Upadhyaya  determined that a portion of the spotted knapweed seed population requires exposure to red light to germinate. This light requirement may permit buried seeds to remain dormant for an extended period of time [29,30]. Experimental evidence indicates that spotted knapweed seeds can remain viable but dormant after 5 years of burial . Perez and others  observed evidence of seed banking of spotted knapweed in Nebraska sandhills prairie.
|Spotted knapweed seed bank data collected on 2 Idaho fescue-bluebunch wheatgrass habitat types near Bozeman, Montana |
|Site (sample year)||Adult plants/m˛||Average number of seeds/m˛ (± SE)||Average number of viable* seeds/m˛ (± SE)|
|Site 1 (1994)||99||51,850+30,600||3,825+1,285|
|Site 1 (1995)||10||47,000+7,900||34+43|
|Site 2 (1994)||177||60,690+13,133||8,466+3,060|
|Site 2 (1995)||151||60,350+13,023||646+850|
When seed production was controlled with herbicide treatments at 2 other heavily infested spotted knapweed sites in Montana, the spotted knapweed seed bank decreased by 72% to 81% after 15 months . After 7 years, only 5% of the original seed bank remained, leaving about 160,000 viable seeds per acre (400,000 seeds/ha, or about 4 viable seeds per 0.09 m˛) [29,30]. Seedlings showed a decline in vigor associated with duration of burial .
Germination: Spotted knapweed seeds have the potential for germination shortly after maturity, and approximately 90% are viable upon dispersal [34,166]. However, many studies indicate a dormancy period for some portion of the annual seed crop. For example, Watson and Renney  observed an increase in germination from 40% to 80% after 25 days of dry storage. Similarly, reports of low germination and emergence under field conditions range from 65% to 85% of the seed crop ungerminated but viable the year following dispersal [23,34,166]. This period of dormancy may be released by seed aging, cool-moist stratification, freezing , or exposure to red light . Nolan and Upadyaya  describe 3 distinct types of germination behavior in spotted knapweed seed, a phenomenon that distributes seed germination over time by facilitating the incorporation of seeds into a seed bank.
Spotted knapweed seeds germinate whenever moisture and temperature are suitable, and both fall and spring seedling emergence is common. Optimal temperatures for germination range from 45 to 93 °F (7-34 °C), and germination is best at 66 °F (19 °C) . Spotted knapweed seeds required at least 55% soil moisture to initiate emergence. Germination increases with increased soil moisture, and 65% to 70% soil moisture content is optimum for germination . Dormancy may prevent germination at higher temperatures when soil moisture status is fluctuating, and at lower temperatures when germination in late fall may make seedlings susceptible to winter kill. Germination after cold stratification provides a strategy for spring emergence and avoidance of environmental extremes . Canopy cover had no effect on emergence rate, with spotted knapweed germinating equally well over a range from 0% to 100% canopy cover, as simulated in a laboratory experiment . Spotted knapweed seeds can germinate in light and dark, with maximum germination in alternating light and dark periods. Optimum germination occurs with the seeds at the soil surface, and decreases with burial depth, with little germination below 2 inches (5 cm) [181,220]. Because of the bare interspace areas between plants and seasonal periods of drought, bunchgrass rangeland provides favorable microsites for spotted knapweed seed germination .
Seedling survival: Spotted knapweed seedling survival depends primarily on environmental conditions at the time of emergence and establishment. Seed weight was positively correlated with initial growth, but its influence decreased over time and disappeared after 8 weeks. Competition (with meadow ryegrass (Lolium pratense)) did not influence growth of spotted knapweed seedlings during early weeks, but strongly suppressed growth after 9 weeks . Spotted knapweed seedling survival is poor when conditions are dry following emergence , and survival is enhanced if precipitation coincides with the time of seedling emergence . Seedling mortality averages 12%, but can be as high as 55% under dry conditions [174,220]. Seedlings emerging in April in Idaho and Washington had a high rate of survival, with most plants flowering the following growing season. Seedlings emerging after May 15 had a very low survival rate and almost no flowerstem production the following season .
Spotted knapweed establishes and dominates on dry, disturbed sites, especially along roads [159,167,206,215,220]. In western Montana, the success of spotted knapweed increases with site disturbance and soil moisture stress. Disturbance intensity has the greatest influence in habitat types moister than the Douglas-fir group, with coarse soil texture and steep slopes adding to success. In grass and shrub habitat types, south aspect and disturbance intensity are important variables for spotted knapweed success . Spotted knapweed is well adapted to open forested areas, especially after logging or other disturbances .
Spotted knapweed is found on soils with a wide range of chemical and physical properties [25,220], and often on "poor" soils . It does especially well in coarse-textured soils [121,162] that are well-drained with low water holding capacity . Spotted knapweed is well adapted to Montana rangelands with "light-textured" soils that receive summer rainfall. In northeastern Washington, it is usually found on glacial till and outwash soils . Spotted knapweed is poorly adapted to irrigated pastures where saturated soil is common [30,220], and does not compete well with vigorously growing grass in moist sites . However, in central Washington spotted knapweed thrives in irrigated land, although it is also found growing on compacted soil in a 10-inch (254 mm) precipitation zone .
Spotted knapweed occurs at elevations ranging from 1,900 to 9,975 feet (578-3,040 m) and in precipitation zones ranging from 7 to 79 inches (200-2000 mm) .
|Spotted knapweed occurrence by elevation and precipitation range by area|
|State or Province||Elevation||Annual precipitation||References|
|MT||2,001 to 8,999 feet
|12 to 30 inches
|WA||up to 6,800 feet
| 16 to 22 inches
|BC||98 to 3,937 feet
|10 to 25.5 inches
In Montana, spotted knapweed is most abundant between 4,000 and 6,000 feet (1,219-1,829 m) and in areas with 10 to 80 inches (250-2,030 mm) annual precipitation with 50 to 120 frost-free days [4,25]. In British Columbia, spotted knapweed is most common below 2,950 feet (900 m) on south-facing slopes .
Spotted knapweed not only readily occupies disturbed sites, but it also invades relatively undisturbed perennial native plant communities in the northern Intermountain region [31,101,173,205,206], and invades wilderness areas all over Montana . In Glacier National Park, spotted knapweed established in undisturbed rough fescue grasslands adjacent to roadside spotted knapweed infestations . The frequent breaks in cover, which provide favorable light conditions for seed germination, and the warm-dry climate of these grassland ecosystems appear to facilitate invasion of spotted knapweed and other nonnative species [130,205].
Spotted knapweed usually establishes shortly after disturbance, especially when overstory species are removed. In western Montana, for example, spotted knapweed invades when the dominant species from ponderosa pine/red-osier dogwood or black cottonwood/red-osier dogwood riparian site types have been removed . Spotted knapweed seeds are able to germinate under full canopy , but mature plants are uncommon in shaded areas . Spotted knapweed is typically found under open canopies [115,162,220]. In Yellowstone National Park, spotted knapweed was always found under <20% canopy cover, and 75% of its occurrence was under <5% canopy cover . In a grand fir/queencup-beadlily (Clintonia uniflora) habitat type in the Selway-Bitterroot Wilderness, spotted knapweed occurred in a 15-year-old stand but not in a 185-year-old stand .
Hironaka  presents a replacement series of weedy species in the Intermountain Region in which summer annuals are replaced by winter annuals (e.g., Russian-thistle (Salsola kali) is replaced by cheatgrass), and the earlier winter annuals are replaced by the later maturing ones (e.g., cheatgrass is replaced by medusahead (Taeniatherum caput-medusae) or knapweeds (Centaurea spp.)).
Spotted knapweed seeds germinate in the fall and early spring and develop into rosettes, though most recruitment is between April and June . Most root growth occurs during this stage .
Spotted knapweed overwinters as roots, rosettes, or seeds. Rosette mortality may occur under extreme winter conditions, in which case root crowns form rosettes in early spring and bolt in early May . In the Intermountain West, rosettes that overwinter also bolt in early May [174,220]. Seeds that overwinter germinate in early spring. Early spring growth gives spotted knapweed a competitive advantage over many natives for soil moisture and mineral nutrients . Flowering buds are formed in early June and flowering occurs from July through September. Mature seeds are formed by mid-August. Spotted knapweed flowerheads that are not infested with larvae open after they dry, about 2 to 3 weeks after seeds mature, usually in late summer , from mid-August through mid-September [34,174]. Most seeds are shed upon maturity; very few overwinter in seedheads . At 2 sites in Montana, seed rain occurred during August and September, with 30% to 62% of the seeds produced reaching the soil surface by October . Rosettes that do not bolt die back to the root crown, which serves as the perennating part of the plant over winter .
In Virginia, flowering of spotted knapweed occurs from June through August
and sometimes as late as November, and fruiting occurs from mid-June to November .
In the Adirondacks, spotted knapweed flowers 19 July to 20 August .
FIRE ECOLOGY OR ADAPTATIONS:
Spotted knapweed has a large, perennial taproot and is likely to survive and sprout after fire if the root crown is not killed. It also produces large quantities of durable, heat-tolerant seed that can probably survive most grassland fires, although high severity fire may kill some spotted knapweed seeds . See Fire Effects for more detail.
Spotted knapweed occurs
primarily in bunchgrass and open ponderosa pine forest community types in the
Intermountain West, especially Montana. The historical fire regimes of these
communities were relatively frequent, low-severity surface fires. Spotted
knapweed did not occur in these communities at the time in which these fire
regimes were functioning, but has established since fire exclusion began. It is
unclear how this type of fire regime might affect spotted knapweed populations.
It is also unclear how the presence of spotted knapweed might affect these fire
regimes, though it has been observed that spotted knapweed does not carry fire
as readily as grasses. Therefore, dense knapweed infestations may alter fuel
characteristics at a given site and thus affect fire regime characteristics such
as return interval and severity [120,234]. See
Management Considerations for more detail.
The following table provides fire regime intervals for communities and ecosystems in which spotted knapweed most commonly occurs. For more information on fire regimes in these communities, see the FEIS review for the dominant species listed here. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".
|Community or Ecosystem||Dominant Species||Fire Return Interval Range (years)|
|grand fir||Abies grandis||35-200 |
|sagebrush steppe||Artemisia tridentata/Pseudoroegneria spicata||20-70|
|plains grasslands||Bouteloua spp.||<35|
|western juniper||Juniperus occidentalis||20-70|
|Rocky Mountain juniper||J. scopulorum||<35 |
|western larch||Larix occidentalis||25-100|
|Engelmann spruce-subalpine fir||Picea engelmannii-Abies lasiocarpa||35 to >200 |
|pinyon-juniper||Pinus-Juniperus spp.||<35 |
|Rocky Mountain lodgepole pine*||P. contorta var. latifolia||25-300+ [6,9,164]|
|Colorado pinyon||P. edulis||10-49 |
|interior ponderosa pine*||P. ponderosa var. scopulorum||2-10 |
|red pine (Great Lakes region)||P. resinosa||10-200 (10**) [32,41]|
|eastern white pine||P. strobus||35-200|
|eastern white pine-northern red oak-red maple||P. strobus-Quercus rubra-Acer rubrum||35-200 |
|mountain grasslands||Pseudoroegneria spicata||3-40 (10**) [6,9]|
|Rocky Mountain Douglas-fir*||Pseudotsuga menziesii var. glauca||25-100 |
|oak-juniper woodland (Southwest)||Quercus-Juniperus spp.||<35 to <200 |
|northern red oak||Q. rubra||10 to <35 |
|elm-ash-cottonwood||Ulmus-Fraxinus-Populus spp.||<35 to 200 [32,216]|
POSTFIRE REGENERATION STRATEGY :
Caudex/herbaceous root crown, growing points in soil
Ground residual colonizer (on-site, initial community)
Secondary colonizer (on-site or off-site seed sources)
IMMEDIATE FIRE EFFECT ON PLANT:
As of this writing (2002), little published information is available regarding direct effects of fire on spotted knapweed. Low-severity fire is not likely to kill spotted knapweed plants or seeds. Fire may top-kill spotted knapweed and stress the plant; however, the sturdy perennial taproot is likely to survive most low-severity fires, and plants can sprout from the root crown. Spotted knapweed seeds are durable and persistent (see Seed banking) and likely to survive all but severe fire .
DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
In a greenhouse study using seeds from a spotted knapweed infested site in Michigan, germination of spotted knapweed seeds was significantly (P<0.05) reduced in seeds exposed to 392 °F (200 °C) for 120 seconds or more, and for seeds exposed to 752 °F (400 °C) for 30 seconds or more .
PLANT RESPONSE TO FIRE:
Much of the available information on spotted knapweed response to fire is anecdotal, and details are lacking. Spotted knapweed is one of the introduced species mentioned as "taking over large tracts of logged, burned, or otherwise disturbed lands in British Columbia" . Olson  observes that spotted knapweed is "seldom impacted by fire." Following a study to test control methods for spotted knapweed on a northeastern Washington rangeland, Sheley and Roche  report in an abstract: "Treatments which did not include herbicide generally yielded the greatest amount of weeds and least amount of forage." No additional information is given on the characteristics of the fire or comparisons to controls; however, Sheley and others  interpret this study as suggesting that "a single, low-intensity fire increased the cover and density of this weed without improving the residual, desirable understory species" . Conversely, a land manager in Michigan has used prescribed fire and spot-burning to stimulate native vegetation and reduce cover of spotted knapweed on prairie and dune sites .
Based on its regeneration strategies, one might assume that fire could promote establishment and spread of spotted knapweed by creating areas of bare soil and increasing the amount of sunlight that reaches the soil surface . Spotted knapweed plants present before burning may sprout from root crowns, and seedlings may emerge from the soil seed bank or establish on bare ground from an off-site seed source following fire. Fire studies and unpublished data from western Montana [8,58,132,199] generally support the observation that where propagules are available, spotted knapweed is likely to establish, persist, and/or spread following fire in that geographic area. Conversely, manager observations in Michigan prairies suggest that spotted knapweed can be controlled by prescribed fire . See details below.
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Data on postfire response of spotted knapweed in western Montana are limited. Available information implies that spotted knapweed spread may be favored by high severity fire and by fall burning versus spring burning; however, data are insufficient for drawing firm conclusions regarding effects of different burn seasons and severities.
In Douglas-fir-ponderosa pine communities in western Montana, spotted knapweed cover increased after both high and low fuel consumption spring prescribed fires following shelterwood cuts. Spotted knapweed cover was highest in the high consumption burn area, both before and after treatment, so it is not clear whether the higher severity burn led to greater increases in spotted knapweed than the low consumption burn. Changes in spotted knapweed cover during the 4 years following treatments were as follows :
|Treatment||Average percent cover of spotted knapweed|
|Prefire||Postfire year 1||Postfire year 2||Postfire year 3||Postfire year 4|
|Low consumption burn||0.5||1.0||3.4||4.9||5.9|
|High consumption burn||1.9||3.6||7.3||11.4||14.1|
Fuel moisture and duff characteristics of the fires were as follows :
|Treatment||Fuel moisture content||Duff characteristics|
|Lower duff||Slash||Initial depth||Depth reduction|
|Low consumption burn||50%||90%||1.1 inch
|High consumption burn||16%||40%||1.4 inch
Spotted knapweed was present on these sites before treatments were carried out, probably as a result of soil scarification during logging activities, as well as big game and livestock use of the site prior to these studies. The harvesting done as a part of this study resulted in scarification of about 11% of the area, which evidently encouraged spotted knapweed establishment. Burning seems to further encourage the spread of spotted knapweed .
Spotted knapweed was present before burning, but was not present in the first year after fall prescribed burning at a western Montana site in a Douglas-fir/ninebark habitat type dominated by snowbush ceanothus (Ceanothus velutinus). Two years following the burn, spotted knapweed volume had doubled compared with preburn volume . On a spring burn of lesser intensity on an adjacent site, no spotted knapweed was recorded before or after burning. Shrubs recovered faster and grasses increased on the spring burn, while herbaceous cover recovered faster and nongraminoid, herbaceous species increased on the fall burn .
Spotted knapweed was reported on a Douglas-fir forest site 5 to 10 years after wildfire in western Montana, though it was not recorded 3 years after the fire. No information on prefire vegetation was given .
The Research Project Summary Vegetation response to restoration treatments in ponderosa pine-Douglas-fir forests provides information on prescribed fire and postfire response of plant community species, including spotted knapweed, that was not available when this species review was originally written.
On prairie sites in Michigan with low to moderate spotted knapweed density and sufficient fine fuels to carry a fire, annual spring broadcast burning under severe conditions (when humidity and dead fine fuel moisture are as low as possible) reduces spotted knapweed populations and favors native prairie vegetation. On some sites, 3 years of this regimen reduced spotted knapweed to the point where it could be controlled by hand-pulling individuals and increasing the fire return interval to 3 to 5 years. In dense infestations (>60 rosettes/m˛), broadcast burning is ineffective due to lack of adequate fuel to carry the fire. In this case, spotted knapweed plants can be killed with repeated spot-burning (using a propane torch) of individuals and resprouts 3 to 4 times during the growing season until root reserves are depleted. Top-killing individuals allows seedlings to emerge in the time between burning treatments, so they are also killed with subsequent burning, thus depleting the seed bank. This treatment does not seem to harm existing prairie natives, but newly-germinating natives may be at risk, and seeding after the last burning may help them to recover .
FIRE MANAGEMENT CONSIDERATIONS:
It is thought that prescribed fire has little potential for spotted knapweed control, because small areas are left unburned and fires are not usually hot enough to eliminate all of the viable seed in the soil or to prevent root crowns from sprouting [97,149,173]. Repeated burning of prairie and dune sites in Michigan was, however, successful at encouraging the growth of native vegetation and reducing spotted knapweed density  (see above). Experimental evidence is limited.
Some authors suggest combining herbicide applications with prescribed fire to increase control of spotted knapweed. Rice  suggests controlling weeds (including spotted knapweed) with herbicides before reintroducing fire for the restoration of native plant communities, to avoid increasing the weed problem by using fire alone. A study is underway in western Montana comparing different combinations and timing of prescribed burning and herbicide applications . It has also been suggested that burning prior to herbicide application can increase the efficacy of the herbicide and stimulate new growth from competitive species . A study in western Montana  did not support this assertion, probably because dry conditions following burning limited spring germination of knapweed prior to herbicide application. A project was begun in the fall of 1996 on the Lolo National Forest in western Montana, in which herbicide treatments were coupled with prescribed burning to improve habitat for elk. Prescribed burning in April 1997, followed by aerial application of picloram in June 1997 resulted in a 95% decrease in weed biomass (primarily spotted knapweed with small amounts of common mullein (Verbascum thapsis) and leafy spurge (Euphorbia esula)), an 86% decrease in native forb biomass, and a 714% increase in grass biomass as of July 1998 . The area was retreated in 1999 with a lower concentration of picloram that was successful in killing spotted knapweed plants missed in 1997 and in killing the young knapweed that had sprouted after the original treatments. These results suggest that treatments were successful at reducing spotted knapweed cover and increasing native grass cover, although native forb density decreased. Changes in plant community composition were not recorded .
Prescribed burning of spotted knapweed can be difficult, especially if no fine fuels are present, because fire does not usually carry through spotted knapweed stems easily. Grass fuel models work poorly for spotted knapweed unless associated grasses exceed 40% to 50% cover . Spotted knapweed fuel loading varies between sites, thus affecting fire behavior. Intense fires in spotted knapweed have been observed under some conditions, and prescribed burning in spotted knapweed for fire hazard reduction may be a consideration .
A fuel model for spotted knapweed and guidelines for prescribed burning are available [233,234]. Calculation of fuel load is based on knapweed plant height and percentage of ground cover (old, standing plants and new plants), and litter depth and cover (including sparse grasses) . The model was developed for early spring burns and is valid only under specific fine fuel loading conditions. It is offered as a guideline to help the user select environmental conditions (wind, dead fuel moisture) that allow safe and effective burns. Fire managers familiar with fire modelling can create their own site-specific models for knapweed infestations using this general knapweed fuel model as it is summarized by Xanthopoulos [233,234].
When prescribed burning in spotted knapweed for fire hazard reduction in spring, sustaining a fire that carries without interruption and without risk of escape generally requires a predicted flame length of at least 20 cm (using a fuel model that does not include standing knapweed), and a controllable flame length less than 120 cm (as predicted using a complete fuel model). Fire behavior problems caused by discontinuous and nonuniform fuels are similar to the ones found in big sagebrush. Use these models with caution, since they have been verified with few actual test burns in spotted knapweed .
The following seasonal moisture data for spotted knapweed were collected on a south-facing slope in the Missoula Valley, Montana :
Mean moisture content
(% of dry weight)
|7/1/86||235||Flowerheads in dough stage|
|7/9/86||210||Few flowers open; grasses drying|
|7/15/86||170||Approximately 20% of the flowers open; grasses mostly dry|
|8/1/86||120||80% of the flowers open; <3% of flowers lost petals and dried; lower knapweed leaves turning yellow; grasses mostly cured|
|8/6/86||93||All flowers open; 5-10% of flowers lost petals; lower knapweed leaves mostly dry|
|8/20/86||45||Spotted knapweed plants look dry; most leaves crumbly and yellow; less than 10% of flowers retain petals|
|8/27/86||30||Plants with any green leaves very rate; less than 2% of flowers retain petals|
The USDA Forest Service's "Guide to noxious weed prevention practices"  provides several fire management considerations for weed prevention in general that apply to spotted knapweed. To prevent infestation, re-establish vegetation on bare ground as soon as possible using either natural recovery or artificial techniques as appropriate to site objectives. When reseeding after wildfires and prescribed burns use only certified weed-free seed. Monitor the burn site and associated disturbed areas after the fire and the following spring for emergence of spotted knapweed, and treat to eradicate any emergent spotted knapweed plants. Regulate human, pack animal, and livestock entry into burned areas at risk for weed invasion until desirable site vegetation has recovered sufficiently to resist weed invasion.
Additionally, when planning a prescribed burn,
preinventory the project area and evaluate cover and phenology of any spotted
knapweed present on or adjacent to the site, and avoid ignition and burning in areas at high risk
for spotted knapweed establishment or spread due to fire effects. Avoid creating soil conditions that promote weed
germination and establishment. Discuss weed status
and risks in burn rehabilitation plans. Also, wildfire managers might consider
including weed prevention education and providing weed identification
aids during fire training; avoiding known weed
infestations when locating fire lines; monitoring camps, staging areas,
helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free;
incorporating weed prevention into fire rehabilitation plans; and acquiring restoration
funding. Additional guidelines and specific recommendations and requirements are
|Photo ©John M. Randall/The Nature Conservancy|
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
The importance of spotted knapweed to livestock and wildlife depends upon the size and density of the infestation, the availability of other forage plants, and the season. Large-scale infestations of spotted knapweed can impede access to more desirable forage for livestock and wildlife, especially when the presence of old, dried knapweed stems creates a dense and spiny overstory . Large reductions in available forage  and wildlife use [10,173] have been reported on knapweed-infested range. Reports of forage losses for elk vary, and probably do not consider the possibility of elk using spotted knapweed as forage [11,231]. In fact, the perception that spotted knapweed has minimal forage value and may even be toxic  has changed since studies have shown that it has good nutritional value , and it is eaten by both livestock  and wildlife [107,228].
Several reports from western Montana indicate some use of spotted knapweed by livestock. In general, use of spotted knapweed by livestock is highest during spring and early summer when plants are green and actively growing in the rosette and bolt stages [96,158]. Use declines as spotted knapweed matures, and protein and digestibility decrease , although flowerbuds and seedheads may be grazed in the late summer [26,96,108]. Domestic sheep may also graze rosettes when present (from regrowth and late season germination) in the fall . Cox [26,27] observed domestic sheep eating large quantities of spotted knapweed in the spring and early summer in preference to grasses and other forbs. Olson and Wallander [139,140] also report sheep readily grazing spotted knapweed in an infested pasture, although they grazed other plants as well and did not consistently graze one plant more than the others. Sheep grazed spotted knapweed leaves and avoided stems. In sagebrush steppe rangeland in southeastern Idaho, spotted knapweed was readily consumed by domestic sheep throughout the year. Grasses were consumed in amounts similar to spotted knapweed, but native forbs were most commonly used . Sheep grazing has been proposed as a potential control method for spotted knapweed [27,53,106] (see Other Management Considerations).
Reports on wildlife use of spotted knapweed are also varied. Bedunah  cites several studies suggesting large potential losses of elk range to spotted knapweed, though he indicates that quantifying the effects of infestation on elk populations is complicated by their mobility. Spotted knapweed infestation is considered more detrimental to elk than to deer because spotted knapweed replaces grasses that are preferred by elk, while deer have a diet of predominantly shrubs and conifers that spotted knapweed does not replace [107,228]. Elk use increased following removal of spotted knapweed with herbicides on sites in Montana [155,198]. In a study in western Montana on 2 game ranges in the Bitterroot Valley, Willard and others  report minimal use of spotted knapweed dominated sites by elk and mule deer. When used, mule deer consumed knapweed flowerheads from December through April, while elk consumed knapweed flowerheads only during winter. Spotted knapweed was preferred by deer and elk over other plant species on sites with dense knapweed cover [107,228]. It was suggested that deer and elk did not frequently use the spotted knapweed sites in this study because cervid densities were relatively low and other forage was available . On spotted knapweed infested bunchgrass range in the Selway-Bitterroot Wilderness, Idaho, Wright and Kelsey  report that elk, mule deer, and white-tailed deer used spotted knapweed infested range as much as or more than uninfested bunchgrass range from 1 December through 25 April. All cervid species consumed both rosettes and seedheads of spotted knapweed, with seedhead consumption greatest during periods of snow cover. The authors suggest that when estimating carrying capacity of a cervid range, spotted knapweed can be considered a potential food source, because when animal densities are high and food choices are limited, elk and deer will consume spotted knapweed . In British Columbia, deer and elk forage on diffuse and spotted knapweed rosettes in late fall and early winter, and again when snow cover recedes and spring green-up commences. Knapweed rosettes and bluegrass comprised 90% of mule and white-tail deer diets in February and early March .
In the Gilpin range, British Columbia, California bighorn sheep utilized diffuse and spotted knapweed seedheads as primary forage when snow depth was in excess of 8 inches (20 cm). As snow cover receded in January and February, knapweed basal rosettes were the largest component (80%) of their diet. Rocky Mountain bighorn sheep also utilized knapweed seedheads and basal rosettes throughout the year in the Robson/Syringa Park area, British Columbia .
Spotted knapweed is a nectar source for the endangered Karner blue butterfly in Wisconsin . Rodent utilization of spotted knapweed seed has been suggested . Deer mice have been observed eating larvae and seeds from spotted knapweed flowerheads infested with seedhead flies (introduced biocontrol agents) .
It has been suggested that the bitter tasting sesquiterpene lactone, cnicin, found primarily in the leaves of spotted knapweed, may make it unpalatable to mammalian herbivores [88,111]. Wright and Kelsey  were unable, however, to correlate changes in cnicin levels to changes in the amount of spotted knapweed consumed by mule deer, white-tailed deer, or elk. Furthermore, observations by Cox  suggest that spotted knapweed is more palatable to domestic sheep than orchardgrass (Dactylis glomerata), timothy, quackgrass (Elytrigia repens), Kentucky bluegrass, sainfoin (Onobrychis viciifolia), or birdsfoot trefoil (Lotus spp.). In a cafeteria trial, domestic sheep readily consumed spotted knapweed in all growth stages, although they preferred rosette and bolting stages somewhat to the flowering stage . Robbins  also observed cattle readily grazing spotted knapweed in the spring, though cattle prefer grasses when available . Mature spotted knapweed plants (with stems) are eaten less frequently than young plants (rosettes), and may be less palatable due to spininess and high fiber content [24,174].
Spotted knapweed has substantial nutritional value that compares favorably to the native plants with which it is commonly associated. By traditional measures of forage nutritive value (e.g., crude protein (CP), neutral detergent fiber (NDF), in-vitro dry matter digestibility (IVDMD), and total nonstructural carbohydrates (TNC)), spotted knapweed (especially leaves and flowerheads) is more nutritious than Idaho fescue . The following table provides measurements of nutritive value of spotted knapweed, harvested before flowering, as reported by Kelsey and Mihalovich :
(% dry wt)
|IVDMD (%)||TNC (%)||Ash (%)||Ether extract (%)||Gross energy (cal/g)|
Values vary with season, plant part, age, and site. Nutritional value declines as summer progresses [140,231], with crude protein and nonstructural carbohydrates most concentrated during the spring. Spotted knapweed becomes more fibrous, with lower protein and carbohydrate levels, as stems mature over the summer . Seedheads are less nutritious than rosettes, but may be available above the snow . Willard and others  measured crude protein, fiber, and lignin content in spotted knapweed flowers and recorded average values of 6.6%, 45.6%, and 14.5%, respectively. Crude protein was similar for open and forested sites, while fiber and lignin values were higher on forested sites than on open sties . Jones and others  provide a detailed study of the forage value of spotted knapweed from a sagebrush/grassland site in southeastern Idaho, comparing different parts of mature and immature plants from May through September.
Secondary compounds in spotted knapweed, such as cnicin, can negatively affect activity and growth of anaerobic rumen microorganisms in domestic sheep, reducing digestibility of spotted knapweed [136,138].
VALUE FOR REHABILITATION OF DISTURBED SITES:
Watson and Renney  noted that "the rapid establishment of (spotted knapweed) cover in the form of rosettes on barren soil prevents soil erosion and leads to accumulation of organic matter." Kelsey  points out that "this characteristic is of marginal utility since the plants inhibit further succession and are difficult to replace with more desirable species." Furthermore, spotted knapweed establishment provides a seed source for invasion of adjacent lands and potential for reduction of native plant diversity . Lacey and others  determined that surface water runoff and stream sediment yield were 56% and 192% higher, respectively, and infiltration rates lower, for spotted knapweed-dominated sites compared to bunchgrass-dominated sites.
Kelsey and Locken  cite studies indicating that the compound cnicin has antimicrobial properties, as well as being active against some human carcinoma cells and L-1210 leukemia. Knapweeds provide substantial pollen and nectar for domestic bees in interior British Columbia , the Intermountain West , and Michigan . Kelsey  recommends short-term research to utilize spotted knapweed biomass for commercial products.
OTHER MANAGEMENT CONSIDERATIONS:
Spotted knapweed is considered a serious threat to rangelands in Montana, Washington, Idaho, Oregon, Wyoming, and British Columbia. Because of its affinity for the climate of western Montana, Chicoine and others  predicted that spotted knapweed had the potential to invade 37 million acres (15 million ha) in that state alone, and Bedunah  speculated that the foothill grasslands in western Montana (the primary habitat for spotted knapweed) are becoming an endangered vegetation type. The ill effects of spotted knapweed are manifold. Spotted knapweed infestations have been associated with reductions in forage production [60,220], plant species richness and diversity [203,205], cryptogam cover , soil fertility [65,136,231], and wildlife habitat , as well as increases in bare ground , surface water runoff, and stream sedimentation . Lesica and Shelly  also found that spotted knapweed reduced seed germination and seedling establishment of a rare Montana endemic forb, Mt. Sapphire rockcress (Arabis fecunda).
Experimental evidence suggests that spotted knapweed gains dominance in part by its ability to out-compete native grasses for nutrients such as nitrogen  and phosphorus . Other evidence suggests that as succession proceeds and nutrients become less available, the competitive advantage shifts from spotted knapweed to native plants such as bluebunch wheatgrass . Allelopathy, primarily from the compound cnicin, has been suggested as a growth inference mechanism in spotted knapweed [19,88]. However, because cnicin concentration in soil is lower than that found to be toxic in in-vitro experiments, allelopathy was not considered as important as resource competition in determining the ecological success of spotted knapweed [87,111]. Allelopathic activity of cnicin may be enhanced, however, when large quantities of stem and leaf tissue from live or dead spotted knapweed plants come in direct contact with the soil surface, as when plants are trampled or mowed. This allelopathic activity could be minimized by burning or removing plant material before it comes in contact with the soil surface . More recent experimental evidence suggests that knapweed's advantage over its North American neighbors could be attributed to differences in the effects of its root exudates and how they, in turn, affect competition for resources , thus linking allelopathy and resource competition.
In many areas, eradication of spotted knapweed is no longer an option. Perhaps small patches can be eradicated with cultural practices or herbicides. Large infestations must be controlled or suppressed with cultural and biological methods, perhaps in conjunction with herbicides, to contain the weed and slow its spread. Before management plans for the control of spotted knapweed can be designed, land use objectives must be defined. A generalized objective may be to develop a plant community that is weed resistant and meets other land-use objectives. Desired plant communities can be designed to maximize niche occupation with desirable species that compete intensely, grow rapidly, and grow during much of the season [73,176]. It is important that the successional effects of spotted knapweed control are considered in this manner to avoid replacing spotted knapweed with another weedy species [69,176,229].
Spotted knapweed control requires a sustained, site-specific commitment over a period of several years [96,205]. Economic considerations for spotted knapweed control have been investigated [50,136], and Griffith  provides a procedure for performing an economic evaluation for noxious weed management on rangeland, with spotted knapweed as an example.
Integrated weed management: Managers are encouraged to integrate different control methods that can complement one another in a given situation. Integrated management includes considerations of not only killing the target weed, but also of establishing desirable species and maintaining weed-free systems over the long-term. Factors to be addressed before a management decision is made include inventory and assessment to identify the target weed and determine the size of the infestations; assessment of non-target vegetation, soil types, climatic conditions and important water resources; and an evaluation of the benefits and limitations of control methods . Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies .
Conceptual models can be developed to determine the probability that the weed management strategy will result in the desired plant community, based on the life histories and population dynamics of the species in the existing plant community [73,176]. A weed management strategy may include designed disturbance (e.g., cultural or chemical control), controlled colonization (e.g., planting competitive species), and controlled species performance (e.g., biological control) [176,229]. Management strategies may include several approaches designed to disrupt the stages in a weed's life cycle that are most vulnerable to stress or control . Jacobs and Sheley  identified juvenile, the transition from juvenile to adult, and adult as critical phases in the life history of spotted knapweed. The key processes associated with these stages are competition, growth, and reproductive allocation. Successful control practices must target one or more of these processes. Sheley and others  provide examples of ecologically based spotted knapweed management systems.
Prevention: Prevention of spotted knapweed establishment is the most cost-effective control strategy . Prevention practices begin with the maintenance of healthy, desirable vegetation that is resistant to weed establishment. This includes minimizing soil disturbance in all activities and reestablishing desirable vegetation promptly whenever soil disturbance leaves areas of bare ground, with continued monitoring and immediate follow-up treatment of colonizing weeds, and/or revegetation with desirable species [31,205,206,208]. In areas where it is critical to maintain native vegetation (e.g., national parks, nature preserves, wilderness), avoid building new roads and trails, since this is the primary habitat for many invasive species including spotted knapweed [3,121,205,206]. Carefully monitor the intensity, frequency, and season of grass defoliation in grazing prescriptions so that grasses can tolerate grazing and resist weed invasion. One greenhouse study suggests that even moderate defoliation of competing grasses (30%) may allow greater spotted knapweed growth on Idaho fescue rangeland . Another study found that spring defoliations increased spotted knapweed cover compared to summer defoliations; that grass defoliation greater than 60% caused an increase in spotted knapweed cover and density; and that more than one grass defoliation in a year increased spotted knapweed cover. The researchers suggest that a single, annual grass defoliation of 60% or less, regardless of the season, will not increase spotted knapweed invasion on rangeland . Rangeland managers must also consider the potential for livestock to introduce spotted knapweed seed in their feces or fur.
Regular removal of newly established spotted knapweed plants at trailheads, campsites and along road corridors is critical to prevent their spread into adjacent natural areas [95,115]. Monitor for weed emergence annually, especially in areas where there is vehicle or livestock movement, in riparian areas, areas of wildlife concentration, public use areas, and locations where sand, gravel, or fill materials have been imported, soil has been disturbed, or vegetation or overstory has been removed . When spotted knapweed plants are found, remove them immediately.
The introduction of spotted knapweed seeds from infested areas to recently disturbed and/or uninfested areas can be limited by monitoring vehicle, livestock, and wildlife movement [205,206,208]. Encourage public land users to avoid driving vehicles through or scheduling livestock use in existing spotted knapweed infestations when seeds are present, to inspect and clean vehicles of weeds and their seeds, to brush and clean animals, tack, and equipment before entering public lands, and to minimize soil disturbance by stock. Regulations promoting minimum impact camping and the use of weed-free feed, hay, straw, and mulch in natural areas may reduce spotted knapweed infestations [115,208]. Encourage the use of certified weed-free feed for several days before entering backcountry [129,206,208,218].
A key component of prevention practices is education and awareness of managers, land owners, and public land users [31,86,129,208]. In Montana, programs have been implemented such as a trust fund for weed research and weed management efforts, organized cooperative weed management programs for landowners [98,129], a curriculum for educating school children , and bounty programs . Examples of successful cooperative spotted knapweed control efforts have been reported in Canada  and Wyoming .
Weed prevention and control can be incorporated into all types of management plans, including logging and site preparation, management of grazing allotments, recreation management, research projects, road building and maintenance, and fire management . See the "Guide to noxious weed prevention practices"  for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions. When prevention fails, or spotted knapweed populations already exist, several management approaches can be used to eradicate small populations or to control larger infestations including cultural, biological and chemical control methods, or some combination of methods timed in such a way as to be complementary. Tu and others  provide a comprehensive review of weed control methods that are applicable for use in natural areas. The information is also available online (Weed Control Methods Handbook).
Physical and mechanical control: Manual control techniques may be preferred in some areas with spotted knapweed infestations. For example, on the Salmon River watershed in the Klamath National Forest in California, it is recognized that manual control methods offer less risk to the high quality waters and high value fisheries than do chemical applications. A cooperative effort among the local residents and land management agencies, orchestrated by the Salmon River Restoration Council (SRRC), has successfully employed the use of several manual control approaches including propane torching of seedlings early in the season, hand digging with small tools, mulching with black plastic, and mowing with weed eaters . For a detailed description of their knapweed control program see the SRRC website.
Spotted knapweed does not persist under annual cultivation or in irrigated alfalfa. This, however, does not present a widely applicable solution for infested wildland, rangeland or prairie . Mowing, hand-pulling, planting competitive species, and good range management may reduce the spread of spotted knapweed, but may not eliminate well established stands . Tillage can reportedly lead to the spread of spotted knapweed . This is especially likely in mature stands since tillage creates an ideal weed seed bed from which individuals in the seed bank may emerge. Tillage may be more successful if followed by seeding with a strongly competitive grass-legume mixture .
Consistent hand pulling can control spotted knapweed, although it is time and labor intensive. Entire plants must be removed before they produce seeds each year, and flowering plants should be removed from the site so no seeds are dispersed . In greenhouse studies, only severe defoliation reduced spotted knapweed root, root crown, and aboveground growth. Some spotted knapweed plants produced flowers even when clipped monthly from June through September . Mowing diffuse and spotted knapweed in Canada at the bud stage, flowering stage, or once at bud stage and again at flowering, reduced the number of plants producing seed by 77%, 99%, and 96% compared to unmowed plants. The latter 2 treatments also reduced germination of the seeds by approximately 79% . Rinella and others  found that a single mowing at the flowering or seed stage resulted in an 83-85% decrease in adult spotted knapweed density at 2 sites in western Montana. This reduction was as much as any treatment consisting of repeated mowing at both sites. The long-term effects on spotted knapweed densities are unknown, but Rinella and others  hypothesize that repeated annual mowing may shift the competitive balance in favor of desired grasses. Rolling plots with a pasture roller, burning, mowing, or harrowing treatments had no effect on the spotted knapweed seedbank at 2 sites in Montana .
Burning: See Fire Management Considerations for information on using prescribed fire and spot burning for spotted knapweed control.
Biological control: The aim of biological control is to stress spotted knapweed and shift the competitive advantage away from the weed to desirable grasses and forbs [31,128,173,229]. Biological control efforts for diffuse and spotted knapweed have been underway in North America since 1970. Wilson and McAffrey  provide a discussion of considerations and safety issues in developing and implementing a biological control program. The objective of biological control efforts is to propagate and redistribute sufficient insect populations to hold spotted knapweed to population levels similar to their populations in Europe . It is believed that 4 agents are necessary to affect 1 plant species, and that 6 established agents would help control both diffuse and spotted knapweeds since most agents attack both plants . To date, 13 Eurasian insects have been introduced for the control of these knapweeds, several of which are demonstrating some impact against 1 or both species. Of these, 8 of the insects attack the flower heads, while 5 attack the roots . The idea is that these agents will work together to reduce viable seed production and stunt the overall growth and strength of the plants . In this sense, there is an additional ecological niche to be filled, since no agents attack the rosette foliage or root crown. These structures are vulnerable to attack for at least 1 year before the plant can reproduce .
Story and Piper  provide a current assessment of the status of biocontrol agents on spotted knapweed. On sites in Montana where Urophora affinis and U. quadrifasciata coexist, spotted knapweed seed production is reduced by at least 50% [192,193], and Harris  reports a 92% reduction in spotted knapweed seed production at locations in British Columbia. He also notes that Agapeta zoenga and Cyphocleonus achates are causing noticeable reductions in density and vigor of spotted knapweed at several locations in western Montana . Success has not been dramatic, and has not developed as expected . One partial explanation for the limited success of introduced biocontrol agents may be that biocontrol agents came from Centaurea maculosa, which is native to central Europe, because the plant that is invasive in North America was initially misidentified and is actually Centaurea stoebe subsp. micranthos, which is native to eastern Europe . Climatic analyses indicate that the climate of western Montana is more similar to the area in eastern Europe, from which Centaurea stoebe ssp. micranthos originated, than the area from which the bulk (12 of the 13) of biocontrol agents came. It is therefore suggested that future explorations in the area of origin of the target plant are necessary to find agents that are adapted to colder climates and a more vigorous plant [169,180].
Biological control efforts may also be hindered by indirect effects of herbivory on nontarget species. Callaway and others  found that herbivory on spotted knapweed had substantial negative, indirect effects on Idaho fescue under 2 very different sets of experimental conditions. They hypothesized that moderate herbivory may have stimulated compensatory growth in spotted knapweed, induced the production of defense chemicals that also had allelopathic effects, or stimulated root exudates that altered the relationship between knapweed and Idaho fescue via soil microbes .
Additional indirect affects of biological control of spotted knapweed have been observed in west-central Montana . Gall fly larvae (Urophora spp.) released as biological control agents for spotted knapweed are the primary food item in native deer mouse diets for most of the year and made up 84-86% of their winter diet. The implications of these findings include the possibility that deer mice and other predators may reduce Urophora populations below a threshold to effectively control knapweed; the unknown effects on deer mouse population dynamics and subsequent effects on food chains; and the effects on seed dispersal, since 9% of deer mouse stomachs also contained knapweed seeds during the period following seed dispersal .
The following table shows insects that have been established in North America for the control of spotted knapweed, and the states or provinces in which they have been established or recovered, and additional references pertaining to each [104,173,189,193,201]:
|Name||Type||States established or recovered||References|
|Sulfur knapweed moth
|root -boring moth||CO, MN, MT, NV, OR, SD, UT, WA, WY||[62,126,127,182]|
|Broad-nosed seedhead weevil
|seedhead weevil||MT, OR, UT|||
|Knapweed peacock fly (Chaetorellia acrolophi)||seedhead weevil||CO, MN, MT, OR||[193,201]|
|Knapweed root weevil (Cyphocleonus achates)||root-boring/gall weevil||CO, MN, MT, OR, SD, UT, WA, WY||[62,182,227]|
|Lesser knapweed flower weevil
|seedhead weevil||ID, MN, MT, NV, OR, SD, UT, WA, WY||[83,103]|
|Blunt knapweed flower weevil
|seedhead weevil||MT, WA, WY||[193,201]|
|Spotted knapweed seedhead moth
|seedhead moth||CO, ID, MN, MT, OR, VA, WA||[46,62,114,118]|
|Brown-winged root moth (Pelochrista medullana)||root-boring moth||MT|||
|Gray-winged root moth (Pterolonche inspersa)||root-boring moth||establishment not yet confirmed||[193,201]|
|Bronze knapweed root borer
|root beetle||MT, OR||[193,201]|
|Green clearwing fly
|seedhead fly||MN, MT, OR, SD, WY||[193,201]|
|Banded gall fly
|seedhead fly||AZ, CA, CO, ID, MI, MN, MT, ND, NE, NV, NY, OR, PA, SD, UT, VA, WA, WI, WY, PQ||[61,62,102,114,118,133,225]|
|UV knapweed seedhead fly (Urophora quadrifasciata)||seedhead fly||AZ, CA, CO, CT, ID, IN, MA, MI, MN, MD, MA, MT, ND, NE, NH, NJ, NV, NY, OR, PA, RI, SD, UT, VA, VT, WA, WI, WV, WY, BC||[61,62,70,102,133,224,225]|
In addition to insect control agents, several microorganisms have been considered for potential control of spotted knapweed including the fungi Sclerotinia sclerotiorum [76,173], Fusarium avenaceum  and F. oxysporum , and the bacteria Pseudomonas syringae . A phytotoxin isolated from the black leaf blight fungus (Alternaria alternata), maculosin, was found to be the active ingredient in this host-specific pathogen of spotted knapweed, and was synthesized in the laboratory [185,196]. Maculosin appears to be highly toxic only to spotted knapweed and was being researched for potential field efficacy in 1993 , though no work has been completed to date .
Grazing: Low to moderate levels of grazing of spotted knapweed by cattle, domestic sheep, and domestic goats have been observed in Montana . Of these, sheep seem to have the most promise for control of spotted knapweed through prescribed grazing [27,53,106,140,141]. Others suggest that livestock grazing is not likely to seriously reduce spotted knapweed populations in native bunchgrass communities because of its many other competitive attributes [90,100].
Two consecutive years of sheep grazing in May to early June and again in late summer on 40 acres (16 ha) in western Montana heavily infested with spotted knapweed completely eliminated spotted knapweed seed production, and the sheep were healthy [26,86]. Olson and others  observed that 1- and 2-year-old spotted knapweed plants were effectively controlled by sheep grazing in Montana. Sheep grazing when grasses are dormant can reduce potential negative impacts on associated grass species  and reduces density of very young spotted knapweed seedlings, thereby limiting seedling recruitment [31,173]. Timing of grazing is important, as sheep were observed to pass viable seed of spotted knapweed up to 7 days after consumption .
Chemical control: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but are rarely a complete or long-term solution to weed management. If chemical control is used it must be incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations . Use of herbicides may be limited in natural areas. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Chemical control of spotted knapweed can be effective in some cases, but may be cost prohibitive because repeat applications are usually necessary to exhaust the seedbank, and because spotted knapweed often infests large tracts of marginal rangeland and rough terrain [50,166,207]. It is important to consider not only the efficacy of the herbicide for control of spotted knapweed, but also its effects on non-target organisms (plants, animals, aquatics, and invertebrates), the environmental persistence of the chemicals used, their decomposition products, and the environmental effects of chemical contaminants and other additives found in herbicide preparations. Chemical control methods are the focus of considerable research. The following summary is current as of July 2001: Clopyralid, dicamba, picloram, and 2,4-D, alone and in combination, can effectively control spotted knapweed on rangeland . Each chemical or combination of chemicals provides different degrees of control for varying periods of time [12,13,14,22,37,42,97,105,112,149,152,171,226], has different effects on non-target organisms [10,12,13,150,151,152,153,154,171,219] (including biocontrol agents [60,77,119,191,213]), and has different degrees of environmental persistence . Rate and timing of application and site conditions (e.g., soil texture and precipitation) will affect the degree of control, the impacts on non-target organisms, and the persistence of the chemicals in the environment.
Germination of spotted knapweed seeds was not affected by the application of wheat gluten meal at any rate . Fertilization trials in western Montana using variable rates of nitrogen fertilizer suggest that N fertilization alone is an impractical control approach, and would likely contribute to an increase in spotted knapweed .
Prescribed burning in combination with herbicide application may increase the efficacy of the herbicide and stimulate growth from competitive species. See Fire Management Considerations for more information. A study examining the effect of the timing and frequency of grass defoliation on spotted knapweed reinvasion in areas treated with picloram and fertilizer combinations found increased densities of spotted knapweed at intermediate fertilizer applications rates, and better grass growth at higher rates. Also, alternating spring/fall defoliation resulted in higher spotted knapweed density and biomass than did annual spring or fall defoliation, while fall defoliation alone appeared to be the best for minimizing spotted knapweed. The site with residual understory of smooth brome and timothy was more responsive to picloram treatments than the site which had a residual understory of Kentucky bluegrass, which was more affected by fertilizer and clipping treatments .
Competition: Reducing spotted knapweed density on wildlife ranges without filling the empty niches with more desirable forage and/or native species may encourage the proliferation of other exotics that are less palatable and more toxic, spiny, or otherwise noxious than spotted knapweed [128,231]. Temporary control of spotted knapweed is an attainable objective; however, the subsequent establishment of a healthy community of desirable plants is required for a more permanent solution [69,172,176]. In a container study, competition with grass had a more negative effect on spotted knapweed growth than did either root herbivory or nitrogen shortage . A healthy, weed-resistant plant community consists of a diverse group of species that occupy diverse niches [74,176]. Establishing desirable species with diverse above- and belowground growth can enhance resource capture by desirables on the site and may limit exotic invasion. In a growth chamber study of competition between spotted knapweed, bluebunch wheatgrass and northern sweetvetch (Hedysarum boreale), Jacobs and Sheley  found evidence to support the idea that maintaining taprooted forbs along with grasses increases niche occupation and may be more effective in minimizing invasion of taprooted weeds than grasses alone.
Due to the limited availability of seed from native species that are capable of rapid development under stress conditions , natural revegetation of spotted knapweed infested rangelands often fails . The initial level of spotted knapweed reduction necessary to shift the competitive relationship between spotted knapweed and bluebunch wheatgrass varies with location, plant densities, and the initial composition and density of the suppressed grasses [76,172]. Jacobs and others  provide evidence that establishment of bluebunch wheatgrass on spotted knapweed infested rangeland may be improved by combining the fungus Sclerotinia sclerotiorum with dense grass seeding, although the fungus has not been approved for use as a biocontrol agent to date.
without a residual understory of desired plants, artificial revegetation is required
for effective control of spotted knapweed [173,176,214]. Revegetation of aggressive species has
been shown to inhibit reinvasion by spotted knapweed [71,173,214]. In a controlled environment experiment, Lindquist and others
 found that smooth brome (an invasive species itself) is capable of suppressing the growth of spotted
knapweed, with the degree of suppression increasing with increasing nitrogen
levels. Bluebunch wheatgrass and Idaho fescue had no impact on spotted knapweed
growth . Vigorous grass (especially
crested wheatgrass) slow knapweed invasion; however, the drought tolerance
and high seed production of spotted knapweed enable it to rapidly occupy dryland range,
vigor is reduced by grazing . Both 'Durar' hard fescue (Festuca
trachyphylla) and 'Covar' sheep fescue (F. ovina) are fairly aggressive
competitors with spotted knapweed in the Columbia River Basin, although both are
introduced grass species .
Reseeding may be limited by the challenge of achieving adequate seed coverage, which is
usually achieved by drilling and not feasible on most rangelands . Some specific approaches to
controlling spotted knapweed by planting competitive species are given by Sheley
and others  and Velegala and others . Combining herbicide
treatments with reseeding and/or fertilizer applications has met with some
1. Abella, Scott R.; MacDonald, Neil W. 2000. Intense burns may reduce spotted knapweed germination. Ecological Restoration. 18(2): 203-204. 
2. Agriculture Canada. 1979. Research Station: Kamloops, British Columbia. In: Research branch report: 1976-1978. Kamloops, BC: Agriculture Canada, Research Station: 325-323. 
3. Ali, Shafeek. 1989. Knapweed eradication program in Alberta. In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 105-106. 
4. Allen, Karen; Hansen, Katherine. 1999. Geography of exotic plants adjacent to campgrounds, Yellowstone National Park, USA. The Great Basin Naturalist. 59(4): 315-322. 
5. Anderson, Loran C. 1988. Noteworthy plants from north Florida. III. SIDA. 13(1): 93-100. 
6. Arno, Stephen F. 1980. Forest fire history in the Northern Rockies. Journal of Forestry. 78(8): 460-465. 
7. Arno, Stephen F. 1996. Percent coverage for selected plant species at Lick Creek under different prescribed fire treatments. Unpublished data on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab, Missoula, MT. 1 p. 
8. Arno, Stephen F. 1999. Undergrowth response, shelterwood cutting unit. In: Smith, Helen Y., Arno, Stephen F., eds. Eighty-eight years of change in a managed ponderosa pine forest. Gen. Tech. Rep. RMRS-GTR-23. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 36-37. [+ Appendix C: Summary of vegetation changes in shelterwood cutting unit]. 
9. Arno, Stephen F. 2000. Fire in western forest ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 97-120. 
10. Bedunah, Don; Carpenter, Jeff. 1989. Plant community response following spotted knapweed (Centaurea maculosa) control on three elk winter ranges in western Montana. In: Fay, Peter K.; Lacey, John R., eds. Proceedings of the knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 205-212. 
11. Bedunah, Donald J. 1992. The complex ecology of weeds, grazing and wildlife. Western Wildlands. 18(2): 6-11. 
12. Bedunah, Donald J.; Moen, Crystal. 1988. Spotted knapweed control and native forb response to various herbicides. In: Montana Forest and Conservation Experiment Station: Biennial report 1987-88. Missoula, MT: University of Montana, School of Forestry: 21. 
13. Belles, W. S.; Wattenbarger, D. W.; Lee, G. A. 1980. Effects of various herbicide treatments and subsequent fertilization on spotted knapweed control and forage production in Bonner County. Western Society of Weed Science--Research Progress Report: 57-60. 
14. Belles, W. S.; Wattenberger, D. W.; Lee. G. A. 1980. Spotted knapweed control on non-cropland. Western Society of Weed Science--Research Progress Report: 55-56. 
15. Bernard, Stephen R.; Brown, Kenneth F. 1977. Distribution of mammals, reptiles, and amphibians by BLM physiographic regions and A.W. Kuchler's associations for the eleven western states. Tech. Note 301. Denver, CO: U.S. Department of the Interior, Bureau of Land Management. 169 p. 
16. Boggs, Keith W.; Story, Jim M. 1987. The population age structure of spotted knapweed (Centaurea maculosa) in Montana. Weed Science. 35: 194-198. 
17. Bultsma, Paul M.; Lym, Rodney G. 1985. Survey for spotted knapweed in North Dakota. North Dakota Farm Research. 43(1): 19-22. 
18. Bussan, Alvin J.; Dyer, William E. 1999. Herbicides and rangeland. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 116-132. 
19. Callaway, Ragan M.; Aschehoug, Erik T. 2001. Mechanisms for the success of invaders: diffuse knapweed interacts differently with new neighbors than with old ones. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 69. Abstract. 
20. Callaway, Ragan M.; DeLuca, Thomas H.; Belliveau, Wendy M. 1999. Biological-control herbivores may increase competitive ability of the noxious weed Centaurea maculosa. Ecology. 80(4): 1196-2101. 
21. Carpenter, Jeffrey L. 1986. Responses of three plant communities to herbicide spraying and burning of spotted knapweed (Centaurea maculosa) in western Montana. Missoula, MT: University of Montana. 110 p. Thesis. 
22. Carrithers, Vanelle F.; Gaiser, Dean R.; Duncan, Celestine; Horton, Denise. 1997. Seed germination of yellow starthistle and spotted knapweed after treatment with picloram or clopyralid. Proceedings, Western Society of Weed Science. 50: 39-40. 
23. Chicoine, T. K.; Fay, P. K. 1984. The longevity of viability of spotted knapweed seeds in Montana. Proceedings, Western Society of Weed Science. 37: 204-207. 
24. Chicoine, Timothy K. 1984. Spotted knapweed (Centaurea maculosa L.) control, seed longevity and migration in Montana. Bozeman, MT: Montana State University. 83 p. Thesis. 
25. Chicoine, Timothy K.; Fay, Peter K.; Nielsen, Gerald A. 1986. Predicting weed migration from soil and climate maps. Weed Science. 34: 57-61. 
26. Cox, James W. 1983. Try sheep to control spotted knapweed. Montana Farmer-Stockman. 3: 64-65. 
27. Cox, James W. 1989. Observations, experiments and suggestions for research on the sheep-spotted knapweed relationship. In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 79-82. 
28. Czembor, E.; Strobel, G. A. 1997. Limitations of exotic and indigenous isolates of Fusarium avenaceum for the biological control of spotted knapweed--Centaurea maculosa. World Journal of Microbiology and Biotechnology. 13(1): 119-123. 
29. Davis, Edward S.; Fay, Peter K. 1989. The longevity of spotted knapweed seed in Montana. In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 67-72. 
30. Davis, Edward S.; Fay, Peter K.; Chicoine, Timothy K.; Lacey, Celestine A. 1993. Persistence of spotted knapweed (Centaurea maculosa) seed in soil. Weed Science. 41: 57-61. 
31. DiTomaso, Joseph M. 2000. Invasive weeds in rangelands: species, impacts, and management. Weed Science. 48(2): 255-265. 
32. Duchesne, Luc C.; Hawkes, Brad C. 2000. Fire in northern ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 35-51. 
33. Duncan, Celestine Lacey. 2001. Knapweed management: another decade of change. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 1-7. 
34. Eddleman, L. E.; Romo, J. T. 1988. Spotted knapweed germination response to stratification, temperature, and water stress. Canadian Journal of Botany. 66: 653-657. 
35. Evans, Francis C. 1986. Bee-flower interactions on an old field in southeastern Michigan. In: Clambey, Gary K.; Pemble, Richard H., eds. The prairie: past, present and future: Proceedings of the 9th North American Prairie Conference; 1984 July 29 - August 1; Moorhead, MN. Fargo, ND: Tri-College University Center for Environmental Studies: 103-109. 
36. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. 
37. Fay, P. K.; Davis, E. S.; Lacey, C. A.; Chicoine, T. K. 1991. Chemical control of Centaurea maculosa in Montana. In: James, Lynn F.; Evans, John O.; Ralphs, Michael H.; Child, R. Dennis, eds. Noxious range weeds. Westview Special Studies in Agriculture Science and Policy. Boulder, CO: Westview Press: 301-315. 
38. Fay, Pete. 1992. The role of herbicides in weed management. Western Wildlands. 18(2): 24-26. 
39. Forcella, Frank; Harvey, Stephen J. 1983. Eurasian weed infestation in western Montana in relation to vegetation and disturbance. Madrono. 30(2): 102-109. 
40. Foster, Bryan L. 1999. Establishment, competition and the distribution of native grasses among Michigan old-fields. Journal of Ecology. 87(3): 476-489. 
41. Frissell, Sidney S., Jr. 1968. A fire chronology for Itasca State Park, Minnesota. Minnesota Forestry Research Notes No. 196. Minneapolis, MN: University of Minnesota. 2 p. 
42. Gaiser, Dean R.; Carrithers, Vanelle F.; Duncan, Celestine. 1997. Efficacy of picloram or clopyralid applications at three timings on spotted knapweed or yellow starthistle. Proceedings, Western Society of Weed Science. 50: 42-44. 
43. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; Lewis, Mont E.; Smith, Dixie R. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. 
44. Gillespie, Donald N.; Hedstrom, Linda. 1979. Aeroallergens of western Montana. Rocky Mountain Medical Journal. [Volume unknown] 79-82. 
45. Gleason, Henry A.; Cronquist, Arthur. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. New York: New York Botanical Garden. 910 p. 
46. Good, William R.; Story, Jim M.; Callan, Nancy W. 1997. Winter cold hardiness and supercooling of Metzneria paucipunctella (Lepidoptera: Gelechiidae), a moth introduced for biological control of spotted knapweed. Environmental Entomology. 26(5): 1131-1135. 
47. Gough, R. E.; Carlstrom, R. 1999. Wheat gluten meal inhibits germination and growth of broadleaf and grassy weeds. HortScience. 34(2): 269-270. 
48. Great Plains Flora Association. 1986. Flora of the Great Plains. Lawrence, KS: University Press of Kansas. 1392 p. 
49. Griffith, Duane. 1999. Economic evaluation procedures for noxious weed management on rangeland. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 44-56. 
50. Griffith, Duane; Lacey, John R. 1991. Economic evaluation of spotted knapweed [Centaurea maculosa] control using picloram. Journal of Range Management. 44(1): 43-47. 
51. Guenther, Glen E.; Wambolt, Carl L.; Frisina, Michael R. 1993. Characteristics of bitterbrush habitats that influence canopy cover and mule deer browsing. Journal of Environmental Management. 36: 175-181. 
52. Haack, Robert A. 1993. The endangered Karner blue butterfly (Lepidoptera: Lycaenidae): biology, management considerations, and data gaps. In: Gillespie, Andrew R.; Parker, George R.; Pope, Phillip E., eds. Proceedings, 9th central hardwood forest conference; 1993 March 8-10; West Lafayette, IN. Gen. Tech. Rep. NC-161. St. Paul, MN: U.S. Department of Agriculture, Forest Service, North Central Forest Experiment Station: 83-100. 
53. Hale, Michael B.; Launchbaugh, Karen L. 2001. Developing prescription grazing guidelines for controlling spotted knapweed with sheep. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 83-84. Abstract. 
54. Hall, Christine N.; Kuss, Fred R. 1989. Vegetation alteration along trails in Shenandoah National Park, Virginia. Biological Conservation. 48: 211-227. 
55. Hansen, Paul L.; Chadde, Steve W.; Pfister, Robert D. 1988. Riparian dominance types of Montana. Misc. Publ. No. 49. Missoula, MT: University of Montana, School of Forestry, Montana Forest and Conservation Experiment Station. 411 p. 
56. Hansen, Paul L.; Pfister, Robert D.; Boggs, Keith; Cook, Bradley J.; Joy, John; Hinckley, Dan K. 1995. Classification and management of Montana's riparian and wetland sites. Miscellaneous Publication No. 54. Missoula, MT: The University of Montana, School of Forestry, Montana Forest and Conservation Experiment Station. 646 p. 
57. Hansen, Paul; Pfister, Robert; Joy, John; Svoboda, Dan; Boggs, Keith; Myers, Lew; Chadde, Steve; Pierce, John. 1989. Classification and management of riparian sites in southwestern Montana. Draft Version 2. Missoula, MT: University of Montana, School of Forestry, Montana Riparian Association. 292 p. 
58. Harrington, Michael G. 1995. [Personal communication]. April 12. Missoula, MT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Fire Sciences Laboratory. 
59. Harris, Grant A. 1991. Grazing lands of Washington state. Rangelands. 13(5): 222-227. 
60. Harris, P.; Cranston, R. 1979. An economic evaluation of control methods for diffuse and spotted knapweed in western Canada. Canadian Journal of Plant Science. 59: 375-382. 
61. Harris, P.; Shorthouse, J. D. 1996. Effectiveness of gall inducers in weed biological control. The Canadian Entomologist. 128(6): 1021-1055. 
62. Harris, Peter. 1990. The Canadian biocontrol of weeds program. In: Roche, Ben F.; Roche, Cindy Talbott, eds. Range weeds revisited: Proceedings of a symposium: A 1989 Pacific Northwest range management short course; 1989 January 24-26; Spokane, WA. Pullman, WA: Washington State University, Department of Natural Resource Sciences, Cooperative Extension: 61-68. 
63. Harrison, R. D.; Chatterton, N. J.; Page, R. J.; [and others]. 1996. Effects of nine introduced grasses on ecological biodiversity in the Columbia Basin. In: West, N. E., ed. Rangelands in a sustainable biosphere: Proceedings, 5th international rangeland congress; 1995 July 23-28; Salt Lake City, UT. Denver, CO: Society for Range Management: 211-212. 
64. Harrod, Richy J.; Taylor, Ronald J. 1995. Reproduction and pollination biology of Centaurea and Acroptilon species, with emphasis on C. diffusa. Northwest Science. 69(2): 97-105. 
65. Harvey, Stephen J.; Nowierski, Robert M. 1989. Spotted knapweed: allelopathy or nutrient depletion? In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 118. 
66. Henry, Charles. 1998. Mormon Ridge Elk Winter Range Restoration Project. Techline: Information about invasive/noxious plant management. August: 1-8. 
67. Hickman, James C., ed. 1993. The Jepson manual: Higher plants of California. Berkeley, CA: University of California Press. 1400 p. 
68. Hillis, Mike. 2001. Grass production monitoring results: Mormon Ridge aerial spray project. 2 leaves. Unpublished data. On file with: U.S. Department of Agriculture, Forest Service, Lolo National Forest. 
69. Hironaka, M. 1990. Range ecology as the basis for vegetation management. In: Roche, Ben F.; Roche, Cindy Talbott, eds. Range weeds revisited: Proceedings of a symposium: A 1989 Pacific Northwest range management short course; 1989 January 24-26; Spokane, WA. Pullman, WA: Washington State University, Department of Natural Resource Sciences, Cooperative Extension: 11-14. 
70. Hoebeke, E. Richard. 1993. Establishment of Urophora quadrifasciata (Diptera: Tephritidae) and Chrysolina quadrigemina (Coleoptera: Chrysomelidae) in portions of eastern United States. Entomological News. 104(3): 143-152. 
71. Hubbard, William A. 1975. Increased range forage production by reseeding and the chemical control of knapweed. Journal of Range Management. 28(5): 406-407. 
72. Jacobs, James S.; Sheley, Roger L. 1997. Relationships among Idaho fescue defoliation, soil water, and spotted knapweed emergence and growth. Journal of Range Management. 50(3): 258-262. 
73. Jacobs, James S.; Sheley, Roger L. 1998. Observation: life history of spotted knapweed. Journal of Range Management. 51(6): 665-673. 
74. Jacobs, James S.; Sheley, Roger L. 1999. Competition and niche partitioning among Pseudoroegneria spicata, Hedysarum boreale, and Centaurea maculosa. The Great Basin Naturalist. 59(2): 175-181. 
75. Jacobs, James S.; Sheley, Roger L.; Carter, Joella R. 2000. Picloram, fertilizer, and defoliation interactions on spotted knapweed reinvasion. Journal of Range Management. 53(3): 309-314. 
76. Jacobs, James S.; Sheley, Roger L.; Maxwell, Bruce D. 1996. Effect of Sclerotinia sclerotiorum on the interference between bluebunch wheatgrass (Agropyron spicatum) and spotted knapweed (Centaurea maculosa). Weed Technology. 10(1): 13-21. 
77. Jacobs, James S.; Sheley, Roger L.; Story, Jim M. 2000. Use of picloram to enhance establishment of Cyphocleonus achates (Coleoptera: Curculionidae). Environmental Entomology. 29(2): 349-354. 
78. Jacobs, James S.; Sheley, Roger, L. 1999. Grass defoliation intensity, frequency, and season effects on spotted knapweed. Journal of Range Management. 52(6): 626-632. 
79. Jennings, James C.; Apel-Birkhold, Patricia C.; Bailey, Bryan A.; Anderson, James D. 2000. Induction of ethylene biosynthesis and necrosis in weed leaves by a Fusarium oxysporum protein. Weed Science. 48(1): 7-14. 
80. Jones, Matt B.; Ganguli, Amy C.; Launchbaugh, Karen L.; Hale, Michael B. 2001. Potential forage value of spotted knapweed. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 83. Abstract. 
81. Kartesz, John T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. Volume I--checklist. 2nd ed. Portland, OR: Timber Press. 622 p. 
82. Kartesz, John T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. 1st ed. In: Kartesz, John T.; Meacham, Christopher A. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Chapel Hill, NC: North Carolina Botanical Garden (Producer). In cooperation with: The Nature Conservancy; U.S. Department of Agriculture, Natural Resources Conservation Service; U.S. Department of the Interior, Fish and Wildlife Service. 
83. Kashefi, J. M.; Sobhian, R. 1998. Notes on the biology of Larinus minutus Gyllenhal (Col., Curculionidae), an agent for biological control of diffuse and spotted knapweeds. Journal of Applied Entomology. 122(9-10): 547-549. 
84. Kearing, S. A.; Nowierski, R. M. 1997. First report of spotted knapweed (Centaurea maculosa) stem dieback caused by Pseudomonas syringae. Plant Disease. 81(1): 113. 
85. Keay, Jeffrey A. 1977. Relationship of habitat use patterns and forage preferences of white-tailed and mule deer to post-fire vegetation, upper Selway River. Moscow, ID: University of Idaho. 76 p. Thesis. 
86. Kelsey, Rick G. 1984. Living with spotted knapweed: minimizing economic impact research possibilities. In: Knapweed symposium: Proceedings; 1984; Great Falls, MT. Bulletin 1315. Bozeman, MT: Montana State University, Cooperative Extension Service: 15-21. 
87. Kelsey, Rick G.; Bedunah, Donald J. 1989. Ecological significance of allelopathy for Centaurea species in the northwestern United States. In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 10-32. 
88. Kelsey, Rick G.; Locken, Laura J. 1987. Phytotoxic properties of cnicin, a sesquiterpene lactone from Centaurea maculosa (spotted knapweed). Journal of Chemical Ecology. 13(1): 19-33. 
89. Kelsey, Rick G.; Mihalovich, Robert D. 1987. Nutrient composition of spotted knapweed (Centaurea maculosa). Journal of Range Management. 40(3): 277-281. 
90. Kennett, Gregory A.; Lacey, John R.; Butt, Curtis A.; [and others]. 1992. Effects of defoliation, shading and competition on spotted knapweed and bluebunch wheatgrass. Journal of Range Management. 45(4): 363-369. 
91. Krueger, Jane; Sheley, Roger; Herron, Gretchen. 2001. Influence of nutrient availability on the interaction between spotted knapweed and native perennials. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 68. Abstract. 
92. Kuchler, A. W. 1964. United States [Potential natural vegetation of the conterminous United States]. Special Publication No. 36. New York: American Geographical Society. 1:3,168,000; colored. 
93. Kudish, Michael. 1992. Adirondack upland flora: an ecological perspective. Saranac, NY: The Chauncy Press. 320 p. 
94. Kulla, Andy. 2001. [Email to Kristin Zouhar]. October 5. Mormon Ridge project. Missoula, MT: U.S. Department of Agriculture, Forest Service, Lolo National Forest. On file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Missoula, MT; RWU 4403 files. 
95. Kummerow, Max. 1992. Weeds in wilderness: a threat to biodiversity. Western Wildlands. 18(2): 12-17. 
96. Lacey, C. A.; Lacey, J. R.; Chicoine, T. K.; [and others]. 1986. Controlling knapweed on Montana rangeland. Circular 311. Bozeman, MT: Montana State University, Cooperative Extension Service. 15 p. 
97. Lacey, C. A.; Lacey, J. R.; Fay, P. K.; [and others]. 1992. Controlling knapweeds on Montana rangeland. Circular 311 [Revised]. Bozeman, MT: Montana State University, Extension Service. 17 p. 
98. Lacey, Celestine. 1990. Knapweeds--the situation in Montana. In: Roche, Ben F.; Roche, Cindy Talbott, eds. Range weeds revisited: Proceedings of a symposium: A 1989 Pacific Northwest range management short course; 1989 January 24-26; Spokane, WA. Pullman, WA: Washington State University, Department of Natural Resource Sciences, Cooperative Extension: 29-31. 
99. Lacey, John R.; Marlow, Clayton B.; Lane, John R. 1989. Influence of spotted knapweed (Centaurea maculosa) on surface runoff and sediment yield. Weed Technology. 3(4): 627-631. 
100. Lacey, John R.; Olson-Rutz, Kathrin M.; Haferkamp, Marshall R.; Kennett, Gregory A. 1994. Effects of defoliation and competition on total non-structural carbohydrates of spotted knapweed. Journal of Range Management. 47(6): 481-484. 
101. Lacey, John; Husby, Peter; Handl, Gene. 1990. Observations on spotted and diffuse knapweed invasion into ungrazed bunchgrass communities in western Montana. Rangelands. 12(1): 30-32. 
102. Lang, R. F.; Richard, R. D.; Hansen, R. W. 1997. Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) released and monitored by USDA, APHIS, PPQ as biological control agents of spotted and diffuse knapweed. The Great Lakes Entomologist. 30(3): 105-113. 
103. Lang, R. F.; Story, J. M.; Piper, G. L. 1996. Establishment of Larinus minutus Gyllenhal (Coleoptera: Curculionidae) for biological control of diffuse and spotted knapweed in the western United States. The Pan-Pacific Entomologist. 72(4): 209-212. 
104. Lang, Ronald F.; Richard, Robert D.; Parker, Paul E.; Wendel, Lloyd. 2000. Release and establishment of diffuse and spotted knapweed biocontrol agents by USDA, APHIS, PPQ, in the United States. The Pan-Pacific Entomologist. 76(4): 197-218. 
105. Lass, L. W.; Callihan, R. H. 1989. Spotted knapweed control in pasture. In: Western Society of Weed Science--Research progress report: 105-106. 
106. Launchbaugh, Karen. 2001. Prescription grazing for Centaurea control on rangelands. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 27-32. 
107. Lavelle, Darlene Anne. 1986. Use and preference of spotted knapweed (Centaurea maculosa) by elk (Cervus elaphus) and mule deer (Odocoileus hemionus) on two winter ranges in western Montana. Missoula, MT: University of Montana. 72 p. Thesis. 
108. Leininger, Wayne C. 1988. Non-chemical alternatives for managing selected plant species in the western United States. XCM-118. Fort Collins, CO: Colorado State University, Cooperative Extension. In cooperation with: U.S. Department of the Interior, Fish and Wildlife Service. 47 p. 
109. Lesica, Peter; Shelly, J. Stephen. 1996. Competitive effects of Centaurea maculosa on the population dynamics of Arabis fecunda. Bulletin of the Torrey Botanical Club. 123(2): 111-121. 
110. Lindquist, John L.; Maxwell, Bruce D.; Weaver, T. 1996. Potential for controlling the spread of Centaurea maculosa with grass competition. The Great Basin Naturalist. 56(3): 267-271. 
111. Locken, Laura J.; Kelsey, Rick G. 1987. Cnicin concentrations in Centaurea maculosa, spotted knapweed. Biochemical Systematics and Ecology. 15(3): 313-320. 
112. Lym, Rodney G.; Messersmith, Calvin G. 1986. Russian and spotted knapweed control by several herbicides in North Dakota. In: Western Society of Weed Science--Research progress report: 6-7. 
113. Maddox, D. M. 1979. The knapweeds: their economics and biological control in the western states, U.S.A. Rangelands. 1(4): 139-141. 
114. Maddox, Donald M. 1982. Biological control of diffuse knapweed (Centaurea diffusa) and spotted knapweed (C. maculosa). Weed Science. 30(1): 76-82. 
115. Marcus, W. Andrew; Milner, Gary; Maxwell, Bruce. 1998. Spotted knapweed distribution in stock camps and trails of the Selway-Bitterroot Wilderness. The Great Basin Naturalist. 58(2): 156-166. 
116. Marler, Marilyn J.; Zabinski, Catherine A.; Wojtowicz, Todd; Callaway, Ragan M. 1999. Mycorrhizae and fine root dynamics of Centaurea maculosa and native bunchgrasses in western Montana. Northwest Science. 73(3): 217-224. 
117. Marler, Marilyn; Zabinski, Catherine A.; Callaway, Ragan M. 1999. Mycorrhizae indirectly enhance competitive effects of an invasive forb on a native grassland. Ecology. 80(4): 1180-1186. 
118. Mays, W. T.; Kok, L. T. 1996. Establishment and dispersal of Urophora affinis (Diptera: Tephritidae) and Metzneria paucipunctella (Lepidoptera: Gelechiidae) in southwestern Virginia. Biological Control. 6(3): 229-305. 
119. McCaffrey, Joseph P.; Callihan, Robert H. 1988. Compatibility of picloram and 2,4-D with Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) for spotted knapweed control. Environmental Entomology. 17(5): 785-788. 
120. McGowan-Stinski, Jack. 2001. [Email to Kristin Zouhar]. October 11. Spotted knapweed and fire. Lansing, MI: The Nature Conservancy, Michigan Chapter. On file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab, Missoula, MT; RWU 4403 files. 
121. Meier, Gretchen; Weaver, T. 1997. Desirables and weeds for roadside management--a northern Rocky Mountain catalogue. Report No. RHWA/MT-97/8115. Final report: July 1994-December 1997. Helena, MT: State of Montana Department of Transportation, Research, Development, and Technology Transfer Program. 145 p. 
122. Miller, Valerie A. 1990. Knapweed as forage for big game in the Kootenays. In: Roche, Ben F.; Roche, Cindy Talbott, eds. Range weeds revisited: Proceedings of a symposium: A 1989 Pacific Northwest range management short course; 1989 January 24-26; Spokane, WA. Pullman, WA: Washington State University, Department of Natural Resource Sciences, Cooperative Extension: 35-37. 
123. Mohlenbrock, Robert H. 1986. [Revised edition]. Guide to the vascular flora of Illinois. Carbondale, IL: Southern Illinois University Press. 507 p. 
124. Mooers, Gloria B.; Willard, E. Earl. 1989. Critical environmental factors related to success of spotted knapweed in western Montana. In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 126-135. 
125. Mueggler, W. F.; Stewart, W. L. 1980. Grassland and shrubland habitat types of western Montana. Gen. Tech. Rep. INT-66. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 154 p. 
126. Muller, H. 1989. Growth pattern of diploid and tetraploid spotted knapweed, Centaurea maculosa Lam. (Compositae), and effects of the root-mining moth Agapeta. Weed Research. 29: 103-111. 
127. Muller-Scharer, Heinz. 1991. The impact of root herbivory as a function of plant density and competition: survival, growth and fecundity of Centaurea maculosa in field plots. Journal of Applied Ecology. 28: 759-776. 
128. Muller-Scharer, Heinz; Schroeder, Dieter. 1993. The biological control of Centaurea spp. in North America: do insects solve the problem? Pesticide Science. 37(4): 343-353. 
129. Mullin, Barbara. 1992. Meeting the invasion: integrated weed management. Western Wildlands. 18(2): 33-38. 
130. Nolan, Daryl G.; Upadhyaya, Mahesh K. 1988. Primary seed dormancy in diffuse and spotted knapweed. Canadian Journal of Plant Science. 68: 775-783. 
131. Norton, Jay; Swant, Gary. 1989. Resource Education Awareness Project (REAP). In: Fay, Peter K.; Lacey, John R., eds. Proceedings: knapweed symposium; 1989 April 4-5; Bozeman, MT. Bozeman, MT: Montana State University: 107-109. 
132. Noste, Nonan V. 1982. Vegetation response to spring and fall burning for wildlife habitat improvement. In: Baumgartner, David M., compiler. Site preparation and fuels management on steep terrain: Proceedings of a symposium; 1982 February 15-17; Spokane, WA. Pullman, WA: Washington State University, Cooperative Extension: 125-132. 
133. Nowierski, Robert M.; Fitzgerald, Bryan C.; McDermott, Gregory J.; Story, Jim M. 2000. Overwintering mortality of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) on spotted knapweed: effects of larval competition versus exposure to subzero temperatures. Physiological and Chemical Ecology. 29(3): 403-412. 
134. Ochsmann, Jorg. 2001. An overlooked knapweed hybrid in North America: Centaurea X psammogena Gayer (diffuse knapweed X spotted knapweed). In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 76. Abstract. 
135. Ochsmann, Jorg. 2001. On the taxonomy of spotted knapweed (Centaurea stoebe L.). In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 33-41. 
136. Olson, Bret E. 1999. Impacts of noxious weeds on ecologic and economic systems. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 4-18. 
137. Olson, Bret E.; Blicker, Pamela S. 2001. Nitrate uptake of spotted knapweed and two native grasses from pulse events. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 67. Abstract. 
138. Olson, Bret E.; Kelsey, Rick G. 1997. Effect of Centaurea maculosa on sheep rumen microbial activity and mass in vitro. Journal of Chemical Ecology. 23(4): 1131-1144. 
139. Olson, Bret E.; Wallander, Roseann T. 1997. Biomass and carbohydrates of spotted knapweed and Idaho fescue after repeated grazing. Journal of Range Management. 50(4): 409-412. 
140. Olson, Bret E.; Wallander, Roseann T. 2001. Sheep grazing spotted knapweed and Idaho fescue. Journal of Range Management. 54(1): 25-30. 
141. Olson, Bret E.; Wallander, Roseann T.; Lacey, John R. 1997. Effects of sheep grazing on a spotted knapweed-infested Idaho fescue community. Journal of Range Management. 50(4): 386-390. 
142. Paysen, Timothy E.; Ansley, R. James; Brown, James K.; Gottfried, Gerald J.; Haase, Sally M.; Harrington, Michael G.; Narog, Marcia G.; Sackett, Stephen S.; Wilson, Ruth C. 2000. Fire in western shrubland, woodland, and grassland ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 121-159. 
143. Pearson, Dean E. 1999. Small mammals of the Bitterroot National Forest: a literature review and annotated bibliography. Gen. Tech. Rep. RMRS-GTR-25. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 63 p. 
144. Pearson, Dean E.; McKelvey, Kevin S.; Ruggiero, Leonard F. 2000. Non-target effects of an introduced biological control agent on deer mouse ecology. Oecologia. 122(1): 121-128. 
145. Perez, Claudio J.; Waller, Steven S.; Moser, Lowell E.; [and others]. 1998. Seedbank characteristics of a Nebraska sandhills prairie. Journal of Range Management. 51(1): 52-62. 
146. Powell, G. W.; Wikeem, B. M.; Sturko, A.; Boateng, J. 1997. Knapweed growth and effect on conifers in a montane forest. Canadian Journal of Forest Research. 27(9): 1427-1433. 
147. Radford, Albert E.; Ahles, Harry E.; Bell, C. Ritchie. 1968. Manual of the vascular flora of the Carolinas. Chapel Hill, NC: The University of North Carolina Press. 1183 p. 
148. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. 
149. Renney, J. A.; Hughes, E. C. 1969. Control of knapweed, Centaurea species, in British Columbia with Tordon herbicide. Down to Earth. 24: 6-8. 
150. Rice, P. M.; Toney, J. C. 1996. Plant population responses to broadcast herbicide applications for spotted knapweed control. Down to Earth. 51(2): 14-19. 
151. Rice, Peter M.; Bedunah, Donald J.; Carlson, Clinton E. 1992. Plant community diversity after herbicide control of spotted knapweed. Res. Pap. INT-460. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 6 p. 
152. Rice, Peter M.; Toney, J. Christopher. 1998. Exotic weed control treatments for conservation of fescue grassland in Montana. Biological Conservation. 85(1-2): 83-95. 
153. Rice, Peter M.; Toney, J. Christopher; Bedunah, Donald J.; Carlson, Clinton E. 1997. Elk winter forage enhancement by herbicide control of spotted knapweed. Wildlife Society Bulletin. 25(3): 627-633. 
154. Rice, Peter M.; Toney, J. Christopher; Bedunah, Donald J.; Carlson, Clinton E. 1997. Plant community diversity and growth form responses to herbicide applications for control of Centaurea maculosa. Journal of Applied Ecology. 34(6): 1397--1412. 
155. Rice, Peter. 2000. Restoration of native plant communities infested by invasive weeds--Sawmill Creek Research Natural Area. In: Smith, Helen Y., ed. The Bitterroot Ecosystem Management Research Project: what we have learned: Symposium proceedings; 1999 May 18-20; Missoula, MT. Proceedings RMRS-P-17. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 29-30. 
156. Rice, Peter; Flynn, Sarah Wilhelm. 2000. Integration of herbicides and prescribed burning for plant community restoration--Pesticide Impact Assessment Program (FS-PIAP). Unpublished study plan on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 11 p. 
157. Rinella, Matthew J.; Jacobs, James S.; Sheley, Roger L.; Borkowski, John J. 2001. Spotted knapweed response to season and frequency of mowing. Journal of Range Management. 54(1): 52-56. 
158. Robbins, John. 1990. Grazing knapweed using holistic resource management. In: Roche, Ben F.; Roche, Cindy Talbott, eds. Range weeds revisited: Proceedings of a symposium: A 1989 Pacific Northwest range management short course; 1989 January 24-26; Spokane, WA. Pullman, WA: Washington State University, Department of Natural Resource Sciences, Cooperative Extension: 39-41. 
159. Roche, Ben F., Jr.; Piper, Gary L.; Talbott, Cindy Jo. 1986. Knapweeds of Washington. Pullman, WA: Washington State University, Cooperative Extension, College of Agriculture and Home Economics. 41 p. 
160. Roche, Ben F., Jr.; Roche, Cindy Talbott. 1991. Identification, introduction, distribution, ecology, and economics of Centaurea species. In: James, Lynn F.; Evans, John O.; Ralphs, Michael H.; Child, R. Dennis, eds. Noxious range weeds. Westview Special Studies in Agriculture Science and Policy. Boulder, CO: Westview Press: 274-291. 
161. Roche, Ben F., Jr.; Talbott, Cindy Jo. 1986. The collection history of Centaureas found in Washington state. Research Bulletin XB 0978. Pullman, WA: Washington State University, College of Agriculture and Home Economics, Agriculture Research Center. 36 p. 
162. Roche, Cindy Talbott. 1990. Knapweed: major populations in Washington. In: Roche, Ben F.; Roche, Cindy Talbott, eds. Range weeds revisited: Proceedings of a symposium: A 1989 Pacific Northwest range management short course; 1989 January 24-26; Spokane, WA. Pullman, WA: Washington State University, Department of Natural Resource Sciences, Cooperative Extension: 23-28. 
163. Rolfsmeier, Steven B.; Steinauer, Robert F.; Sutherland, David M. 1999. New floristic records for Nebraska--5. Transactions, Nebraska Academy of Sciences. 25: 15-22. 
164. Romme, William H. 1982. Fire and landscape diversity in subalpine forests of Yellowstone National Park. Ecological Monographs. 52(2): 199-221. 
165. Salmon River Restoration Council, Noxious Weed Management Program. 2001. Salmon River non-chemical knapweed control, [Online]. Available: http://www.srrc.org/weeds/commapproach.html [2001, September 29]. 
166. Schirman, Roland. 1981. Seed production and spring seedling establishment of diffuse and spotted knapweed. Journal of Range Management. 34(1): 45-47. 
167. Schwartz, Mark W.; Porter, Daniel J.; Randall, John M.; Lyons, Kelly E. 1996. Impact of nonindigenous plants. In: Status of the Sierra Nevada. Sierra Nevada Ecosystem Project: Final report to Congress. Volume II: Assessments and scientific basis for management options. Wildland Resources Center Report No. 37. Davis, CA: University of California, Centers for Water and Wildland Resources: 1203-1218. 
168. Seymour, Frank Conkling. 1982. The flora of New England. 2nd ed. Phytologia Memoirs 5. Plainfield, NJ: Harold N. Moldenke and Alma L. Moldenke. 611 p. 
169. Sforza, Rene; Story, Jim; Hufbauer, Ruth; [and others]. 2001. New foreign explorations for classical biocontrol of spotted knapweed. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 77-78. Abstract. 
170. Sheley, R. L.; Roche, B. F. 1982. Rehabilitation of spotted knapweed infested rangeland in northeastern Washington. Western Society of Weed Science. [Volume unknown] 31. Abstract. 
171. Sheley, Roger L.; Duncan, Celestine A.; Halstvedt, Mary B.; Jacobs, James S. 2000. Spotted knapweed and grass response to herbicide treatments. Journal of Range Management. 53(2): 176-182. 
172. Sheley, Roger L.; Jacobs, James S. 1997. Response of spotted knapweed and grass to picloram and fertilizer combinations. Journal of Range Management. 50(3): 263-267. 
173. Sheley, Roger L.; Jacobs, James S.; Carpinelli, Michael F. 1998. Distribution, biology, and management of diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea maculosa). Weed Technology. 12(2): 353-362. 
174. Sheley, Roger L.; Jacobs, James S.; Carpinelli, Michael L. 1999. Spotted knapweed. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 350-361. 
175. Sheley, Roger L.; Jacobs, James S.; Lucas, Daniel E. 2001. Revegetating spotted knapweed infested rangeland in a single entry. Journal of Range Management. 54(2): 144-151. 
176. Sheley, Roger L.; Svejcar, Tony J.; Maxwell, Bruce D.; Jacobs, James S. 1996. Successional rangeland weed management. Rangelands. 18(4): 155-159. 
177. Sheley, Roger; Manoukian, Mark; Marks, Gerald. 1999. Preventing noxious weed invasion. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 69-72. 
178. Shiflet, Thomas N., ed. 1994. Rangeland cover types of the United States. Denver, CO: Society for Range Management. 152 p. 
179. Smith, Jane Kapler; Fischer, William C. 1997. Fire ecology of the forest habitat types of northern Idaho. Gen. Tech. Rep. INT-GTR-363. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 142 p. 
180. Smith, Lincoln. 2001. Considerations for resuming foreign exploration for natural enemies of spotted and diffuse knapweed. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 18-26. 
181. Spears, B. M.; Rose, S. T.; Belles, W. S. 1980. Effect of canopy cover, seeding depth, and soil moisture on emergence of Centaurea maculosa and C. diffusa. Weed Research. 20: 87-90. 
182. Steinger, Thomas; Muller-Scharer, Heinz. 1992. Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability. Oecologia. 91: 141-149. 
183. Stephenson, John R.; Calcarone, Gena M. 1999. Factors influencing ecosystem integrity. In: Stephenson, John R.; Calcarone, Gena M. Southern California mountains and foothills assessment: Habitat and species conservation issues. Gen. Tech. Rep. PSW-GTR-172. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station: 61-109. 
184. Stickney, Peter F. 1989. Seral origin of species comprising secondary plant succession in Northern Rocky Mountain forests. FEIS workshop: Postfire regeneration. Unpublished draft on file at: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Fire Sciences Laboratory, Missoula, MT. 10 p. 
185. Stierle, Andrea C.; Cardellina, John H., II; Strobel, Gary A. 1989. Phytotoxins from Alternaria alternata, a pathogen of spotted knapweed. Journal of Natural Products. 52(1): 42-47. 
186. Story, J. M.; Boggs, K. W.; Good, W. R. 1991. First report of the establishment of Agpeta zoegana L.(Lepidoptera: Cochylidae) on spotted knapweed, Cenaurea maculosa Lamarck, in the United States. The Canadian Entomologist. 123(2): 411-412. 
187. Story, J. M.; Good, W. R.; White, L. J. 1994. Propagation of Agapeta zoegana L. (Lepidoptera: Cochylidae) for biological control of spotted knapweed: procedures and cost. Biological Control. 4(2): 145-148. 
188. Story, J. M.; Good, W. R.; White, L. J.; Smith, L. 2000. Effects of the interaction of the biocontrol agent Agapeta zoegana L. (Lepidoptera: Cochylidae) and grass competition on spotted knapweed. Biological Control. 17(2): 182-190. 
189. Story, J. M.; Quimby, P. C.; Piper, G. L.; [and others]. 1996. Spotted knapweed: Centaurea maculosa. In: Rees, Norman E.; Quimby, Paul C., Jr.; Piper, Gary L.; [and others], eds. Biological control of weeds in the West. Bozeman, MT: Western Society of Weed Science. In cooperation with: U.S. Department of Agriculture, Agricultural Research Service; Montana Department of Agriculture; Montana State University: Section II. 
190. Story, Jim M. 1992. Biological control of weeds: selective, economical and safe. Western Wildlands. 18(2): 18-23. 
191. Story, Jim M.; Boggs, Keith W.; Good, William R. 1988. Optimal timing of 2,4-D applications for compatibility with Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) for control of spotted knapweed. Environmental Entomology. 17(5): 911-914. 
192. Story, Jim M.; Boggs, Keith W.; Nowierski, Robert M. 1989. Effect of two introduced seed head flies on spotted knapweed. Montana Ag Research. 5(1): 14-17. 
193. Story, Jim M.; Piper, Gary L. 2001. Status of biological control efforts against spotted and diffuse knapweed. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 11-17. 
194. Strang, R. M.; Lindsay, K. M.; Price, R. S. 1979. Knapweeds: British Columbia's undesirable aliens. Rangelands. 1(4): 141-143. 
195. Strausbaugh, P. D.; Core, Earl L. 1977. Flora of West Virginia. 2nd ed. Morgantown, WV: Seneca Books, Inc. 1079 p. 
196. Strobel, Gary A. 1991. Biological control of weeds. Scientific American. 265(1): 72-78. 
197. Svejcar, Tony. 1999. Implications of weedy species in management and restoration of pinyon and juniper woodlands. In: Monsen, Stephen B.; Stevens, Richard, compilers. Sustaining and restoring a diverse ecosystem: Proceedings: ecology and management of pinyon-juniper communities within the Interior West; 1997 September 15-18; Provo, UT. Proceedings RMRS-P-9. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 394-396. 
198. Thompson, Michael J. 1996. Winter foraging response of elk to spotted knapweed removal. Northwest Science. 70(1): 10-19. 
199. Toth, Barbara L. 1991. Factors affecting conifer regeneration and community structure after a wildfire in western Montana. Corvallis, OR: Oregon State University. 124 p. Thesis. 
200. Tu, Mandy; Hurd, Callie; Randall, John M., eds. 2001. Weed control methods handbook: tools and techniques for use in natural areas. Davis, CA: The Nature Conservancy. 194 p. 
201. Turner, C. E.; Story, J. M.; Rosenthal, S. S.; Rees, N. E. 1996. The knapweeds. In: Rees, Norman E.; Quimby, Paul C., Jr.; Piper, Gary L.; Coombs, Eric M.; Turner, Charles E.; Spencer, Neal R.; Knutson, Lloyd V., eds. Biological control of weeds in the West. Bozeman, MT: Western Society of Weed Science. In cooperation with: U.S. Department of Agriculture, Agricultural Research Service; Montana State University, Department of Agriculture: Section II. 
202. Turner, Nancy J. 1999. "Time to burn": Traditional use of fire to enhance resource production by aboriginal peoples in British Columbia. In: Boyd, Robert, ed. Indians, fire and the land in the Pacific Northwest. Corvallis, OR: Oregon State University Press: 185-218. 
203. Tyser, Robin W. 1990. Ecology of fescue grasslands in Glacier National Park. In: Boyce, Mark S.; Plumb, Glenn E., eds. National Park Service Research Center, 14th annual report. Laramie, WY: University of Wyoming, National Park Service Research Center: 59-60. 
204. Tyser, Robin W. 1992. Vegetation associated with two alien plant species in a fescue grassland in Glacier National Park, Montana. The Great Basin Naturalist. 52(2): 189-193. 
205. Tyser, Robin W.; Key, Carl H. 1988. Spotted knapweed in natural area fescue grasslands: an ecological assessment. Northwest Science. 62(4): 151-160. 
206. Tyser, Robin W.; Worley, Christopher A. 1992. Alien flora in grasslands adjacent to road and trail corridors in Glacier National Park, Montana (U.S.A.). Conservation Biology. 6(2): 253-262. 
207. U.S. Department of Agriculture, Animal and Plant Health Inspection Service. 1994. Biological control of spotted and diffuse knapweeds. Program Aid Number 1529. Washington, DC: U.S. Department of Agriculture, Animal and Plant Health Inspection Service. 15 p. 
208. U.S. Department of Agriculture, Forest Service. 2001. Guide to noxious weed prevention practices. Washington, DC: U.S. Department of Agriculture, Forest Service. 25 p. Available online: https://www.fs.fed.us /rangelands/ftp/invasives/documents/GuidetoNoxWeedPrevPractices_07052001.pdf [2005, October 25]. 
209. U.S. Department of Agriculture, Natural Resources Conservation Service. 2007. PLANTS Database, [Online]. Available: https://plants.usda.gov /. 
210. U.S. Department of Agriculture, Soil Conservation Service. 1994. Plants of the U.S.--alphabetical listing. Washington, DC: U.S. Department of Agriculture, Soil Conservation Service. 954 p. 
211. University of Montana, Division of Biological Sciences. 2001. INVADERS Database System, [Online]. Available: http://invader.dbs.umt.edu/ [2001, June 27]. 
212. Upadhyaya, Mahesh K. 1986. Induction of bolting by gibberellic acid in rosettes of diffuse (Centaurea diffusa) and spotted (C. maculosa) knapweed. Canadian Journal of Botany. 64(11): 2428-2432. 
213. Vander Meer, Dennis; Six, Diana L.; Sturdevant, Nancy. 2001. Survival of the root mining biological control agents Agapeta zoegana and Cyphocleonus achates in spotted knapweed treated with three concentrations of the herbicides Tordon and Transline. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 78. Abstract. 
214. Velagala, Rajendra P.; Sheley, Roger L.; Jacobs, James S. 1997. Influence of density on intermediate wheatgrass and spotted knapweed interference. Journal of Range Management. 50(5): 523-529. 
215. Voss, Edward G. 1996. Michigan flora. Part III: Dicots (Pyrolaceae--Compositae). Bulletin 61: Cranbrook Institute of Science; University of Michigan Herbarium. Ann Arbor, MI: The Regents of the University of Michigan. 622 p. 
216. Wade, Dale D.; Brock, Brent L.; Brose, Patrick H.; Grace, James B.; Hoch, Greg A.; Patterson, William A., III. 2000. Fire in eastern ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 53-96. 
217. Wagner, Robert G.; Petersen, Terry D.; Ross, Darrell W.; Radosevich, Steven R. 1989. Competition thresholds for the survival and growth of ponderosa pine seedlings associated with woody and herbaceous vegetation. New Forests. 3(2): 151-170. 
218. Wallander, Roseann T.; Olson, Bret E.; Lacey, John R. 1995. Spotted knapweed seed viability after passing through sheep and mule deer. Journal of Range Management. 48(2): 145-149. 
219. Wambolt, Carl L. 2004. Browsing and plant age relationships to winter protein and fiber of big sagebrush subspecies. Journal of Range Management. 57(6): 620-623. 
220. Watson, A. K.; Renney, A. J. 1974. The biology of Canadian weeds. 6. Centaurea diffusa and C. maculosa. Canadian Journal of Plant Science. 54: 687-701. 
221. Watson, V. J.; Rice, P. M.; Monning, E. C. 1989. Environmental fate of picloram used for roadside weed control. Journal of Environmental Quality. 18(2): 198-205. 
222. Weiner, J.; Martinez, S.; Muller-Scharer, H.; [and others]. 1997. How important are environmental maternal effects in plants? A study with Centaurea maculosa. Journal of Ecology. 85: 133-142. 
223. Westbrooks, Randy G. 1998. Invasive plants: changing the landscape of America. Fact Book. Washington, DC: Federal Interagency Committee for the Management of Noxious and Exotic Weeds. 109 p. 
224. Wheeler, A. G., Jr. 1995. Urophora quadrifasciata (Diptera: Tephritae), an introduced seedhead fly new to midwestern North America. The Great Lakes Entomologist. 28(3&4): 235-236. 
225. Wheeler, A. G.; Stoops, Craig A. 1996. Establishment of Urophora affinis on spotted knapweed in Pennsylvania, with new eastern U.S. records of U. quadrifasciata (Diptera: Tephritidae). Proceedings, Entomological Society of Washington. 98(1): 93-99. 
226. Whitson, T. D.; Costa, Robert; Campbell, Steve. 1986. Evaluation of various herbicide application times and treatments for control of knapweed spp. In: Western Society of Weed Science--Research progress reports: 26-27. 
227. Wikeem, Brian M.; Powell, George W. 1999. Biology of Cyphocleonus achates (Coleoptera: Curculionidea), propagated for the biological control of knapweeds (Asteraceae). Canadian Entomologist. 131(2): 243-250. 
228. Willard, E. Earl; Bedunah, Donald J.; Marcum, C. Les; Mooers, Gloria. 1988. Environmental factors affecting spotted knapweed. Biennial Report 1987-1988. Missoula, MT: University of Montana, School of Forestry, Montana Forest and Conservation Experiment Station. 21 p. 
229. Wilson, Linda M.; McCaffrey, Joseph P. 1999. Biological control of noxious rangeland weeds. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 97-115. 
230. Wofford, B. Eugene. 1989. Guide to the vascular plants of the Blue Ridge. Athens, GA: The University of Georgia Press. 384 p. 
231. Wright, Anthony L.; Kelsey, Rick G. 1997. Effects of spotted knapweed on a cervid winter-spring range in Idaho. Journal of Range Management. 50(5): 487-496. 
232. Wunderlin, Richard P. 1998. Guide to the vascular plants of Florida. Gainesville, FL: University Press of Florida. 806 p. 
233. Xanthopoulos, G. 1986. A fuel model for fire behavior prediction in spotted knapweed (Centaurea maculosa L.) grasslands in western Montana. Missoula, MT: University of Montana. 100 p. Thesis. 
234. Xanthopoulos, Gavriil. 1988. Guidelines for burning spotted knapweed infestations for fire hazard reduction in western Montana. In: Fischer, William C.; Arno, Stephen F., compilers. Protecting people and homes from wildfire in the Interior West: proceedings of the symposium and workshop; 1987 October 6-8; Missoula, MT. Gen. Tech. Rep. INT-251. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 195-198. 
235. Yule, Darcy A. 1987. Knapweed infestation. Rangelands. 9(6): 249-250. 
236. Flora of North America Editorial Committee, eds. 2018. Flora of North America north of Mexico, [Online]. Flora of North America Association (Producer). Available: http://www.efloras.orage.aspx?flora_id=1.