SPECIES: Asclepias speciosa

Table of Contents


INTRODUCTORY


 

  ©Dave Powell, U.S.D.A. Forest Service forestryimages.org Yosemite Valley. Charles Weber ©California Academy of Sciences

AUTHORSHIP AND CITATION:
Ulev, Elena D. 2005. Asclepias speciosa. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
ASCSPE

SYNONYMS:
Asclepias giffordii Eastw. [21,28]
A. douglasii W. Hooker [17,30,36,63]

NRCS PLANT CODE [14]:
ASSP

COMMON NAMES:
showy milkweed
Greek milkweed

TAXONOMY:
The scientific name of showy milkweed is Asclepias speciosa Torr.(Asclepidaceae) [6,12,30,37,38,39,44,60,61,63].

Natural hybridization between Asclepias spp. is rare due to mechanical and physiological barriers. Differing anther wings may be one of the mechanical barriers to interspecific pollination [17,63]; however, common milkweed (A. syriaca) × showy milkweed hybrids have been found in Michigan, Iowa, Minnesota, and the Dakotas [22,27,59,63,65].

LIFE FORM:
Forb

FEDERAL LEGAL STATUS: None

OTHER STATUS:
Showy milkweed is listed as threatened in Iowa [57].

DISTRIBUTION AND OCCURRENCE

SPECIES: Asclepias speciosa
GENERAL DISTRIBUTION:
Showy milkweed is native to and widely distributed in western North America. It occurs from southern Manitoba west to British Columbia and south to Minnesota, to northwestern Texas, and California [17,44,61,63]. Plants database provides a distributional map of showy milkweed.

Precise distribution information is unavailable for all locations where showy milkweed may occur.  The following lists are therefore speculative and not exhaustive, and showy milkweed may be present in other vegetation types.

ECOSYSTEMS [20]:
FRES17 Elm-ash-cottonwood
FRES21 Ponderosa pine
FRES28 Western hardwoods
FRES34 Chaparral-mountain shrub
FRES37 Mountain meadows
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands
FRES42 Annual grasslands

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES

AZ AR CA CO ID IL IA KS MI MN
MT NE NV NM ND OK OR SD TX UT
WA WI WY

CANADA
AB BC MB SK

BLM PHYSIOGRAPHIC REGIONS [7]:
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands

KUCHLER [41] PLANT ASSOCIATIONS:
K010 Ponderosa shrub forest
K011 Western ponderosa forest
K030 California oakwoods
K035 Coastal sagebrush
K065 Grama-buffalo grass
K070 Sandsage-bluestem prairie

SAF COVER TYPES [19]:
63 Cottonwood
222 Black cottonwood-willow
244 Pacific ponderosa pine-Douglas-fir
245 Pacific ponderosa pine
246 California black oak
250 Blue oak-foothills pine
255 California coast live oak

SRM (RANGELAND) COVER TYPES [52]:
201 Blue oak woodland
202 Coast live oak woodland
203 Riparian woodland
204 North coastal shrub
205 Coastal sage shrub
214 Coastal prairie
215 Valley grassland
310 Needle-and-thread-blue grama
605 Sandsage prairie
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
611 Blue grama-buffalo grass
614 Crested wheatgrass
704 Blue grama-western wheatgrass

HABITAT TYPES AND PLANT COMMUNITIES:
In the Willamette Valley of Oregon, showy milkweed grows primarily in open grasslands. It sometimes occurs in riparian zones and savannas with small trees [27].

In plains grasslands of western Nebraska, showy milkweed is found in grama-buffalo grass (Bouteloua spp.-Buchloe dactyloides), grama-needle-and-thread grass-western wheatgrass (Hesperostipa comata-Pascopyrum smithii), and sandsage-bluestem (Artemisia filifolia-Andropogon/Schizachyrium spp.) grasslands. Green needlegrass (Stipa viridula) and threadleaf sedge (Carex filifolia) are common codominants within these grasslands [31].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Asclepias speciosa

 

  Charles Weber ©California Academy of Sciences
GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available [6,12,17,22,28,30,34,38,39,44,60,61,63].

Showy milkweed is a warm-season perennial forb [17,22,29,38,61,63]. Stems are ascending to erect and 19.7 to 47.2 inches (50.0-119.9 cm) tall [6,17,22,24,28,38,44,61,63]. Leaves are 2.4 to 7.9 inches (8-20 cm) long and ovate-lanceolate [6,22,24,44,61]. The inflorescence is a large, showy umbelliform cymes [6,22,63]. At 0.59 to 1.10 inch (15-28 mm) wide, they are the largest of all Asclepias species [38,61]. The fruit is a 2.4- to 4.7- inch (6-12 cm) follicle [6,17,22,24,44]. Seeds are elliptic and 0.24 to 0.35 inch (6-9 mm) long [6,17,22,63], with hair-like tufts (see photo above). Showy milkweed has "deep" rhizomes [10,22].

RAUNKIAER [49] LIFE FORM:
Geophyte
Hemicryptophyte

REGENERATION PROCESSES:
Showy milkweed reproduces both sexually and vegetatively.

Breeding system: Showy milkweed is monoecious. Most Asclepias spp. are self-incompatible, requiring crosses between genetically different individuals to produce viable seeds [11,63,65].

Pollination: Showy milkweed is pollinated by wind and insects. Insect pollination is accomplished by lepidopterans and hymenopterans. These insects remove the pollen packet, or pollinarium, when a groove in the plant's corpusculum entraps the hairs or appendages of the insect. The arms of the pollinarium twist, facilitating insertion of the pollinarium into the stigmatic chamber of another flower as the pollinating insect continues foraging [9,27,63,65]. The flowers of Asclepias spp. produce large amounts of nectar, which serves as a germination medium for the pollen [65].

Seed production: Stevens [54] determined that showy milkweed produces an average of 630 seeds per stem with a weight of 0.208 ounce (5.890 g) per 1,000 seeds.

Seed dispersal:  Seeds are dispersed when wind catches the hair-like seed tufts. [10]. Showy milkweed's persistence along waterways [6,12,17,28,30,34,38,42,44,53,60,61] also suggests dispersal by water [14].

Seed banking: Seed may be stored in water or in soil, as shown in the following studies. In a study by Bruns [14] in the Chandler Power Canal in Prosser, Washington, showy milkweed seeds survived in fresh water for several months. Three sets of seeds were submerged in fresh water up to 12 months to determine viability.  Extended viability of seeds indicates the potential of seeds trapped or carried in irrigation water as sources of potential sources of colonization. The following table represents the percentage of seeds that were decomposed after 3 to 12 months of submergence in fresh water, and germination of the submerged and dry-stored seeds after the same periods [14]:

Months after test initiated 0 3 6 9 12
Percent of submerged seeds that decomposed ---- 1 2 48 48
Percent germination of submerged seeds ---- 96 90 5 45
Percent germination of dry-stored seeds 4 14 14 26 12

Chepil's [15] study suggests that showy milkweed has a short-lived soil seedbank. In a study performed in Saskatchewan, showy milkweed seeds were tested for longevity, periodicity of germination, and viability of seeds in cultivated soil. One thousand seeds were mixed in a 2.5-inch (6.4 cm) layer of sterilized soil in the field. From 15 November to 31 March, the soil was turned with a trowel to a 3-inch (7.6 cm) depth 2-3 times, then sown to spring wheat (Triticum aestivum) or cereal barley (Hordeum vulgare) in alternate years. Ninety-five percent or more of the showy milkweed seeds germinated during the 1st year, with the greatest germination from 7 May to 31 May, though substantial amounts of seeds germinated until 31 July. Overall, showy milkweed seed dormancy did not exceed 2 years [15].

Germination: Seeds can germinate in soil or when submerged in water, although submergence may lower the germination rate if the seeds remain in water over 36 months. The longevity of showy milkweed seeds in fresh water was tested by Comes and others [16] in the Chandler Power Canal in Prosser, Washington. One hundred showy milkweed seeds were collected from the field; dried; placed separately into nylon cloth envelopes; placed in plastic screen bags; and submerged 11.8 inches (30 cm) beneath the water surface. Comparative germination tests were made on seeds that had been stored dry in glass bottles at room temperature [16]:

Months after test initiated

 

3 6 9 12 24 36 48 60

%

Germination of submerged seeds 13 39 38 5 7 1 0 0
Submerged seeds remaining firm 55 47 32 14 7 3 0 0
Germination of dry-stored seeds 72 70 68 69 58 65 45 71

Germination in water was at a maximum level of 55% between 3 and 12 months, and no germination occurred after 48 months. Germination of dry-stored seeds at the 3- to 6-month period ranged from 45% to 72% [16].

Seedling establishment/growth: During the seedling stage, showy milkweed directs most of its energy into root development. This contributes to drought tolerance, but the aboveground portions of showy milkweed grow "very slowly" [2].

Asexual regeneration: Showy milkweed spreads clonally from deep rhizomes [10,22], and probably from from sprouting from the root crown.

SITE CHARACTERISTICS:
Showy milkweed prefers wet areas of prairie habitats and moist, sandy soils along waterways, ditches, streambanks, floodplains, and washbottoms.  It is common along the banks of irrigation ditches [1]. It can become weedy in cultivated fields, roadsides, railways, "wastelands," and fence rows [6,12,17,28,30,34,38,42,44,53,60,61].

Elevation: Showy milkweed occurs at the following elevations:

Arizona 6,000-9,000 feet (1,800-2,700 m) [39]
California  0-600 feet (0-1,900 m) [28]
Colorado 3,500-7,500 feet (1,100-2,300 m) [24]
Nevada 2,200-8,500 feet (700-2,600 m) [38]
New Mexico 6,000-9,000 feet (1,800-2,700 m) [44]
Utah  2,700-8,500 feet (800-2,600 m) [61]

Soil: Showy milkweed prefers moist soil [6,12,17,28,30,34,38,42,44,53,60,61]. It grows in all soil textures [53] and tolerates alkaline soils [17].

SUCCESSIONAL STATUS:
Little research has been done addressing the successional status of showy milkweed; however, it is considered a "weedy" species [1,17] that occurs in early and mid-succession. It has the ability to quickly colonize disturbed sites (Riser, personal communication [50]) in open grasslands and partially shaded habitats.

SEASONAL DEVELOPMENT:
Across showy milkweed's range, flowering occurs from May to August [10,17,22,39,44,45,63]. Follicles dry during August and September, and seeds disperse in September and October [10].

FIRE ECOLOGY

SPECIES: Asclepias speciosa
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: There is no information in the literature regarding fire adaptations of showy milkweed; however, showy milkweed likely regenerates from both seed and by sprouting from eeply buried rhizomes and the root crown after fire. Prolific production of hairy, light seeds allows for wind dispersal onto burns, with plants on adjacent unburned sites serving as seed sources after fire. Showy milkweed seed can be stored in the soil for up to 2 years [15]. Seeds also have the ability to float and survive up to 36 months in water [16], allowing another mechanism of travel after a burn. Further research is needed on fire adaptations of showy milkweed.

Fire regimes: Historically, fire has been an important natural component of the savanna and grassland communities where showy milkweed occurs [32]. Ponderosa pine and oak savannas typically experienced fire at less than 10-year intervals [4,23]. Across the Great Plains, fires may have occurred as frequently as every 1 to 10 years prior to European settlement [40,47]. Frequent fires in plains grasslands affects species composition and vegetation dynamics [47], and may favor sprouting species such as showy milkweed.

Riparian zones within mountain and plains grasslands historically had longer fire-return intervals than surrounding vegetation. Typically burning during drought, riparian zones usually have heavier fuel loads and burn with higher severity than surrounding plant communities [4,5].

The following table provides fire return intervals for plant communities and ecosystems where showy milkweed is important. For further information, see the FEIS review of the dominant species listed below. This list may not be inclusive for all plant communities in which showy milkweed occurs. If you are interested in plant communities or ecosystems that are not listed below, see the complete FEIS fire regime table.

Community or ecosystem Dominant species Fire return interval range (years)
bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium <10 [40,47]
coastal sagebrush Artemisia californica <35 to <100 [47]
plains grasslands Bouteloua spp. <35 [47,64]
blue grama-needle-and-thread grass-western wheatgrass Bouteloua gracilis-Hesperostipa comata-Pascopyrum smithii <35 [47,51,64]
California steppe Festuca-Danthonia spp. <35 [47,56]
wheatgrass plains grasslands Pascopyrum smithii <5-47+ [47,48,64]
Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47
California oakwoods Quercus spp. <35 [3]
coast live oak Quercus agrifolia 2-75 [23]
blue oak-foothills pine Quercus douglasii-P. sabiniana <35 [3]
California black oak Quercus kelloggii 5-30 [47]

POSTFIRE REGENERATION STRATEGY [55]:
Rhizomatous herb, rhizome in soil
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: Asclepias speciosa
IMMEDIATE FIRE EFFECT ON PLANT:
The immediate effect of fire on showy milkweed is not documented; however, fire likely top-kills showy milkweed. Soil-stored seed and rhizomes are probably insulated from fire damage.

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

PLANT RESPONSE TO FIRE:
Limited information suggests that showy milkweed establishes from seed after fire [13]. As in many prairie plants, dormant-season fire stimulates postfire flowering [8], so it is likely that showy milkweed established from on-site seed as well as seed blown in from off-site seed sources.

Showy milkweed probably also sprouts from rhizomes and the root crown after fire, although such documentation is lacking. Research is needed on the fire ecology of showy milkweed.

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Research conducted on milkweeds of similar growth form may also apply to showy milkweed. Mead's milkweed (Asclepias meadii) is a closely related, rhizomatous forb that grows in the prairie states. In a study by Bowles and others [13], prescribed burning during the dormant season (before May) had either positive or neutral effects on seedlings on Preserves in Missouri, Illinois, Iowa, and Kansas. Mead's milkweed plants on burn plots produced more cymes, as well as more flowers per cyme, compared to plants on unburned control plots [13].

FIRE MANAGEMENT CONSIDERATIONS:
With light, wind-blown seed, deep rhizomes, and early successional status, showy milkweed has adaptations that usually promote fire survivorship and establishment in early postfire communities [55]. However, since data on postfire regeneration of showy milkweeds are lacking, caution is recommended when using prescribed burning if promoting showy milkweed is a fire management objective. Small-scale burning and postfire monitoring can help managers determine how showy milkweed responds to prescribed burning in a given management area.

MANAGEMENT CONSIDERATIONS

SPECIES: Asclepias speciosa
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Showy milkweed is toxic to most grazing animals. It possesses moderate concentrations of cardenolides (cardiac glycosides) [42], and can poison or even kill grazing animals [22,34].

Showy milkweed is an important host plant for monarch butterflies. Monarch butterflies are specialist herbivores of plants in the milkweed family (Asclepiadaceae), and sequester cardenolides to repel predators [28].

Palatability/nutritional value: Showy milkweed is distasteful to livestock [18,22,34].

Showy milkweed contains 16.3% crude protein, which is similar to alfalfa hay (16%), and contains a higher concentration of essential amino acids than corn (Zea mays) [1]. The following table is a comparison of amino acid composition of showy milkweed residue, alfalfa (Medicago sativa), and corn [2].

Amino Acid Showy milkweed residue
(mg/g)
Alfalfa
(mg/g)
Corn grain
(mg/g)
Alanine 8.9 9.9 7.9
Arginine 8.9 7.0 4.0
Aspartic acid 15.8 17.0 2.0
Cystine 0.9 3.0 1.0
Glutamic acid 15.3 12.6 27.0
Glycine 8.7 8.0 5.0
Histidine 3.7 3.0 2.0
Isoleucine 8.3 8.0 5.0
Leucine 14.4 10.0 12.0
Lysine 8.4 6.0 3.0
Methionine 1.9 1.0 2.0
Phenylaline 8.3 6.0 5.0
Proline 7.3 8.2 8.0
Serine 7.2 7.8 1.0
Threonine 6.8 7.0 3.0
Tryptophan 2.3 1.0 1.0
Tyrosine 4.2 5.0 5.0
Valine 9.6 7.0 5.0

Cover value: Showy milkweed is rated as providing poor cover for small mammals, upland game birds, waterfowl, and small nongame birds [18].

VALUE FOR REHABILITATION OF DISTURBED SITES:
No information is available on this topic.

OTHER USES:
Showy milkweed is being investigated as a potential multipurpose crop for chemical feedstocks. Plants in the genus Asclepias biosynthesize toxic cardenolides, which have toxic effects on the heart, lungs, kidneys, gastrointestinal tract, and brain of animals.  However, after exhaustive extraction with hexane and methanol, showy milkweed residue seems to be equivalent to alfalfa (Medicago sativa) hay in digestibility by domestic sheep [1].

Processed showy milkweed residue is a possible source of a variety of other products. After extraction, showy milkweed produces the following products [1]:

Product % Yield
Hexane extract 3.8
     Pigments 0.4
     Natural rubber 0.1
     Tri-terpenoids, esters and related compounds 3.3
Methanol extract 17.5
     Sucrose 6.0
     Inositol 0.9
     Polyphenolics 1.1
Residue 79.0
     Pectin   4.0
     Fibers 5.0
     Livestock feed 70.0

Honeybees obtain nectar from milkweed flowers, and showy milkweed could be used in honeybee pastures. The defatted seed meal can be used as an effective pesticide for army worms [1,25,35]. More research concerning weed control needs to be done for the economical production of showy milkweed, as well as research on agronomy, harvesting, and storage of showy milkweed crops[1].

Showy milkweed has been used for medicines for centuries by Native American tribes such as the northern Cheyenne of eastern Montana. The latex was used as an antiseptic for cuts, as well for treating ringworm, corns, and calluses. Tea made from the roots was used for treating measles, coughs, diarrhea, and rheumatism. Native Americans also used showy milkweed for food and utility items. The bitter sap was removed from showy milkweed by boiling, and the stems, leaves, flowers and young fruits could then be eaten. Young stalks were eaten like asparagus. Upon maturity, showy milkweed stems were used for rope, cord and bowstrings [17,26,34].

Showy milkweed is used in modern medicine to control heart contractions [28].

OTHER MANAGEMENT CONSIDERATIONS:
Control: Showy milkweed interferes with crops, and is an agricultural pest [1]. In a study by Whitson and Schwope [62], 8 herbicides were tested on showy milkweed on an irrigated grass pasture near Lovell, Wyoming. The effects of differing rates of the herbicides are provided in [62].

REFERENCES

SPECIES: Asclepias speciosa
1. Adams, Robert P. 1983. Chemicals from arid/semiarid land plants: whole plant use of milkweeds. In: Plants: the potentials for extracting protein, medicines, and other useful chemicals--workshop proceedings. OTA-BP-F-23. Washington, DC: U.S. Congress, U.S. Government Printing Office, Office of Technology Assessment: 126-137. [Milkweed: a potential new crop for the western United States. III.--Summary and discussion of each workshop paper: 24-28.]. [55144]

2. Adams, Robert P.; Balandrin, Manuel F.; Martineau, Jess R. 1984. The showy milkweed, Asclepias speciosa: a potential new semi-arid land crop for energy and chemicals. Biomass. 4: 81-104. [55052]

3. Arno, Stephen F. 2000. Fire in western forest ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 97-120. [36984]

4. Arno, Stephen F. 2000. The 100-year crusade against fire: its effect on western forest landscapes. In: Pioneering new trails: Proceedings of the Society of American Foresters 1999 national convention; 1999 September 11-15; Portland, OR. SAF Publication 00-1. Bethesda, MD: Society of American Foresters: 312-315. [37563]

5. Arno, Stephen F.; Harrington, Michael G. 1998. The interior West: managing fire-dependent forests by simulating natural disturbance regimes. In: Forest management into the next century: what will make it work?; 1997 November 19-21; Spokane, WA. Madison, WI: Forest Products Society: 53-62. [43185]

6. Bare, Janet E. 1979. Wildflowers and weeds of Kansas. Lawrence, KS: The Regents Press of Kansas. 509 p. [3801]

7. Bernard, Stephen R.; Brown, Kenneth F. 1977. Distribution of mammals, reptiles, and amphibians by BLM physiographic regions and A.W. Kuchler's associations for the eleven western states. Tech. Note 301. Denver, CO: U.S. Department of the Interior, Bureau of Land Management. 169 p. [434]

8. Betz, Robert F. 1989. Ecology of Mead's milkweed (Asclepias meadii Torrey). In: Bragg, Thomas A.; Stubbendieck, James, eds. Prairie pioneers: ecology, history and culture: Proceedings, 11th North American prairie conference; 1988 August 7-11; Lincoln, NE. Lincoln, NE: University of Nebraska: 187-191. [14042]

9. Bookman, Susan Stone. 1981. The floral morphology of Asclepias speciosa (Aslepiadaceae) in relation to pollination and a clarification in terminology for the genus. American Journal of Botany. 68(5): 675-679. [54527]

10. Bookman, Susan Stone. 1983. Costs and benefits of flower abscission and fruit abortion in Asclepias speciosa. Ecology. 64(2): 264-273. [54528]

11. Bookman, Susan Stone. 1984. Evidence for selective fruit production in Asclepias. Evolution. 38(1): 72-86. [55056]

12. Booth, W. E.; Wright, J. C. 1962. [Revised]. Flora of Montana: Part II--Dicotyledons. Bozeman, MT: Montana State College, Department of Botany and Bacteriology. 280 p. [47286]

13. Bowles, M. L.; McBride, J. L.; Betz, R. F. 1998. Management and restoration ecology of the federal threatened Mead's milkweed, Asclepias meadii (Asclepiadaceae). Annals of the Missouri Botanical Garden. 85: 110-125. [55808]

14. Bruns, V. F. 1965. The effects of fresh water storage on the germination of certain weed seeds. Weeds. 13: 38-39. [48293]

15. Chepil, W. S. 1946. Germination of seeds. I. Longevity, periodicity of germination, and vitality of seeds in cultivated soil. Scientific Agriculture. 26: 307-346. [48281]

16. Comes, R. D.; Bruns, V. F.; Kelley, A. D. 1978. Longevity of certain weed and crop seeds in fresh water. Weed Science. 26(4): 336-344. [50697]

17. Cronquist, Arthur; Holmgren, Arthur H.; Holmgren, Noel H.; [and others]. 1984. Intermountain flora: Vascular plants of the Intermountain West, U.S.A. Vol. 4. Subclass Asteridae, (except Asteraceae). New York: The New York Botanical Garden. 573 p. [718]

18. Dittberner, Phillip L.; Olson, Michael R. 1983. The plant information network (PIN) data base: Colorado, Montana, North Dakota, Utah, and Wyoming. FWS/OBS-83/86. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service. 786 p. [806]

19. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. [905]

20. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; Lewis, Mont E.; Smith, Dixie R. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. [998]

21. Gilmartin, Amy Jean. 1980. Numerical phenetic determination of the taxonomic status of Asclepias giffordii. Bulletin of the Torrey Botanical Club. 107(4): 496-505. [55058]

22. Great Plains Flora Association. 1986. Flora of the Great Plains. Lawrence, KS: University Press of Kansas. 1392 p. [1603]

23. Greenlee, Jason M.; Langenheim, Jean H. 1990. Historic fire regimes and their relation to vegetation patterns in the Monterey Bay area of California. The American Midland Naturalist. 124(2): 239-253. [15144]

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25. Harry-O'kuru, R. E.; Mojtahedi, H.; Vaughn, S. F.; Dowd, P. F.; Santo, G. S.; Holser, R. A.; Abbott, T. P. 1999. Milkweed seedmeal: a control for Meloidogyne chitwoodi on potatoes. Industrial Crops and Products. 9: 145-150. [55051]

26. Hart, Jeffrey A. 1981. The ethnobotany of the Northern Cheyenne Indians of Montana. Journal of Ethnopharmacology. 4: 1-55. [35893]

27. Hatfield, Emerin; Kephar, Susan R. 2003. Reproductive isolation and hybridization between two milkweeds (Asclepias fascicularis and A. speciosa, Asclepiadaceae). Madrono. 50(3): 170-180. [55053]

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29. Higgins, Kenneth F.; Barker, William T. 1982. Changes in vegetation structure in seeded nesting cover in the prairie pothole region. Special Scientific Report--Wildlife No. 242. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service. 27 p. [37248]

30. Hitchcock, C. Leo; Cronquist, Arthur. 1973. Flora of the Pacific Northwest. Seattle, WA: University of Washington Press. 730 p. [1168]

31. Huddle, Julie A.; Tichota, Gina R.; Stubbendieck, James; Stumpf, Julie A. 2001. Evaluation of grassland restoration at Scotts Bluff National Monument. In: Bernstein, Neil P.; Ostrander, Laura J., eds. Seeds for the future; roots of the past: Proceedings of the 17th North American prairie conference; 2000 July 16-20; Mason City, IA. Mason City, IA: North Iowa Community College: 125-135. [46518]

32. Jackson, A. S. 1965. Wildfires in the Great Plains grasslands. In: Proceedings, 4th annual Tall Timbers fire ecology conference; 1965 March 18-19; Tallahassee, FL. Tallahassee, FL: Tall Timbers Research Station: 241-259. [1239]

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35. Jones, D.; Von Bargen, K. L. 1992. Some physical properties of milkweed pods. Transactions of the American Society of Agricultural Engineers. 35(1): 243-246. [55066]

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