Asclepias speciosa, showy milkweed
Ulev, Elena D. 2005. Asclepias speciosa, showy milkweed. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://research.fs.usda.gov/feis/species-reviews/ascspe
| Abbreviation | Common Name | Scientific Name | Classification | Status |
|---|---|---|---|---|
| Plants | ||||
| ASCSPE | showy milkweed | Asclepias speciosa | Life Form: Plants/Forb Kingdom: Plantae Class: Dicot Order: Gentianales Family: Asclepidaceae Genus: Asclepias | Fed. Protected: No Nativity: Native Invasiveness: Noninvasive |
Yosemite Valley.
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].
Synonyms
- Asclepias giffordii Eastw. [21,28]
- A. douglasii W. Hooker [17,30,36,63]
Other Common Names
Greek milkweed
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.
States and Provinces
- 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
Showy milkweed growing in Hot Springs County, Wyoming (CC BY 4.0).
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].
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].
A seed pod of showy milkweed.
Botanical Description
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). Showy milkweed has "deep" rhizomes [10,22].
Raunkiaer Life Form
- Geophyte
- Hemicryptophyte [49]
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].
Flowering showy milkweed.
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 (Triticumaestivum) 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 and 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].
Vegetative Regeneration
Showy milkweed spreads clonally from deep rhizomes [10,22], and probably from from sprouting from the root crown.
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.
Immediate Fire Effects
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.
Postfire Regeneration Strategy
- 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) [55]
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 deeply 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.
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.
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 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. For additional fire regime information, search FEIS for this species by entering the species name on the Advanced Search page and selecting “Fire Regime” as the publication type.
| 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 | Jan-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] |
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.
Federal Status
None
Other Status
Showy milkweed is listed as threatened in Iowa [57].
Importance to Wildlife and Livestock
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 and 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].
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 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 can be used as an antiseptic for cuts, as well for treating ringworm, corns, and calluses. Tea made from the roots can be used for treating measles, coughs, diarrhea, and rheumatism. Native Americans have also used showy milkweed for food and utility items. The bitter sap can be removed from showy milkweed by boiling, and the stems, leaves, flowers and young fruits can then be eaten. Young stalks can be eaten like asparagus. Upon maturity, showy milkweed stems can be 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].
Table A1— Forest and range ecosystems, Bureau of Land Management (BLM) physiographic regions, Kuchler plant associations, Society for American Foresters (SAF) forest cover types, and Society for Rangeland Management (SRM) rangeland cover types in which this species occurs.
Forest and Range Ecosystems (Garrison et al. 1977)
- 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
BLM Physiographic Regions (Bernard and Brown 1977)
- 1 Northern Pacific Border
- 2 Cascade Mountains
- 3 Southern Pacific Border
- 4 Sierra Mountains
- 5 Columbia Plateau
- 6 Upper Basin and Range
- 7 Lower Basin and Range
- 8 Northern Rocky Mountains
- 9 Middle Rocky Mountains
- 10 Wyoming Basin
- 11 Southern Rocky Mountains
- 12 Colorado Plateau
- 13 Rocky Mountain Piedmont
- 14 Great Plains
- 15 Black Hills Uplift
- 16 Upper Missouri Basin and Broken Lands
Kuchler Plant Associations (Kuchler 1964)
- 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 (Eyre 1980)
- 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 (Shiflet 1994)
- 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
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]
24. Harrington, H. D. 1964. Manual of the plants of Colorado. 2d ed. Chicago: The Swallow Press, Inc. 666 p. [6851]
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]
28. Hickman, James C., ed. 1993. The Jepson manual: Higher plants of California. Berkeley, CA: University of California Press. 1400 p. [21992]
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]
33. Jackson, Laura L. 1999. Establishing tallgrass prairie on grazed permanent pasture in the upper Midwest. Restoration Ecology. 7(2): 127-138. [36676]
34. Johnson, James R.; Nichols, James T. 1970. Plants of South Dakota grasslands: A photographic study. Bull. 566. Brookings, SD: South Dakota State University, Agricultural Experiment Station. 163 p. [18483]
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]
36. Jones, Stanley D.; Wipff, Joseph K.; Montgomery, Paul M. 1997. Vascular plants of Texas. Austin, TX: University of Texas Press. 404 p. [28762]
37. Kartesz, John T.; Meacham, Christopher A. 1999. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Available: North Carolina Botanical Garden. In cooperation with: The Nature Conservancy, Natural Resources Conservation Service, and U.S. Fish and Wildlife Service [2001, January 16]. [36715]
38. Kartesz, John Thomas. 1988. A flora of Nevada. Reno, NV: University of Nevada. 1729 p. [In 3 volumes]. Dissertation. [42426]
39. Kearney, Thomas H.; Peebles, Robert H.; Howell, John Thomas; McClintock, Elizabeth. 1960. Arizona flora. 2d ed. Berkeley, CA: University of California Press. 1085 p. [6563]
40. Kucera, Clair L. 1981. Grasslands and fire. In: Mooney, H. A.; Bonnicksen, T. M.; Christensen, N. L.; Lotan, J. E.; Reiners, W. A., technical coordinators. Fire regimes and ecosystem properties: Proceedings of the conference; 1978 December 11-15; Honolulu, HI. Gen. Tech. Rep. WO-26. Washington, DC: U.S. Department of Agriculture, Forest Service: 90-111. [4389]
41. Kuchler, A. W. 1964. United States [Potential natural vegetation of the conterminous United States]. Special Publication No. 36. New York: American Geographical Society. 1:3,168,000; colored. [3455]
42. Ladner, Deborah T.; Altizer, Sonia. 2005. Oviposition preference and larval performance of North American monarch butterflies on four Asclepias species. Entomologia Experimentalis et Applicata. 116: 9-20. [55055]
43. Love, Askell; Love, Doris. 1954. Vegetation of a prairie marsh. Bulletin of the Torrey Botanical Club. 81(1): 16-34. [18104]
44. Martin, William C.; Hutchins, Charles R. 1981. A flora of New Mexico. Volume 2. Germany: J. Cramer. 2589 p. [37176]
45. Mohlenbrock, Robert H. 1986. [Revised edition]. Guide to the vascular flora of Illinois. Carbondale, IL: Southern Illinois University Press. 507 p. [17383]
46. Parker, Robert, compiler. 1982. Reaction of various plants to 2,4-D, MCPA, 2,4,5-T, silvex and 2,4-DB. EM 4419 [Revised]. Pullman, WA: Washington State University, College of Agriculture, Cooperative Extension. 61 p. In cooperation with: U.S. Department of Agriculture. [1817]
47. Paysen, Timothy E.; Ansley, R. James; Brown, James K.; [and others]. 2000. Fire in western shrubland, woodland, and grassland ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-volume 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 121-159. [36978]
48. Quinnild, Clayton L.; Cosby, Hugh E. 1958. Relicts of climax vegetation on two mesas in western North Dakota. Ecology. 39(1): 29-32. [1925]
49. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. [2843]
50. Riser, James. 2006. [Email to Elena Ulev]. January 3. Showy milkweed. Missoula, MT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory. On file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT; RWU 4403 files. [55858]
51. Rowe, J. S. 1969. Lightning fires in Saskatchewan grassland. Canadian Field-Naturalist. 83: 317-324. [6266]
52. Shiflet, Thomas N., ed. 1994. Rangeland cover types of the United States. Denver, CO: Society for Range Management. 152 p. [23362]
53. Stevens, O. A. 1945. Asclepias syrica and A. speciosa, distribution and mass collections in North Dakota. American Midland Naturalist. 34(2): 368-374. [55060]
54. Stevens, O. A. 1957. Weights of seeds and numbers per plant. Weeds. 5: 46-55. [44071]
55. Stickney, Peter F. 1989. FEIS postfire regeneration workshop--April 12: Seral origin of species comprising secondary plant succession in Northern Rocky Mountain forests. 10 p. Unpublished draft on file at: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Fire Sciences Laboratory, Missoula, MT. [20090]
56. Stomberg, Mark R.; Kephart, Paul; Yadon, Vern. 2001. Composition, invasibility, and diversity in coastal California grasslands. Madrono. 48(4): 236-252. [41371]
57. U.S. Department of Agriculture, Natural Resources Conservation Service. 2006. PLANTS database (2006), [Online]. Available: https://plants.usda.gov /. [34262]
58. Van Emon, Jeanette; Seiber, James N. 1985. Chemical constituents and energy content of two milkweeds, Asclepias speciosa and A. curassavica. Economic Botany. 39(1): 47-55. [55057]
59. Voss, Edward G. 1996. Michigan flora. Part III: Dicots (Pyrolaceae--Compositae). Cranbrook Institute of Science Bulletin 61/University of Michigan Herbarium. Ann Arbor, MI: The Regents of the University of Michigan. 622 p. [30401]
60. Weber, William A.; Wittmann, Ronald C. 1996. Colorado flora: eastern slope. 2nd ed. Niwot, CO: University Press of Colorado. 524 p. [27572]
61. Welsh, Stanley L.; Atwood, N. Duane; Goodrich, Sherel; Higgins, Larry C., eds. 1987. A Utah flora. The Great Basin Naturalist Memoir No. 9. Provo, UT: Brigham Young University. 894 p. [2944]
62. Whitson, T. D.; Schwope, M. L. 1988. Showy milkweed (Asclepias speciosa Torr.) control with various herbicides. In: Proceedings, Western Society of Weed Science: 1988 Research Progress Report; 1988 March 8-10; Fresno, CA. [Newark, CA]: Western Society of Weed Science: 36. [55141]
63. Woodson, Robert E., Jr. 1954. The North American species of Asclepias L. Annals of the Missouri Botanical Garden. 41(1): 1-211. [55067]
64. Wright, Henry A.; Bailey, Arthur W. 1982. Fire ecology: United States and southern Canada. New York: John Wiley & Sons. 501 p. [2620]
65. Wyatt, Robert; Broyles, Steven B. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics. 25: 423-441. [55059]