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Lepidium latifolium


Table of Contents


INTRODUCTORY


 

Photo ©John M. Randall/The Nature Conservancy


AUTHORSHIP AND CITATION:
Zouhar, Kris. 2004. Lepidium latifolium. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/forb/leplat/all.html [].

FEIS ABBREVIATION:
LEPLAT

SYNONYMS:
Cardaria latifolia (L.) Spach [89]

NRCS PLANT CODE [85]:
LELA2

COMMON NAMES:
broadleaved pepperweed
perennial pepperweed
tall whitetop

TAXONOMY:
The scientific name of broadleaved pepperweed is Lepidium latifolium L. (Brassicaceae) [5,23,31,33,38,40,43,44,45].

LIFE FORM:
Forb

FEDERAL LEGAL STATUS:
None

OTHER STATUS:
As of this writing (2004), broadleaved pepperweed is designated a noxious or prohibited weed or weed seed in at least 13 states in the United States and 1 Canadian province [86]. See the Invaders, Plants, or APHIS databases for more information.

DISTRIBUTION AND OCCURRENCE

SPECIES: Lepidium latifolium
GENERAL DISTRIBUTION:
Broadleaved pepperweed is native to western Asia and southeastern Europe and now occurs from North Africa north through Europe to Norway and east to the western Himalayas. It has been introduced to Australia, Mexico, and throughout much of the U.S. Broadleaved pepperweed was probably introduced into North America several times, possibly as a contaminant of sugar beet (Beta vulgaris) seed, imported from eastern Europe ([41,102,103] and references therein).

In the U.S., broadleaved pepperweed occurs in a few states along the eastern seaboard, in several midwestern states, and in all far western states. Plants database provides a state distribution map of broadleaved pepperweed. It is also found in Quebec, western Canada, and Mexico. Broadleaved pepperweed has greatly increased in distribution and dominance in western North America during the past 2 decades ([102] and references therein).

Broadleaved pepperweed occurs throughout California, except coastal rainforest in the northwest, and low elevation desert in the southeast. Broadleaved pepperweed is a serious problem in the Modoc Plateau [83]. Small infestations of broadleaved pepperweed occur along roadsides in the Sierra Nevada. It is found east of the Sierra Nevada in native hay meadows and managed alkaline wetlands. Its range in southern California is not well documented. According to observations by land managers, broadleaved pepperweed populations in California have expanded, and it has increased its overall range during the last 15 years [41,104]. In the Intermountain Area broadleaved pepperweed occurs along river systems from the lower edge of coniferous forests to saline/alkaline deltas and sinks, and is adapted to string meadows characteristic of the big sagebrush zone. It is not yet a major pest in high mountain meadows in the coniferous forest zone [104]. Broadleaved pepperweed is among several nonnative species that pose "critical weed management concerns" in the San Luis Valley in Colorado [75], where it is particularly problematic in Monte Vista and Alamosa National Wildlife refuges [13].

The following lists include North American ecosystems, habitats, and forest and range cover types in which broadleaved pepperweed is known or thought to be invasive, as well as some types that may be invaded by broadleaved pepperweed following disturbances in which vegetation is killed and/or removed and/or soil disturbed (e.g. cultivation, logging, fire, grazing, herbicide application, flooding). These lists are not necessarily exhaustive, as habitat information for broadleaved pepperweed in the midwestern and eastern states is not available.

Broadleaved pepperweed is invasive primarily in riparian areas and wetlands and may invade adjacent areas once established [10,102]. Some ecosystems and plant communities are included in the following lists because wetland and riparian areas within these types may be susceptible to invasion by broadleaved pepperweed. More information is needed regarding of particular ecosystems and plant communities where broadleaved pepperweed is invasive.

ECOSYSTEMS [29]:
FRES17 Elm-ash-cottonwood
FRES19 Aspen-birch
FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES23 Fir-spruce
FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES34 Chaparral-mountain shrub
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES39 Prairie
FRES41 Wet grasslands
FRES42 Annual grasslands

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES
AZ CA CO CT ID IL IN KS MA MO
MT NE NV NH NM NY OR TX UT WA
WY

CANADA
AB BC PQ

MEXICO

BLM PHYSIOGRAPHIC REGIONS [6]:
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
16 Upper Missouri Basin and Broken Lands

KUCHLER [48] PLANT ASSOCIATIONS:
K025 Alder-ash forest
K026 Oregon oakwoods
K030 California oakwoods
K035 Coastal sagebrush
K036 Mosaic of K030 and K035
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K040 Saltbush-greasewood
K048 California steppe
K049 Tule marshes
K050 Fescue-wheatgrass
K051 Wheatgrass-bluegrass
K055 Sagebrush steppe
K056 Wheatgrass-needlegrass shrubsteppe
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K069 Bluestem-grama prairie
K070 Sandsage-bluestem prairie
K073 Northern cordgrass prairie
K074 Bluestem prairie
K076 Blackland prairie
K098 Northern floodplain forest
K101 Elm-ash forest

SAF COVER TYPES [26]:
16 Aspen
63 Cottonwood
217 Aspen
220 Rocky Mountain juniper
222 Black cottonwood-willow
235 Cottonwood-willow
246 California black oak
249 Canyon live oak
250 Blue oak-foothills pine
255 California coast live oak

SRM (RANGELAND) COVER TYPES [80]:
101 Bluebunch wheatgrass
102 Idaho fescue
103 Green fescue
105 Antelope bitterbrush-Idaho fescue
107 Western juniper/big sagebrush/bluebunch wheatgrass
201 Blue oak woodland
202 Coast live oak woodland
203 Riparian woodland
210 Bitterbrush
214 Coastal prairie
215 Valley grassland
217 Wetlands
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
309 Idaho fescue-western wheatgrass
314 Big sagebrush-bluebunch wheatgrass
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
404 Threetip sagebrush
406 Low sagebrush
408 Other sagebrush types
409 Tall forb
411 Aspen woodland
413 Gambel oak
414 Salt desert shrub
418 Bigtooth maple
419 Bittercherry
422 Riparian
501 Saltbush-greasewood
601 Bluestem prairie
602 Bluestem-prairie sandreed
604 Bluestem-grama prairie
605 Sandsage prairie
606 Wheatgrass-bluestem-needlegrass
607 Wheatgrass-needlegrass
609 Wheatgrass-grama
611 Blue grama-buffalo grass
615 Wheatgrass-saltgrass-grama
701 Alkali sacaton-tobosagrass
702 Black grama-alkali sacaton
712 Galleta-alkali sacaton
724 Sideoats grama-New Mexico feathergrass-winterfat
725 Vine mesquite-alkali sacaton
802 Missouri prairie
803 Missouri glades
805 Riparian

HABITAT TYPES AND PLANT COMMUNITIES:
Broadleaved pepperweed seems to be most problematic in riparian areas and wetlands in California, Nevada, and the Intermountain Area. Information on habitat types and plant communities in which broadleaved pepperweed occurs comes primarily from literature focused on these areas. More information on habitat types and plant communities where broadleaved pepperweed occurs outside these areas is needed to better understand and predict its invasive potential.

In California, broadleaved pepperweed is most common in coastal areas, beaches, tidal shores, inland marshes, riparian areas, wetlands, grasslands, and roadsides, and has the potential to invade montane wetlands [16,38]. Broadleaved pepperweed occurs at the Honey Lake Wildlife Refuge in northeastern California, where native vegetation includes black greasewood (Sarcobatus vermiculatus), saltgrass (Distichlis spicata), basin wildrye (Leymus cinereus), and rushes (Juncus spp.) [12]. At the Cosumnes River Preserve, small populations of broadleaved pepperweed establish in grasslands "in areas that have at least some sun" [74]. Broadleaved pepperweed has invaded pickleweed- (Salicornia spp.) dominated marshes in some areas in California, although, in most areas, it typically prefers sites slightly higher in elevation than those dominated by pickleweed [41].

Broadleaved pepperweed is considered "highly invasive and competitive" in sagebrush (Artemisia spp.) ecosystems in the Intermountain Area, where it is found in locally dense populations in transitions from meadow to upland [64]. It also commonly infests native hay meadows in this area, where plant communities consist of native and introduced perennial grasses, sedges (Carex spp.), and rushes [102].

At Malheur National Wildlife Refuge in Oregon, broadleaved pepperweed occurs in dense stands with trace amounts of beardless wildrye (L. triticoides), bottlebrush squirreltail (Elymus elymoides), basin wildrye, saltgrass, cheatgrass (Bromus tectorum), flixweed tansymustard (Descurainia sophia), rushes, and sedges in lower areas [46]. At Diamond Pond in southeastern Oregon, amid plant communities dominated by shadscale (Atriplex confertifolia), black greasewood, spiny hopsage (Grayia spinosa), basin big sagebrush (Artemisia tridentata ssp. tridentata), rabbitbrush (Chrysothamnus spp.), and horsebrush (Tetradymia spp.), broadleaved pepperweed codominates with stinging nettle (Urtica dioica) near the western margin of the pond among stands of native and nonnative weeds [92].

Broadleaved pepperweed occurs in riparian areas east of the Cascades in Oregon and Washington where plant associates include desert false indigo (Amorpha fruticosa), sagebrush, knapweed (Centaurea spp.), reed canarygrass (Phalaris arundinacea), willow (Salix spp.), Fuller's teasel (Dipsacus fullonum ssp. sylvestris), dock (Rumex spp.), cheatgrass, and tumble mustard (Sisymbrium spp.) [30].

Cox [20] reported in 1997 that the heaviest infestations of broadleaved pepperweed in Idaho were in the southwestern portion of the state, with small infestations in southern and eastern Idaho. The heavy infestations occurred primarily near rivers, canals, and in other areas with high water tables. It was not considered a problem in cropland, but was considered a potential pest in pastures if not controlled. While the author states that county weed control superintendents indicated broadleaved pepperweed was not rapidly spreading, it did appear to be getting denser in some areas [20].

In Nevada, broadleaved pepperweed is a problem for ranchers and natural resource managers along the Humboldt and lower Truckee Rivers in the western part of the state [83]. It is also reported on Nature Conservancy preserves such as Ash Meadows, where it occurs along edges (e.g. roads, streams, etc.), and in areas where changes in hydrology (i.e. water release timing) and loss of native cottonwood (Populus spp.) recruitment enables broadleaved pepperweed establishment. Here broadleaved pepperweed affects cottonwood and willow communities and oxbow meadows [74].

In Utah, broadleaved pepperweed is found in riparian and wetland habitats and, less commonly, in dry barrow pits and roadsides [90]. Along the Green River in Uintah County, broadleaved pepperweed occurs in areas that would naturally consist of saltgrass, alkali sacaton (Sporobolus airoides), and povertyweed (Iva axillaris) [67].

On a study site in northwestern Colorado, dominated by basin big sagebrush, broadleaved pepperweed occurs with twisted moss (Tortula ruralis) and desert goosefoot (Chenopodium pratericola) in valley bottoms near streams, but not away from streams [19].

Broadleaved pepperweed is less common in the eastern U.S. On the Atlantic coast, broadleaved pepperweed is rare, but occurs on beaches and tidal shores from New York to Massachusetts [25,78], and at widely scattered locations elsewhere in the northeastern U.S. [31].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Lepidium latifolium

 

©John M. Randall/The Nature Conservancy


GENERAL BOTANICAL CHARACTERISTICS:
The following description of broadleaved pepperweed is a compilation of information from several sources [31,38,39,40,41,90] unless otherwise cited. It provides characteristics that may be relevant to fire ecology, and is not meant for identification. Correct identification of nonnative invasive species is critical before control measures are implemented. Keys for identification of broadleaved pepperweed are available (e.g. [23,31,33,40,45]). According to Young and others [104], none of the Lepidium species native to North America is similar in size and growth habit to broadleaved pepperweed.

Broadleaved pepperweed is a nonnative, perennial forb with 1 to many aboveground stems, 3 to 8 feet (1-2.5 m) tall. Initially, shoots form a rosette near the soil surface (see Seasonal Development). Basal leaves are long-petioled, 4 to 12 inches (10-30 cm) long and 1 to 3 inches (2.5-8 cm) wide. Older stems have alternate cauline leaves, 0.4 to 1.5 inches (1-4 cm) wide. Lower leaves are petioled and upper leaves are sessile. Leaf size decreases up the stem. Leaf area of broadleaved pepperweed is highest at the flowerbud stage when it reaches values over 26,528 cm2 leaf area/m2. Leaf area is not evenly distributed within the broadleaved pepperweed canopy, with about one half of the leaf area in the top third of the canopy during the flowerbud to fruiting stages. Total leaf area decreases as broadleaved pepperweed stems flower and fruit (Renz and DiTomaso, unpublished data, as cited by [69]).

Broadleaved pepperweed has a panicle inflorescence 5 to 6 inches (25-27.5 cm) wide and composed of many small flowers, about 3 mm wide, in dense clusters at the tops of stems. Fruits are 2-chambered pods (silicles), about 2 mm long at maturity and slightly flattened. Fruits contain 1 seed per chamber, about 1mm long and 0.5 mm wide.

Broadleaved pepperweed roots are typically highly elongated and thick, with minimal branching. Some roots creep horizontally below the soil and others penetrate deep into the soil, but neither type forms dense clusters of roots. Roots are coarse and widely spaced [9]. Excavation of broadleaved pepperweed belowground biomass in a riparian habitat revealed that 19% of broadleaved pepperweed roots occurred in the top 4 inches (10 cm) of soil, and 85% in the top 24 inches (60 cm) [73]. Some broadleaved pepperweed roots may extend much deeper. In excavations at Honey Lake National Wildlife Refuge, Blank and Young [12] observed broadleaved pepperweed rooting depth in excess of 9 feet (3 m). Belowground biomass constitutes about 40% of broadleaved pepperweed's total biomass [73]. This extensive creeping root system is thought to enhance the belowground competitiveness of broadleaved pepperweed for water and nutrients while increasing the carbohydrate reserve important for rapid shoot development in the spring [10,69]. Several florae describe rhizomes in broadleaved pepperweed; however, researchers indicate that it has no rhizomes, and that the underground portion of broadleaved pepperweed is technically creeping roots [72,98]. Also, no florae describe a woody caudex in broadleaved pepperweed, but several authors indicate that the base of the stem is woody, and that roots enlarge at the soil surface, forming a semi-woody crown (e.g. [41,69,72,104]).

Broadleaved pepperweed may occur as spotty, scattered populations, or as large, dense, nearly monospecific stands [69,104]. Dense colonies are most common under moist conditions, as roots creep out from initial plants and form new shoots, eventually merging into closed canopy stands [102]. The aerial portion of broadleaved pepperweed stems dies back to the ground in fall and winter, leaving a thick thatch of dry, semi-woody stems that may persist for several years [104]. According to Renz [69], large amounts of litter may build up in dense infestations, with litter layers reaching upward of 4 inches (10 cm) in depth. Old stems and deep litter form layers that are impenetrable to light and prevent the emergence of other plant species, as few plants besides broadleaved pepperweed have enough stored energy to grow through broadleaved pepperweed's dense litter layer [69].

To date, there is no verifiable evidence of allelopathy in broadleaved pepperweed (Blank and young, unpublished data, as cited by [103]).

RAUNKIAER [66] LIFE FORM:
Hemicryptophyte
Geophyte

REGENERATION PROCESSES:
Broadleaved pepperweed reproduces from seed, creeping roots, and semi-woody crowns [72,98].

Breeding system: No information is available on this topic.

Pollination: According to DiTomaso and Healy [22], broadleaved pepperweed is insect pollinated.

Seed production: According to a review by Howald [41], each mature broadleaved pepperweed plant has the capacity to produce thousands of seeds each year. Seed production is reported as 16 billion seeds per hectare per year from stands with 200 broadleaved pepperweed stems per m2 ([11,69] and references therein).

According to Young and others [103], flowering of broadleaved pepperweed is profuse in dry years, but seed set and maturity is minimal. In very wet years, infection with the white rust (Albugo spp.) appears to largely inhibit seed production [103].

Seed dispersal: According to a review by Howald [41], broadleaved pepperweed seeds have no special adaptations for long-distance dispersal, although they could be transported by wind, water, and possibly by waterfowl and other animals. Broadleaved pepperweed seeds may also be transported in agricultural products and by vehicles and machinery.

Broadleaved pepperweed seeds do not dehisce from the pods at maturity, but fall at irregular intervals during the winter [104], and some seeds may remain in pods until spring [22].

Spread to new areas may occur when seeds are transported in water from upstream sources [67]. According to Young [97], broadleaved pepperweed seeds initially sink when immersed in water. Then a layer of mucilage forms on the seed surface making them buoyant.

The role of domestic livestock and waterfowl in dispersing broadleaved pepperweed seeds is not fully understood [103].

In hay fields where broadleaved pepperweed is a contaminant, its seeds may be dispersed via contaminated hay or machinery [28]. There is concern that broadleaved pepperweed seeds are transported in alfalfa (Medicago sativa) grown in the Intermountain Area. However, broadleaved pepperweed does not appear to be a contaminant in well-managed alfalfa or other hay fields, and the cutting and harvesting practices for alfalfa usually preclude seed contamination of hay. Long distance transportation of straw from grain crops may be a source of broadleaved pepperweed spread. Even if the straw is weed free (as is likely for broadleaved pepperweed in straw), the storage sites for the straw may provide inoculum, with transportation vehicles serving as the vector [103].

Seed banking: It is unclear how long broadleaved pepperweed seeds may persist under field conditions and whether broadleaved pepperweed seeds in the soil seed bank may establish seedlings following disturbance. Laboratory tests indicate that broadleaved pepperweed seeds may remain viable for 2 years or more. Research is needed to determine longevity and viability of broadleaved pepperweed seeds in the field.

Miller and others [55] found germinability of broadleaved pepperweed seeds tested 1, 6, and 12 months after harvest did not change with time, and concluded that broadleaved pepperweed seeds can be stored under laboratory conditions for at least a year with no special precautions. More importantly, there seems to be no inherent dormancy system (e.g. a hard seed coat) present in broadleaved pepperweed seeds. Additionally, the temperature conditions that produce optimum germination for broadleaved pepperweed seeds do not change within a year after harvest, as would be the case if an afterripening requirement existed. This evidence suggests that buried seeds of broadleaved pepperweed may not be a prolonged source of reinfestation once a population is controlled. To be certain of this, however, the fate of deeply buried seeds that are not exposed to diurnal temperature fluctuations necessary for germination (see Germination) must be determined.

According to Young [97], the seed and seedbed ecology of broadleaved pepperweed is poorly understood. Weekly bioassay of surface soils collected for 2 years from dense broadleaved pepperweed stands at Reno, Nevada, demonstrated peak emergence of broadleaved pepperweed seedlings from samples collected in February and March. Emergence from bioassay samples indicates a potential field emergence of 25 to 35 seedlings per square foot. The original bioassay samples were then alternately wet and dried for 4 week periods for 2 years, and continued to produce occasional broadleaved pepperweed seedlings during that time. In the stands where the seedbed bioassay samples were collected, seedlings were never observed, even in plots where all surface vegetation was removed by hand or rototilling. In 2 to 5 acre (0.8-2 ha) field plots broadleaved pepperweed seedling establishment is not a serious problem after the established population is suppressed. Numerous, isolated roadside infestations of broadleaved pepperweed suggest, however, that seedling establishment may play an important role in establishment of new populations.

Germination: Germination studies [55] (Robbins et al 1951,as cited by [41]) have shown high germination rates (64%-100%) for broadleaved pepperweed under a variety of conditions. However broadleaved pepperweed seedlings are rarely observed in the field (e.g. [22,97]).

Miller and others [55] found that very cold temperatures and constant temperatures between 32 and 104 °F (0-40 °C) failed to support broadleaved pepperweed germination. The highest germination rates (96% to 100%) for broadleaved pepperweed seed occurred under alternating temperature regimes with low night temperatures (32, 36, or 41 °F (0, 2, or 5 °C)) and high day temperatures (95 to 104 °F (35-40 °C)). These fluctuating regimes represent realistic temperatures of seedbeds in the Intermountain Area during fall or late spring when diurnal fluctuations are characterized by low night temperatures and high day temperatures. Broadleaved pepperweed seeds would probably have to be on or near the soil surface to experience temperature fluctuations of this magnitude. Deep burial of broadleaved pepperweed seeds may greatly reduce emergence because of poor germination due to more constant and cooler temperature regimes with depth.

No significant (p<0.01) differences in germination of broadleaved pepperweed seed were attributed to year of production, duration of storage after harvest, or seed source [55].

According to Howald [41], broadleaved pepperweed seeds typically germinate in spring in wet sand or mud, although the source of this information is not given.

Seedling establishment/growth: Little is known about broadleaved pepperweed seedling establishment under field conditions. Some authors indicate that broadleaved pepperweed seedlings are rarely found in the field [22,97]. This may be because broadleaved pepperweed seedlings are difficult to recognize in the field as they are easily confused with many other adventitious members of the mustard family [103]. Seedling establishment may, however, play an important role in establishment of new populations [97]. More research is needed in this area.

Once established, it seems that broadleaved pepperweed plants grow and spread rapidly. Preliminary experiments have shown that broadleaved pepperweed roots perennialize rapidly (by the 8-leaf stage) (Renz and DiTomaso, unpublished data as cited by [69]). A single established plant can become a small population, several meters in diameter, in 2 seasons. After 5 years, areas infested by broadleaved pepperweed may be near monocultures, with stem densities approaching 150 per m2 [8,12].

A conceptual model of broadleaved pepperweed spread is presented by Blank and Young [11]. Broadleaved pepperweed seeds are dispersed, plants establish and spread by creeping roots, establishing single-species colonies. A visual estimate of broadleaved pepperweed coverage at this initial stage of invasion would be about 2%. The colonies expand and eventually merge (see Successional status) [11].

Blank and Young [11] also provide an example that illustrates the rate of spread of broadleaved pepperweed colonies. An 80 acre (32 ha) field within the Honey Lake Wildlife Refuge in California had no broadleaved pepperweed plants in 1993. In 1994 a 431 ft2 (40 m2) plot was established to monitor spread of broadleaved pepperweed, at which time there were 2 colonies in the plot, each less than 11 ft2 (1 m2), with stem densities less than 10 stems per m2. In 2000, most of the 431 ft2 (40 m2) plot had become invaded, with stem densities greater than 100 stems per m2 in some areas. Broadleaved pepperweed densities began to decline in 2000 and the decline continued to 2002. The authors suggest that this decline was due to below normal winter precipitation. A graphical summarization of these data can be seen at the EIWRU website [11,12].

In a study to determine the rate of spread of broadleaved pepperweed at 3 locations in California, Renz and others [71] found that undisturbed broadleaved pepperweed infestations spread clonally in a predictable pattern along the leading edge, about 3 to 6 feet (1-2 m) per year. Initial size and area of infestations were found to influence the rate of spread, with infestations of small area and large perimeter expanding at the greatest rate. Rosette and stem density was highest at the center of infestations. Similar rates of expansion were observed at all undisturbed plots at Colusa and Grizzly Island sites, with populations expanding from 44 to 129% from 1999 to 2001. Thus it was concluded that in the absence of disturbance, broadleaved pepperweed infestations will continue to expand and increase in density over time.

Asexual regeneration: Rather than expansion from seedlings, it is much more common to see broadleaved pepperweed populations expand by creeping roots [98,103] (also see Seedling establishment/growth). Broadleaved pepperweed also regenerates from roots and semi-woody crowns [72,103] when tops are removed, and from root fragments that produce buds and sprout new plants [95].

Broadleaved pepperweed has a deep, extensive root system with a high reproductive potential that allows it to sprout from previously dormant buds near the soil surface following removal of aboveground parts via cutting or mowing [71], grazing [4], or herbicide treatments [103].

New broadleaved pepperweed plants can also establish from root fragments. Pieces of rootstock less than 0.8 cm in diameter and less than 1 inch (2.5 cm) long can form new broadleaved pepperweed plants. One-inch-long (2.5 cm) root segments can produce more than 1 shoot sprout. Shoots produced by segments that were planted in the greenhouse in early summer produced flowers and seed in late summer. There was a significant (p<0.05) difference in sprouting between root fragments from plants that had been treated with 2,4-D at flower bud stage during the previous summer, and roots fragments taken from untreated broadleaved pepperweed plants. Five percent of herbicide-treated roots sprouted, and 50% of the control roots spouted, indicating that considerable amounts of 2,4-D were translocated to the roots. The authors note that, based on observations of field plots, 5% sprouting is still sufficient to result in complete recolonization of treated plots by the end of the 1st growing season following 1 herbicide treatment [95].

Roots are buoyant and when broken from eroding banks can be transported long distances by water and establish new populations [103]. Broadleaved pepperweed roots also have the ability to tolerate dry conditions and resist desiccation. Research is needed to understand the growth patterns and longevity of broadleaved pepperweed roots and what soil depths they can emerge from [69].

SITE CHARACTERISTICS:
In its native range, broadleaved pepperweed grows in a wide variety of habitats, including fresh, brackish, and saltwater wetlands, in and around agricultural fields, in waste places, and even on stony slopes, from sea level to above 10,000 feet (3,050 m) (May 1995, unpublished report, as cited by [41]).

Broadleaved pepperweed also tolerates a wide variety of environmental conditions in North America. Many sites have dense infestations in one area and no plants invading into nearby locations, indicating that broadleaved pepperweed spread may be limited by environmental, physical, and/or geographical factors, although it is unclear what these factors are [69]. In California, for example, broadleaved pepperweed typically grows in full sun in heavy, moist soils that are often saline or alkaline, but it also grows on drier sites and on other soil types. Its precise tolerance limits for aridity, alkalinity, and salinity are unknown [41].

Broadleaved pepperweed seems to be most problematic in riparian areas, marshes, estuaries, irrigation channels, wetlands, and floodplains, but is not exclusive to these areas. According to Muldavin and others [58], broadleaved pepperweed is a facultative plant; i.e. it has about equal probability of occurring in wetland or non-wetland sites.

Broadleaved pepperweed is tolerant to halomorphic soils and thus is adaptable in coastal areas, interior salt marshes, and sinks such as brackish marshes of San Francisco Bay, and in saline/alkaline sinks in the Carson Desert of western Nevada. Once established in wetlands, infestations often follow irrigation structures into irrigated pastures, native hay meadows, agronomic fields, and urban landscaping [4,10,69,102,103]. Once established, broadleaved pepperweed can persist in roadsides, native hay meadows, alfalfa fields, and rangeland habitats [69]. According to Trumbo [83] broadleaved pepperweed is highly invasive in areas of California that were formerly under intensive agriculture and then sold to the State of California for use as wildlife refuges. Broadleaved pepperweed populations in these areas, unchecked by frequent cultivations and crop competition, then expanded at the expense of recovering native plants.

In the Intermountain Area the major sites of infestation are native hay meadows. These meadows generally have not been leveled or plowed, and they vary considerably in site characteristics and topography. They are irrigated by "wild-flooding" in the spring, with depressions flooded for prolonged periods. The soils of these meadows range from slightly to highly influenced by salts [102]. In the Susan River Valley of northeastern California, broadleaved pepperweed has invaded beyond native meadows to where it is now a serious pest in more intensive agricultural rotation crops such as cereal grains and alfalfa [103].

Research by Blank and others [9] demonstrates that broadleaved pepperweed achieves optimal growth within a narrow range of soil water potential. Accumulated biomass was greatest when broadleaved pepperweed was grown at -0.02 MPa soil matric water potential. At higher potential (saturation) broadleaved pepperweed grew poorly, but all plants survived. At lower potential (more negative), broadleaved pepperweed biomass decreased substantially, yet all plants survived. The authors suggest that broadleaved pepperweed primarily invades wetlands because the high soil water content reduces tortuosity and allows efficient transport of nutrients to this sparsely rooted species. When soil moisture and/or the nutrient supplying capacity of the soil declines, plants with greater root density may out-compete broadleaved pepperweed (see Successional status).

According to Young and others [99,103], broadleaved pepperweed is adapted to but not restricted to salt-affected soils, alkali soils, and soils with sodium hazards. Broadleaved pepperweed invades brackish to saline or alkaline wetlands throughout California, from the coast to the interior and north and eastward into the Great Basin and Columbia Basin. It is also found in native hay meadows and as a weed in agricultural fields where the soil is slightly alkaline or saline (Young and Turner 1995, as cited by [41]). In Wyoming, broadleaved pepperweed is found on soils of high alkalinity (pH 9.2), and it appears to tolerate, but not require, saline conditions. Patches of broadleaved pepperweed occurred right up to the edge of white alkali patches [4]. Roots of broadleaved pepperweed are not hindered by root-restricting layers or high water tables [11,12]. More research is needed to understand the mechanism(s) that allow broadleaved pepperweed to cope with varying soil salinities [8,12,69].

At Honey Lake Wildlife Refuge in northeastern California, field infestations of broadleaved pepperweed occur in almost pure colonies with few other species. Soils in the area are slightly saline and sodic, generally fine-textured, and have compact and hard natric subsoils [12]. Broadleaved pepperweed invaded areas have thick organic and debris-rich O horizons that are lacking in uninvaded areas (occupied primarily by tall wheatgrass (Thinopyrum ponticum)). Researchers also noted that natric horizons in invaded areas were modified both physically and chemically (see Successional status and Impacts for more details) compared with uninvaded sites [8,10,12]. The authors state that while many of the differences in soil attributes observed between sites are attributable to broadleaved pepperweed invasion, it is also possible that antecedent soil differences favored invasion by broadleaved pepperweed in some areas [12].

Disturbance: Broadleaved pepperweed is often described as occurring on roadsides and in "waste places." Initial establishment of broadleaved pepperweed in the intermountain Area has often occurred in association with structures for diversion of irrigation water from streams. Construction or repair of such physical facilities usually provides bare soil for colonization by pioneer species. The undercarriage of track-laying heavy construction equipment is an ideal mechanism for transporting vegetative propagules of weeds such as broadleaved pepperweed from site to site [103].

Elevation: Elevation ranges for broadleaved pepperweed occurrence are reported by area as follows:

State Elevation range References
CA <8,200 feet (2,500 m) [5,93]
CO 5,500-8,000 feet (1,700-2,440 m) [36]
NV 3,900-8,000 feet (1,200-2,440 m) [45]
NM 5,000-8,000 feet (1,500-2,440 m) [53]
UT 4,100-7,900 feet (1,250-2,410 m) [90]

SUCCESSIONAL STATUS:
While there is some anecdotal evidence that disturbance favors establishment of broadleaved pepperweed (e.g. [83,101]) it is unclear whether broadleaved pepperweed requires disturbance for initial establishment. Once established, broadleaved pepperweed may remain as scattered, isolated plants or populations [69,97], or it may form dense colonies which eventually merge into closed-canopy stands that are practically monospecific [102]. The root architecture of broadleaved pepperweed is such that monoculture stands may be self-limiting as soil fertility levels decline, but longevity of broadleaved pepperweed populations is unknown. In some areas, invasion of broadleaved pepperweed introduces a radically different plant community such that soil physical properties and biogeochemical cycling are altered [9,12]. Invasion of an area by broadleaved pepperweed may alter soil properties to such an extent that different successional trajectories are triggered, possibly altering subsequent soil evolution [11,12].

Evidence presented by Blank and Young [11] suggests that succession proceeds differently in disturbed compared with undisturbed broadleaved pepperweed-invaded sites. Where broadleaved pepperweed invades undisturbed native vegetation, the population expands along a narrow front, with the dominant mode of invasion via creeping roots. In more disturbed areas, the invasion front is wide with small colonies scattered throughout. Broadleaved pepperweed stem densities increase, and individual colonies expand and eventually merge. When broadleaved pepperweed stem densities exceed about 20 stems per m2, other plant species are excluded.

In established monocultures, large root reserves, shading, and accumulation of plant litter may contribute to the competitiveness of broadleaved pepperweed [11]. Results presented by Blank and others [9,11] suggest that competitiveness of broadleaved pepperweed may decline over time. Wetland and riparian environments that broadleaved pepperweed invades are often occupied by shallow-rooted species such as saltgrass, rushes, and sedges, and many areas are saline and/or alkaline with root-restricting soil layers (natric horizons). In the environments studied, root length density (root length per volume of soil) of broadleaved pepperweed is typically less than that of the native community. However, broadleaved pepperweed rooting depth often exceeds 9 feet (3 m) and it is capable of exploring a nutrient niche in these deep soil layers, even below the dense, compact natric horizons, that has been minimally explored by the vegetation it is replacing. Optimal nutrient uptake by sparsely-rooted broadleaved pepperweed requires efficient transport of nutrients through the soil, which is fostered by a wet soil. Biogeochemical cycling in which natric horizons are altered and nutrients are taken from deep soil layers and deposited on the surface through litter fall, enriches surface soil with nutrients. High nutrient levels in surface soil will be of minimal value to broadleaved pepperweed because the surface soil dries relatively early in the growing season, thereby limiting mass flow of nutrients [11].

In a greenhouse study, Blank and others [9] investigated the influence of soil nutrient depletion on plant growth and plant competition between broadleaved pepperweed and cheatgrass. As nutrient supplying capacity of the soil declined through growth cycles, aboveground mass of broadleaved pepperweed decreased significantly (p<0.05) and growth potential of cheatgrass surpassed that of broadleaved pepperweed. These findings are explained by a combination of differences in root architecture, processes involved with soil nutrient bioavailability, and soil nutrient depletion. It is unclear whether nutrient depletion will eventually lead to a decline of broadleaved pepperweed in the field. The oldest monocultures of broadleaved pepperweed observed by Blank and others are about 15 years old, and they had not yet noticed any decline in its vigor and stature.

Invasion by broadleaved pepperweed has the potential to alter soil properties and processes relative to uninvaded sites to favor its own growth and survival, and possibly alter the trajectory of soil evolution [12]. Observed differences between broadleaved pepperweed-invaded sites and similar, noninvaded sites include a thick, nitrogen-rich litter layer; greater nitrogen availability and nitrogen-mineralization potentials; increased enzyme activities; increased biogeochemical fluxes of carbon, nitrogen, phosphorus, calcium, magnesium, and sulfur; lower sodium absorption ratios; and less compact, more friable natric horizons in sites occupied by broadleaved pepperweed as compared to sites dominated by tall wheatgrass [8,11,12]. Amelioration of sodic soils, including those with hard and compact subsoils (natric horizons) could give these soils greater effective rooting depth and more favorable physical properties that would make them likely to support a richer, more productive plant community if broadleaved pepperweed is controlled. The potential for excessive salt accumulation at the soil surface via litter decomposition cautions that long-term invasion by broadleaved pepperweed may increase the osmotic potential of the soil surface, thereby reducing seed germination and growth of salt-intolerant species [12].

All differences in soil attributes observed between broadleaved pepperweed invaded and uninvaded sites are difficult to attribute solely to broadleaved pepperweed invasion. It is possible that antecedent soil differences favored invasion by broadleaved pepperweed in particular areas. However, the case can be made that some differences in soils occupied by broadleaved pepperweed are a direct consequence of plant invasion through a combination of differential biogeochemical cycling and rhizosphere interactions. Where broadleaved pepperweed has converted diverse plant communities to monocultures, it is reasonable to conjecture that this conversion will promote divergent soil evolution [12].

SEASONAL DEVELOPMENT:
As early as mid-winter in the western Intermountain Area, careful examination of broadleaved pepperweed root crowns reveals multiple buds that are green and slowly developing. Shoot growth can begin at varying periods, depending on timing of the last frost, but generally shoots emerge in late winter to early spring before those of most native species [22,103]. In coastal areas, where frost is infrequent, rosette leaves persist through winter. Observations of broadleaved pepperweed seedlings are rare in the field, "but germination appears to occur late winter/early spring" [69].

Broadleaved pepperweed shoots initially form a rosette near the soil surface and remain in rosette form for several weeks before stems elongate [69]. Rosettes may remain largely hidden by persistent, semi-woody herbage from previous years' growth. Stem elongation is rapid during May, and by the 1st of June stems are 1.6 to 2.6 feet (0.5-0.8 m) tall [103]. Day length is believed to be a main factor in the regulation of stem elongation [69].

Flowering dates are given by area are as follows:

Area Flowering dates References
CA May-August [41,60]
NV June-August [45]
NM June-August [53]
Atlantic coast May-June [25]
Great Plains June-August [33]
Intermountain Area begins mid-June [103]
New England August [78]
Pacific Northwest June-September [39]

Flowering time varies from May to August in different parts of California [41,60], and peak bloom lasts for several weeks [41].

As broadleaved pepperweed flowers develop, the shoot apical meristem loses its apical dominance and axillary buds elongate and form secondary panicles with many clusters of flowers. This combination creates a dense canopy of stems, flowers, and fruit throughout much of the summer. Flowering and fruit set may occur for several months [69]. At flowering, stalks are 3 to 6 feet (1-2 m) in height. The basal rosette leaves are nearly senescent and stands are usually so thick that virtually no light reaches the soil surface at flowering [103].

Broadleaved pepperweed seeds mature between June and mid-August [41,103]. Plants mowed in hay fields or injured in control treatments may flower late and have seeds still maturing at 1st frost. Mature seeds do not immediately disperse from the pods, and some seeds may remain on senesced plants until the following season [103]. Aboveground parts typically die in late fall and winter. Dead stems of broadleaved pepperweed degrade slowly and may persist for more than a year [22]. Rosette leaves often emerge from dormant buds below the soil in the late summer/early fall and persist until the initial frost [69].

Broadleaved pepperweed can store large amounts of energy in its perennial roots. When spring growth is initiated, total nonstructural carbohydrates (TNC) in the top 16 inches (40 cm) of root material rapidly decrease and reach a minimum at the bolting stage before flowerbuds develop. Broadleaved pepperweed begins allocating large amounts of photosynthate below ground during the flowerbud stage. The rate of translocation of photosynthates to belowground structures is greatest from the full flowering to seed filling stages. As stems senesce in the late summer/early fall, a decrease in stored TNC is seen. Researchers believe that a flush of new root growth causes this reduction in stored energy, but further research is necessary. After this decrease, stored carbohydrates in perennial roots remain constant until early spring growth begins [70,71].

Leaf area distribution of broadleaved pepperweed also fluctuates during the growing season. Broadleaved pepperweed leaf area is not distributed evenly within the canopy, appears to be dependent on environmental factors, and is altered in plants that sprout after mowing [69]. These factors can have consequences for herbicide movement (see Physical/mechanical control).

FIRE ECOLOGY

SPECIES: Lepidium latifolium
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: There is no information in the literature regarding adaptations of broadleaved pepperweed to fire. Broadleaved pepperweed has a deep, extensive root system with a high reproductive potential that allows it to sprout repeatedly following removal of aboveground growth. Physical and mechanical control methods such as mowing and disking, for example, are unlikely to control broadleaved pepperweed because new plants quickly regenerate from roots and root crowns (see Regeneration Processes). Broadleaved pepperweed is likely, therefore, to similarly re-establish after fire. There is no information in the literature regarding the response of broadleaved pepperweed seed to heat, smoke or fire.

Fire regimes: Broadleaved pepperweed is often found in riparian and wetland communities. There is little quantitative information on prehistoric frequency, seasonality, severity and spatial extent of fire in North American riparian ecosystems. Fire frequency probably varied with drought cycles, prevalence of lightning strikes, prevalence of burning by Native Americans, and fires in surrounding uplands. Broadleaved pepperweed was not widespread in these communities when historic fire regimes were functioning, but has established since habitat alteration and fire exclusion began. It is unclear how historic fire regimes might affect broadleaved pepperweed populations, and it is unclear how the presence of broadleaved pepperweed in native ecosystems might affect fire regimes.

In general, in ecosystems where broadleaved pepperweed replaces plants similar to itself (in terms of fuel characteristics), it may slightly alter fire intensity or slightly modify an existing fire regime. However, if broadleaved pepperweed is qualitatively unique to the invaded ecosystem, it has the potential to completely alter the fire regime [21]. No examples of fire regimes altered by broadleaved pepperweed invasion are described in the available literature.

The following table provides fire return intervals for plant communities and ecosystems in which broadleaved pepperweed may occur. 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)
bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium < 10 [47,62]
silver sagebrush steppe Artemisia cana 5-45 [37,65,96]
sagebrush steppe A. tridentata/Pseudoroegneria spicata 20-70 [62]
basin big sagebrush A. tridentata var. tridentata 12-43 [77]
mountain big sagebrush A. tridentata var. vaseyana 15-40 [3,14,56]
Wyoming big sagebrush A. tridentata var. wyomingensis 10-70 (40**) [87,100]
coastal sagebrush A. californica < 35 to < 100
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus < 35 to < 100
desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 [62]
plains grasslands Bouteloua spp. < 35 [62,96]
blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii < 35 [62,76,96]
blue grama-buffalo grass B. gracilis-Buchloe dactyloides < 35 [62,96]
grama-galleta steppe B. gracilis-Pleuraphis jamesii < 35 to < 100
blue grama-tobosa prairie B. gracilis-P. mutica < 35 to < 100 [62]
cheatgrass Bromus tectorum < 10 [63,91]
northern cordgrass prairie Distichlis spicata-Spartina spp. 1-3 [62]
California steppe Festuca-Danthonia spp. < 35 [62,82]
Rocky Mountain juniper Juniperus scopulorum < 35 [62]
wheatgrass plains grasslands Pascopyrum smithii < 5-47+ [62,65,96]
galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea < 35 to < 100 [62]
eastern cottonwood Populus deltoides < 35 to 200 [62]
aspen-birch P. tremuloides-Betula papyrifera 35-200 [24,88]
quaking aspen (west of the Great Plains) P. tremuloides 7-120 [2,35,54]
mountain grasslands Pseudoroegneria spicata 3-40 (10**) [1,2]
California oakwoods Quercus spp. < 35 [2]
oak-juniper woodland (Southwest) Quercus-Juniperus spp. < 35 to < 200 [62]
coast live oak Q. agrifolia 2-75 [34]
canyon live oak Q. chrysolepis <35 to 200
blue oak-foothills pine Q. douglasii-P. sabiniana <35
Oregon white oak Q. garryana < 35 [2]
California black oak Q. kelloggii 5-30 [62]
blackland prairie Schizachyrium scoparium-Nassella leucotricha < 10 [88]
little bluestem-grama prairie S. scoparium-Bouteloua spp. < 35 [62]
elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. < 35 to 200 [24,88]
**mean

POSTFIRE REGENERATION STRATEGY [81]:
Caudex/herbaceous root crown, growing points in soil
Geophyte, growing points deep in soil
Ground residual colonizer (on-site, initial community)
Initial off-site colonizer (off-site, initial community)
Secondary colonizer (on-site or off-site seed sources)

FIRE EFFECTS

SPECIES: Lepidium latifolium
IMMEDIATE FIRE EFFECT ON PLANT:
There is little information in the literature regarding the immediate effect of fire on broadleaved pepperweed plants. Based on a single experiment [46], field observations during another study [71], and other unknown sources, several authors [41,69,71,83] have suggested that broadleaved pepperweed infestations are difficult to ignite and may not sustain a burn. When broadleaved pepperweed does burn, one might assume that the aboveground growth would be killed, but that much of the perennial root system would be unharmed by fires of low to moderate severity. It is unclear what kind of damage a high severity fire might have on broadleaved pepperweed roots. There is no information in the literature regarding the effects of heat or fire on broadleaved pepperweed seed.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
No additional information is available on this topic.

PLANT RESPONSE TO FIRE:
There is no information in the literature regarding broadleaved pepperweed's response to fire that includes measurements beyond 1 year following fire, and only 1 study in which the effects of fire on broadleaved pepperweed were among the objectives of the study [46]. This study did not include data on the effects of fire alone on broadleaved pepperweed. More research is needed to understand the response of broadleaved pepperweed to fires of varying severity, in various ecosystems over long time periods.

Observations by Renz and others [71] suggest that high severity fire may "temporarily" reduce broadleaved pepperweed cover and spread, although the site that burned was also flooded in the same and the preceding years, and changes in total area infested were measured for only 1 year following the burn. The effects of fire were not the focus of the study, rather the study was designed to measure the rate of spread of broadleaved pepperweed at 3 different sites in California, all seasonal wetlands. It is also unclear what the objectives of the prescribed burn were (see Fire Management Considerations).

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
No additional information is available on this topic.

FIRE MANAGEMENT CONSIDERATIONS:
Fire as a control agent: Most anecdotal evidence in the literature suggests that fire is not a viable control agent for broadleaved pepperweed (e.g. [41,69,71,83]). Conversely, 1 reference suggests that periodic mowing and spring burning have reduced broadleaved pepperweed density in Utah (Hansen, K.S. 1988, personal communication as cited by [49]).

Only 1 study was found in which the effects of fire on broadleaved pepperweed were among the objectives of the study [46]. The study took place at 3 locations in Malheur National Wildlife Refuge, Oregon, and was designed to test the combined effects of herbicides and disking or fire on broadleaved pepperweed density and basal cover. For both herbicide-fire treatments, herbicides were sprayed in early summer and then vegetation was cut to a height of 10 cm with a brush mower 1 week before burning "to increase fire heat at the soil surface for broadleaved pepperweed control." For fire only treatments, plots were similarly mowed 1 week before burning. All burns were conducted on October 17 with back fires ignited by drip torches. Fuels consisted of live grass/forb, dead grass/forb, live broadleaved pepperweed, and dead broadleaved pepperweed. Fuel moisture content and dry weight were determined in the laboratory. Fuel consumption by fire for each plot was calculated as the difference between the average biomass of all fuel types pre- and postfire. Broadleaved pepperweed and native vegetation response were measured pretreatment and 1-year posttreatment. Broadleaved pepperweed density, basal cover of live plant species, bare ground, and residual plant material were determined by sampling along transects within each plot.

A lack of adequate fuels resulted in unsuccessful burns at 2 of these sites. The 2 plots that had a higher proportion of live and dead broadleaved pepperweed biomass and a lower proportion of live grass/forb biomass, lacked adequate fuels for successful burns [46].

Site Fuel biomass
(g/m2 DW)
Live grass/forb biomass
(%)
Live and dead broadleaved pepperweed
(%)
Dead grass forb biomass
(%)
Big Sage 69 55 15 30
Oliver Springs 48 8 62 30
Skunk Farm 46 15 56 30

At the site that did burn (Big Sage), fuel consumption ranged from 82% to 94%.

After 1 posttreatment year, herbicides alone and in combination with fire generally reduced cover of forbs and increased cover of grasses (predominantly beardless wildrye). Beardless wildrye on the herbicide-fire treatments was at least twice as tall as that on the herbicide only plots. The authors speculate that vigorous stands of beardless wildrye on herbicide-fire plots likely resulted from release of nutrients and or stimulation of root buds by fire. Although beardless wildrye was prevalent on the burn-only plot, its stand was not as vigorous as those for herbicide-fire treatments and resembled those of herbicide only treatments at that site [46].

In a study of the rate of spread of broadleaved pepperweed within seasonal wetlands in California, 2 of the study sites were burned under prescription during the study period. At Colusa National Wildlife Refuge, 2 plots were burned in December 1999. At Lower Klamath National Wildlife Refuge all plots were completely burned in October 2000, and all plots were annually flooded from October to May, for 3 years (1999-2001) during the study period. Stem and rosette densities of broadleaved pepperweed were compared from year to year to calculate rate of spread. In the absence of disturbance, broadleaved pepperweed infestations continued to spread and invade new areas. Colusa plots that were burned in winter, 1999, had varying results. After 1 year, the broadleaved pepperweed infested area of burned plots was reduced at Colusa 1 by 6%, and increased at Colusa 2 by 55%. The author speculates that the variability observed may be dependent on other factors such as the intensity of the fire, which was "strong" at Colusa 1 compared to Colusa 2 (data not shown). Two years after the burn, the broadleaved pepperweed-invaded area at both sites had increased; 7% and 128% at Colusa 1 and 2, respectively. Postfire increases in broadleaved pepperweed resulted from clonal spread; almost no broadleaved pepperweed seedlings were observed. At the Lower Klamath site, infestations declined in all but 1 plot, which had a 6% increase during the 2 years of the experiment. One plot had a reduction in infested area after the 1st year (all plots flooded). All plots had reduction in area infested in the 2nd year (all plots flooded and burned). All but 1 plot had a reduction in infested area over the period of study. The authors conclude, "while burning may provide temporary reductions in infestation size, unless repeated annually, burning alone is ineffective in long-term management of broadleaved pepperweed" [71].

Postfire colonization potential: It is unclear whether fire increases the probability of broadleaved pepperweed establishment if seeds or roots are present. Establishment of broadleaved pepperweed by seed under field conditions is poorly understood [69]. When planning a prescribed burn, it is a good idea to survey the surrounding area for broadleaved pepperweed and control plants that may disperse seed or root material into the burn area.

Preventing postfire establishment and spread: The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [84] provides several fire management considerations for weed prevention in general that apply to broadleaved pepperweed.

Preventing invasive plants from establishing in weed-free burned areas is the most effective and least costly management method. This can be accomplished through careful monitoring, early detection and eradication, and limiting invasive plant seed dispersal into burned areas by [32,84]:

In general, early detection is critical for preventing establishment of large populations of invasive plants. Monitoring in spring, summer, and fall is imperative. Managers should eradicate established broadleaved pepperweed plants and small patches adjacent to burned areas to prevent or limit dispersal into the site [32,84].

The need for revegetation after fire can be based on the degree of desirable vegetation displaced by invasive plants prior to burning and on postfire survival of desirable vegetation. Revegetation necessity can also be related to invasive plant survival as viable seeds, root crowns, or root fragments capable of reproduction. In general, postfire revegetation should be considered when desirable vegetation cover is less than about 30% [32].

When prefire cover of broadleaved pepperweed is absent to low, and prefire cover of desirable vegetation is high, revegetation is probably not necessary after low- and medium-severity burns. After a high-severity burn on a site in this condition, revegetation may be necessary (depending on postfire survival of desirable species), and intensive monitoring for invasive plant establishment is necessary to detect and eradicate newly established invasives before they spread [32].

When prefire cover of broadleaved pepperweed is moderate (20-79%) to high (80-100%), revegetation may be necessary after fire of any severity if cover of desired vegetation is less than about 30%. Intensive weed management is also recommended, especially after fires of moderate to high severity [32].

Fall dormant broadcast seeding into ash will cover and retain seeds. If there is insufficient ash, seedbed preparation may be necessary. A seed mix should contain quick-establishing grasses and forbs (exclude forbs if broadleaf herbicides are anticipated) that can effectively occupy available niches. Managers can enhance the success of revegetation (natural or artificial) by excluding livestock until vegetation is well established (at least 2 growing seasons) [32]. See Integrated Noxious Weed Management after Wildfires for more information.

When planning a prescribed burn, managers should preinventory the project area and evaluate cover and phenology of any broadleaved pepperweed and other invasive plants present on or adjacent to the site, and avoid ignition and burning in areas at high risk for broadleaved pepperweed establishment or spread due to fire effects. Managers should also avoid creating soil conditions that promote weed germination and establishment. Weed status and risks must be discussed 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 available [84].

MANAGEMENT CONSIDERATIONS

SPECIES: Lepidium latifolium
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Most domestic livestock generally do not prefer broadleaved pepperweed as forage; however, domestic sheep and goats are known to graze thick stands of broadleaved pepperweed in some areas (see Palatability) [4,101]. Broadleaved pepperweed is apparently inferior food and cover for wildlife compared to native vegetation that it replaces [41,83], although there are no data to support these observations.

Palatability/nutritional value: Little information is available on the palatability of broadleaved pepperweed. Observations of researchers in Nevada suggest that cattle and domestic sheep will graze it when it grows amid other plants, but they do not eat broadleaved pepperweed growing in pure dense stands [94]. According to Baker [4], in Wyoming, pastures with broadleaved pepperweed rapidly become useless to cows and horses, but sheep readily eat broadleaved pepperweed, and even heavily infested pastures appear weed free when grazed by domestic sheep. In Montana, horses and mules were observed eating around broadleaved pepperweed leaf and seed flakes in grass hay [28].

Cattle reportedly graze broadleaved pepperweed rosette leaves in early spring. However, these leaves cannot be reached by livestock unless the accumulation of previous years' stalks is removed. When broadleaved pepperweed plants occur as occasional plants in saltgrass meadows, flowerstalks may be eaten by grazing cattle. In dense stands of broadleaved pepperweed in perennial pastures, there was no evidence of utilization by "grazing animals" even when drought had seriously depleted preferred forage species [103]. Domestic goats graze thick stands of broadleaved pepperweed and seem to prefer the young, tender, more digestible regrowth following grazing and mowing. For example, domestic goats ate about 75% of regrowth, compared to about half of the vegetation in older stands, on which they ignored the semiwoody stems and nibbled only leaves and soft tips (Jim Young, personal communication in [94]).

Cover value: Observations and/or reports of some authors (e.g. [12,41,83,102]) suggest that broadleaved pepperweed provides inferior cover for many bird species compared with the vegetation that it replaces.

OTHER USES:
Broadleaved pepperweed has been used extensively in traditional medicine for over 2,000 years as a diuretic, stomachic tonic, and for antilithiasis. Sulphurated essences have been isolated from its roots, seeds, and leaves; mirosin is also reported as present in its seeds (Front Quer 1973, as cited by [61]). Several sterols and polyphenols have been isolated from its leaves (Navarro et al 1992, as cited by [61]). The leaves are often given as an infusion in the treatment of renal disorders, and several commercial laboratories sell the dried powdered leaves [61].

In a pharmacological screening for diuretic activity in rats using an aqueous extract of broadleaved pepperweed given orally and intraperitoneally, enhanced urinary excretion was observed as compared to control groups. A slight increase in ion excretion was also observed [61].

IMPACTS AND CONTROL:
Impacts: Broadleaved pepperweed is listed by the California Invasive Plant Council (Cal-IPC) on List A-1: a widespread, aggressive invader that displaces natives and disrupts natural habitats. These are the most invasive wildland pest plants in their classification [16]. Little research is available documenting or quantifying impacts of broadleaved pepperweed. However, several authors indicate observed impacts, especially in wetland and riparian settings. Observed and/or suggested impacts include altered species diversity, structure and function [10,67,83,103], displaced native species [74,83] including rare plant populations (Skinner and Pavlik 1994, as cited by [41]), decreased food and habitat for several wildlife species [41,46,83,102,104], changes in biogeochemical cycles [8,11,12] including emission of mercury from contaminated soils into the atmosphere [50], increased streamside soil erosion (personal communications with Susan Donaldson and Jim Young, as cited by [69]), and economic losses through reduced forage quantity and hay quality [4,28,41,46,102,103,104].

Observations of researchers and managers (e.g. [10,74,83,103,104]) suggest that broadleaved pepperweed has altered species diversity, structure, function, and succession in many wetland and riparian areas in the western U.S. Because broadleaved pepperweed is highly competitive, grows in dense patches that are near monocultures, and results in a buildup of heavy thatch and litter that may be rich in salts (depending on the site), seedling recruitment and productivity of important, native species may be adversely affected [10,103,104]. Few data are available to support these observations. Reports of broadleaved pepperweed replacing quackgrass (Elytrigia repens), another highly competitive, nonnative species, attest to the competitiveness of broadleaved pepperweed [11,104]. An inventory of rare and endangered plants in California indicates that broadleaved pepperweed is encroaching on several rare plant populations at Grizzly Island Wildlife Area in Suisun Marsh, including soft bird's-beak (Cordylanthus mollis ssp. mollis), Suisun thistle (Cirsium hydrophilum var. hydrophilum), and Suisun Marsh aster (Symphyotrichum lentum) (Skinner and Pavlik 1994 as cited by [41]).

Changes in vegetation structure caused by broadleaved pepperweed may interfere with management objectives and reduce habitat for various wildlife species. For example, observations along the Green River in Utah indicate that because of broadleaved pepperweed's increased canopy height and density as compared to native vegetation, it directly interferes with mosquito control efforts in the area (Steven V. Romney, personal communication as cited by [67]). Broadleaved pepperweed's tall stature, dense growth pattern, and accumulations of semiwoody stems (see General Botanical Characteristics) are also purported to negatively impact nesting habitat for wildlife [83,102,104]. Observations by Blank and Young [12] suggest that when broadleaved pepperweed populations reach a density of 50 stems per m2, no waterfowl nesting occurs. According to Howald [41], broadleaved pepperweed outcompetes grasses that provide food for waterfowl. Broadleaved pepperweed has invaded pickleweed-dominated marshes in some areas in California, and thus poses a threat to the habitat of the endangered salt marsh harvest mouse, California black rail, and California clapper rail [41,83]. No data are provided to support these observations. At the Malheur National Wildlife Refuge in Oregon, broadleaved pepperweed has displaced 5 and 10% of the meadow and grass/shrub uplands, respectively, that are critical habitats for nesting aquatic and neotropical birds (US Fish and Wildlife Service, unpublished data, as cited by [46]). Because broadleaved pepperweed makes hay from infested pastures unmarketable, broadleaved pepperweed jeopardizes the haying program on the Malheur National Wildlife Refuge, which provides short and medium grasses for sandhill cranes, shorebirds, and waterfowl [46]. 

Observations at the Honey Lake Wildlife Refuge in northeastern California, indicated "striking differences" in soil profiles in broadleaved pepperweed infested areas compared with soils in similar, noninvaded areas of native hayland [10]. These observations led to a series of research projects that indicated many differences in soil physical and chemical properties between broadleaved pepperweed infested sites and similar noninvaded sites [8,10,11,12] (see Successional Status).

Broadleaved pepperweed can take up mercury from contaminated soils and emit about 70% of that taken up during the growing season into the atmosphere (for every one molecule retained in broadleaved pepperweed foliage, 12 molecules were emitted) [50]. The most critical factors governing mercury flux from plants are mercury concentration in the soil, leaf area index, temperature, and irradiance [51]. See Leonard and others [50,51] for more details.

The combination of low root density and easily-fragmented perennial roots allows soil erosion to occur during flooding events or other high waterflow events along riverbanks infested with broadleaved pepperweed. The water will also carry root pieces (which float) downstream where they can establish new populations (personal communications with Susan Donaldson and Jim Young as cited by [69]).

Broadleaved pepperweed invasion also causes economic losses when it persists in hay meadows, pastures, and/or cropland. Where broadleaved pepperweed invades native hay meadows (e.g. in the Humboldt River Valley of Nevada and Lassen County, California), it reportedly lowers the quality of hay in terms of protein content and digestibility [103]. Rumors that broadleaved pepperweed may be poisonous are usually based on horses being fed hay containing broadleaved pepperweed under confined conditions, but no data are available to confirm its toxicity [104]. In infested pastures that are not mowed annually, the accumulation of broadleaved pepperweed stems inhibits grazing [4,104]. Fence rows and "waste areas" within fields may become dense, impermeable thickets of broadleaved pepperweed [102].

Control: Eradication of broadleaved pepperweed is no longer an option in western North America, and control and quarantine efforts for broadleaved pepperweed have been largely unsuccessful. Biological suppression may be a viable goal that is likely to require an integrated management approach, as no single technique is likely to control broadleaved pepperweed [103]. Broadleaved pepperweed has a deep, extensive root system with a high reproductive potential that allows it to sprout repeatedly following removal of aboveground growth. Perennial roots must be killed or removed to prevent reinfestation by broadleaved pepperweed. According to Renz [69], these roots may remain dormant in the soil for several years, resist desiccation, and have been found more than 9 feet (3 m) deep in the soil profile (personal communication from Jim Young as cited by [69]). Strategies to control broadleaved pepperweed must include removing aboveground growth and perennial roots, preventing seed production, monitoring for broadleaved pepperweed re-establishment for several years, locating and controlling potential sources of reinfestation (e.g. populations upstream, down the road, next door, etc.), and establishing desirable vegetation. Timing control efforts to coincide with vulnerable stages in broadleaved pepperweed phenology may increase the probability of success (see Seasonal development). It is also important to consider how different control techniques may affect broadleaved pepperweed phenology and distribution of energy stores in broadleaved pepperweed (see Physical/mechanical control and Chemical control) [27,69]. More research is needed in these areas, especially long-term research, as many studies report results for only 1 year after treatment.

If resources are available to control an entire infestation of broadleaved pepperweed, including large stands, efforts should be made to do so. If only part of an infestation can be treated, modeling and experience indicate that controlling outlying patches and the leading edge of infestations are most important [57]. For smaller, scattered populations, an early response strategy can lead to reduced long-term cost of control. If possible, early detection and eradication of small satellite populations is the least expensive and most effective way to control broadleaved pepperweed [69,74]. Management of broadleaved pepperweed will likely be more intensive and costly as infestations age. Without management, infestations are expected to increase in density, store energy in belowground tissues, and close the canopy structure. All of these factors increase the difficulty of managing broadleaved pepperweed [71]. Therefore, intense monitoring, early detection, and rapid removal of broadleaved pepperweed increase the probability of successful control.

Diligent monitoring in areas where broadleaved pepperweed is being managed is important since roots are difficult to kill. Areas should be monitored in early spring and late summer whenever possible. In many places broadleaved pepperweed is one of the first plants to emerge in the spring and can be identified early in the growing season. Later in the season, as other plants senesce, broadleaved pepperweed will be one of the last remaining plants alive and green. Rosettes can be difficult to detect, but they may form the leading edge of an infestation and so are important to detect and control [57]. The best time to detect new rosettes is late summer. Monitoring can also be done in fall/winter by looking for senesced stems [69]. Nearby populations should also be located and controlled in an effort to limit off-site propagule sources [69,83].

With all control methods, it is important to restore desirable vegetation [99]. When broadleaved pepperweed is controlled, it may be necessary to also remove its litter in order to stimulate germination and growth of desirable plants. Previously infested land can recover, but costs incurred will vary depending upon location, density, and length of time infested.  If soil salinities are dramatically increased by broadleaved pepperweed infestation, an intensive soil remediation program may be necessary before desirable native species can re-establish [69]. More research is needed to identify plants that can effectively compete with broadleaved pepperweed.

Prevention: The most efficient and effective method of managing invasive species is to prevent their invasion and spread [79]. Preventing the establishment of weeds in natural areas is achieved by avoiding management activities that encourage invasion, maintaining healthy natural communities, and conducting aggressive monitoring several times each year. Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along roadsides, parking lots, fencelines, and waterways. When a broadleaved pepperweed infestation is found, the location can be recorded and the surrounding area surveyed to determine the size and extent of the infestation, so these sites can be revisited on follow-up surveys [42]. New infestations should be controlled promptly to prevent further spread [7,104], followed by monitoring and some combination of control methods. Prevention of new invasions is much less costly than postinvasion control [52].

Sources of infestations must be controlled to prevent further spread. Equipment used in broadleaved pepperweed infested areas must be thoroughly cleaned before transport. Water sources, imported soil, and hay bales used for erosion control should be monitored to ensure they do not contain broadleaved pepperweed roots or seeds. Many infestations of broadleaved pepperweed have been initiated by one of these sources (CalEPPC 1999, as cited by [69]). Seed sources of revegetation species should be checked to ensure that there is no broadleaved pepperweed contamination.

Integrated management: A combination of complementary control methods may increase effectiveness of control efforts for broadleaved pepperweed. Integrated management includes not only killing the target plant, but establishing desirable species and discouraging nonnative, invasive species over the long term. Components of any integrated weed management program are sustained effort, constant monitoring and evaluation, and the adoption of improved strategies. An integrated management plan includes efforts to place continual stress on undesirable plants while promoting growth of desirable plants.

Integrated broadleaved pepperweed control strategies consisting of mowing, disking, or burning combined with herbicide applications before or after treatment, have been studied (see Physical/mechanical and Chemical control sections). Kilbride and others [46] examined the potential to restore native vegetation in infested meadows in the Malheur National Wildlife Refuge using integrated management techniques including herbicides, disking, fire, and combinations thereof. Study plots were predominantly broadleaved pepperweed interspersed with trace amounts of beardless wildrye, squirreltail, basin wildrye, saltgrass, cheatgrass, and forbs (e.g. flixweed tansymustard), as well as rushes and sedges in lower (wet) areas. Percent reduction of broadleaved pepperweed density 1 year after treatment was reported as follows [46]:

Site chlorsulfuron metsulfuron methyl disk chlorsulfuron-disk metsulfuron methyl-disk fire chlorsulfuron-fire metsulfuron methyl-fire
Big Sage 100 90 46 100 99 not reported 100 97
Oliver Springs 100 100 -2 100 99 lack of fuels lack of fuels lack of fuels
Skunk Farm 100 not reported 32 100 98 lack of fuels lack of fuels lack of fuels

Herbicide treatments alone or in combination with disking or fire resulted in 90% to 100% reduction in broadleaved pepperweed density 1 year after treatment, with chlorsulfuron providing slightly greater reductions than metsulfuron methyl. All herbicide treatments were more effective than disking alone. It is unclear what effectiveness fire alone had on broadleaved pepperweed density, as no data are given. For more information on the constraints and effects of fire treatments, see Fire Management Considerations. Disking in combination with herbicide treatments reduced cover of native forbs and grasses and resulted in the establishment of undesirable, nonnative species (cheatgrass and Canada thistle (Cirsium arvense)). Combining herbicide treatments with fire or disking did not increase effectiveness over herbicide treatment alone [46]. However, data from only 1 year may be insufficient to judge long-term effectiveness of control measures.

With all control methods, it is important to encourage growth of desirable vegetation. According to Young and others [99] when herbicidal weed control is used on near monocultures of broadleaved pepperweed in native hay meadows in the Intermountain Area, spontaneous regeneration of meadows is slow and reinvasion of broadleaved pepperweed likely. This makes seeding of desirable species necessary to maintain suppression of broadleaved pepperweed [99] (see Cultural control).

Discussion of other combinations of control methods is included in the following sections when these were encountered in the literature.

Physical/mechanical: Physical and mechanical control methods such as mowing and disking alone are unlikely to control broadleaved pepperweed because new plants quickly regenerate from both undisturbed and fragmented roots in the soil (see Regeneration Processes). Small infestations of broadleaved pepperweed can be controlled by repeated removal of above- and belowground plant material. Care must be taken to remove as much of the root as possible as small pieces can sprout. If repeated several times this process can be successful, but it is labor intensive [69]. For larger infestations, combining mowing or disking with other control strategies may improve success (e.g. [71]). Neither mowing nor disking is usually appropriate in natural areas, as they are likely to damage desirable plants, expose soil, and increase erosion potential.

While it is generally accepted that mowing will not control broadleaved pepperweed (e.g. [41,69,83]), Baker [4] notes that haying (i.e. repeated mowing) broadleaved pepperweed infested fields in Wyoming, seems to prevent it from developing into a monoculture. There are no examples in the literature where repeated mowing was tested as a control method for broadleaved pepperweed.

Timing manual defoliation or other disturbances of above ground tissues during periods when minimum pools of stored energy are present can deplete stores of energy for future growth and thus enhance long-term control. Research has shown that minimum amounts of stored energy are in belowground tissues of broadleaved pepperweed at the bolting stage [70] (see Seasonal Development), indicating this as the optimal time to mow stems. Unfortunately, broadleaved pepperweed quickly recovers from mowing and produces leaves from previously dormant buds near the soil surface [71]. Sprouting may require less than 14 days (unpublished data as cited by [68]), and total nonstructural carbohydrate (TNC) pools in the top 16 inches (40 cm) of roots in mowed plants were not different than unmowed plants 7 and 19 days after mowing at 2 study sites, respectively. The authors speculate that TNC from roots deeper than 16 inches (40 cm) may have been mobilized, or that reserves were replaced through photosynthesis by new leaves [70,71].

Mowing changes the architecture of a broadleaved pepperweed stand. Stem density is reduced (64 stems/m2 in mowed plots compared to 142 stems/m2 in plots not mowed), as well as stem height (19.4 inches (49.21 cm) in mowed plots compared to 38.0 inches (96.42 cm) in unmowed areas), and leaf area distribution is altered within the stand (see General Botanical Characteristics) [69]. Unmowed plants have the majority of leaf area in the top 3rd of the canopy, whereas in mowed areas, 84-86% of broadleaved pepperweed leaf area was found within the lower 3rd of the canopy. Sprouting stems also had 21-59% less total leaf area than plants not mowed at the flowerbud stage. According to Renz and DiTomaso [71], this change may increase effectiveness of herbicide sprays used after mowing by depositing more herbicide on basal leaves where it can preferentially be translocated to roots. Also, broadleaved pepperweed plants sprouting after mowing are more uniformly synchronized in growth stage, so herbicide application at a time of maximal below ground translocation is consistent throughout the stand [71]. According to Renz [69], a potential drawback of this approach is that broadleaved pepperweed sprouting is limited in dry sites and/or low precipitation years.

Renz and DiTomaso [71] tested the effects of mowing and herbicide treatments, alone and in combination, in 3 contrasting sites (high desert, roadside, and floodplain) in California. Dense, monospecific stands with >85% broadleaved pepperweed cover were mowed to a height of 1 to 2 inches (2-5 cm) when flowerbuds were present on the main shoot and shoots from axillary buds. Shoots quickly sprouted after mowing, resulting in a dense stand of rosette plants. The majority of these remained as rosettes throughout the season. Herbicide treatments (glyphosate, 2,4-D, and chlorsulfuron) were applied to mowed plants when bolting shoots reached the flowerbud stage. Broadleaved pepperweed biomass and density were measured 1 year after treatments. Mowing alone did not significantly (p<0.1) reduce broadleaved pepperweed biomass or density 1 year after treatment. Chlorsulfuron was equally effective (97-100% biomass reduction) with or without mowing on the floodplain site, and mowing improved effectiveness on the high desert and roadside sites. Glyphosate was equally effective with or without mowing (83.5-87.4% biomass reduction) on the high desert site, while mowing enhanced effectiveness on the roadside and floodplain sites. Effectiveness of 2,4-D was not significantly (p<0.10) enhanced by mowing, and was the least effective of the herbicides tested, with or without mowing.

A similar experiment compared the effects of combined mowing and herbicide treatments on dense and sparse broadleaved pepperweed infestations [71]. Mowing enhanced the effectiveness of herbicides in reducing broadleaved pepperweed biomass 1 year after treatments in the dense infestation, but not in the sparse infestation. Following control measures, response of resident plants was limited in the dense infestation, but extensive in the site with the sparse infestation. Nonnative annual grass cover increased at both sites. The authors conclude that dense broadleaved pepperweed populations may require an integrated approach, while less dense or establishing populations may be controllable with chemicals alone. Additionally, recovery of resident plant populations increases when management programs are initiated before broadleaved pepperweed infestations become dense, monospecific stands [71]. Renz [69] presents data on various herbicides used in this manner.

Disking alone is also not thought to be an effective control method for broadleaved pepperweed because new plants sprout from root fragments [95]. However, incorporating tillage with other management approaches may improve control [69]. For example, tillage after herbicide application may be an effective way of bringing the treated roots to the soil surface where they will desiccate [97].

Disking broadleaved pepperweed may increase the density of an infestation (Renz and DiTomaso unpublished data, as cited by [69]). Periodic disking during the growing season over a 2-year period resulted in no permanent reduction in broadleaved pepperweed cover in native hay meadows in Nevada [102]. Disking broadleaved pepperweed at Grizzly Island Wildlife Area resulted in a serious increase in its distribution (Feliz, personal communication as cited by [41]).

Control following spring herbicide applications at the flowerbud stage was slightly improved in disked areas relative to areas not disked. When previously disked areas are mowed the following spring at the flowerbud stage and herbicides are applied to sprouting plants in the flowerbud stage, greatly enhanced control by herbicides is observed. The incorporation of disking into a control strategy had been shown to enhance plant diversity the following year (Renz and DiTomaso, unpublished data), perhaps by stimulating seeds in the seed bank [69]. Renz [69] presents data on various herbicides used in this manner.

Inundation: Case studies described by Howald [41] and Renz [69] suggest that broadleaved pepperweed may be intolerant of prolonged inundation.

Research presented by Chen and others [17,18] suggests that broadleaved pepperweed may tolerate and survive saturated conditions, but does not grow well under these conditions. This may be an adaptation to arid or semiarid riparian habitats where spring flooding and summer drought are characteristic. After 7 days of flooding, total biomass (p<0.001) and root/shoot ratio (p=0.002) of flooded plants were significantly less than those of unflooded controls (maintained at -20 kPa soil matric water potential) [18]. Further study of anaerobic metabolism in roots of broadleaved pepperweed seedlings indicates that broadleaved pepperweed roots have metabolically adaptive strategies to anoxia, but there is evidence of oxidative stress under anoxia and of postanoxic injury from free radicals upon re-exposure to air. Results suggest that broadleaved pepperweed exhibits a mixture of characteristics typical of hydrophytic, facultative, and anoxia intolerant species [17].

Fire: See the Fire Management Considerations section of this summary.

Biological: Development of a biological control program for broadleaved pepperweed seems unlikely because of risks to many important crop plants that are members of the mustard family [7]. Additionally, several native pepperweed species from the western U.S. are either listed as endangered or are considered for listing [7,41]. Based on molecular phylogeny, broadleaved pepperweed is more closely related to Californian species than are other members of the genus [59]. Acknowledging these difficulties, Birdsall and others [7] point out the limitations of other control methods for broadleaved pepperweed, and suggest that both classical and augmentative biological control approaches warrant further examination, especially the potential for use in conjunction with other available techniques.

According to Young and others [97,99] broadleaved pepperweed can be suppressed by grazing, and there are examples in both Colorado and Nevada where grazing management suppressed broadleaved pepperweed. They provide no data or specific examples, nor do they mention which grazing animals are effective. According to Wood [94], in a preliminary test, 13 goats ate broadleaved pepperweed with no ill effects, and domestic cattle and sheep graze broadleaved pepperweed growing amid other plants, but they do not eat pure, dense stands of broadleaved pepperweed. According to Baker [4], domestic sheep readily eat broadleaved pepperweed in Wyoming, and even heavily infested pastures appear weed free. Once the sheep are removed, however, broadleaved pepperweed comes back (see Palatability/nutritional value). Grazing management is most effective in long term suppression of broadleaved pepperweed when initiated before all perennial grasses are lost from the community [97]. More research is needed on the use of livestock for broadleaved pepperweed suppression.

Chemical: Before using herbicides for control of invasive plants, managers must consider the effectiveness of the herbicide on the target plant, appropriate timing and rates of application, the potential impacts on nontarget organisms, and residual activity and toxicity of the herbicide. 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 [15]. Use of herbicides may be restricted in some areas. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

According to Howald [41], attempts have been made to control broadleaved pepperweed with chemical herbicides in California, Oregon, Wyoming, Idaho, and Utah. The shoot portion of broadleaved pepperweed is susceptible to several herbicides. However, even with 98% initial control, sprouting broadleaved pepperweed plants may result in total stand dominance by the end of the next growing season [103]. The most effective herbicides appear to be chlorsulfuron, metsulfuron methyl, and imazapyr [4,20,67,71,102]. Glyphosate is effective when applied after mowing to sprouting stems at the flowerbud stage [71]. At Malheur National Wildlife Refuge in Oregon, chlorsulfuron and metsulfuron methyl were tested alone and in combination with either fire or disking, with chlorsulfuron reducing broadleaved pepperweed densities by 100% in all 3 sites tested, and metsulfuron methyl resulting in density reductions of 90 to 100% [46] (also see Integrated management and Fire management considerations).

Chlorsulfuron delivers the most consistent long-term control of broadleaved pepperweed. Metsulfuron methyl appears to work well but is less studied. Imazapyr controls broadleaved pepperweed but is a fairly nonselective herbicide and is therefore more likely to damage desirable plants. Chlorsulfuron is not registered for use in many areas where broadleaved pepperweed occurs, particularly areas adjacent to water [69]. Other problems with chlorsulfuron include its adverse effects on valuable woody species [103], and difficulty in establishing perennial grass seedlings following control of broadleaved pepperweed [99] (see Cultural control).

Based on broadleaved pepperweed's seasonal carbon allocation pattern (see Seasonal Development), one might expect the optimal stage for herbicide application to be full-flowering to fruiting stages. However, control seems to be maximized when herbicide is applied at the flowerbud stage [102]. Application in late summer, after the haying operation has removed much of the top growth, may also be effective. "Excellent control" was also obtained with early spring or late fall applications in native hay meadows in Nevada [101]. However, fall applications of 9 herbicide treatments had minimal effects on broadleaved pepperweed in Utah [67].

Longer term studies are needed to better evaluate control potential of herbicides for broadleaved pepperweed [67]. The potential for broadleaved pepperweed to develop a resistance to particular herbicides/families also needs to be investigated [69]. For a more detailed synopsis of chemical control and more detail on particular herbicides, rates, timing, and other considerations see Renz [69].

Cultural: Any lasting biological suppression of broadleaved pepperweed requires establishment and persistence of desirable plants that are capable of competing successfully with broadleaved pepperweed in managed ecosystems. Competitive ecotypes of native species are suggested. An example might be the use of saltgrass in halomorphic wetland areas [103]. Broadleaved pepperweed is, however, highly competitive, as evidenced by its ability to establish and spread in vigorous, well-managed alfalfa or tall wheatgrass stands, and its reputed ability to displace quackgrass [97].

In order to give perennial grass plants a chance to biologically suppress broadleaved pepperweed, repeated applications of selective herbicides may be necessary to help grasses establish. The choice of perennial species for revegetation of seasonally dry meadows with salt affected soils in the Intermountain Area is limited, and tall wheatgrass is the most widely used species [99]. Young and others [99] compared seedling establishment of tall wheatgrass on sites where broadleaved pepperweed was controlled with 2,4-D or chlorsulfuron. Broadleaved pepperweed was controlled with applications of 2,4-D, and tall wheatgrass seedlings established on some of these 2,4-D-treated plots. Plots treated with chlorsulfuron remained virtually free of broadleaved pepperweed, but no seedling establishment of tall wheatgrass occurred. Even when these plots were seeded for 4 consecutive years after herbicide application, tall wheatgrass seedlings never established. The plots remained weed free except for occasional broadleaved pepperweed plants until the 4th year, when Russian-thistle (Salsola kali), lambsquarters (Chenopodium album) and summer-cypress (Kochia scoparia) plants established in the treated area. The authors speculate that the apparent persistence of chlorsulfuron residues may be heightened by the high pH of the salt affected soils.

Where broadleaved pepperweed was initially controlled with applications of 2,4-D a few seedlings of tall wheatgrass were initially present, but no tall wheatgrass plants were present the 2nd year after seeding. Where broadleaved pepperweed was initially controlled with 2,4-D and followed with a lower rate of 2,4-D over tall wheatgrass seedlings the spring after the seeds were planted, good stands of tall wheatgrass established.

Results from competition tests performed with broadleaved pepperweed and cheatgrass presented by Blank and others [9] (see Successional Status) suggest that a similarly aggressive, densely rooted native grass may successfully compete with broadleaved pepperweed. Information about which species can best compete with broadleaved pepperweed and/or prevent new invasions is needed [69].

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