|FEIS Home Page|
Table of Contents
|©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: https://www.fs.fed.us/database/feis/plants/forb/ascspe/all.html .
Asclepias giffordii Eastw. [21,28]
A. douglasii W. Hooker [17,30,36,63]
NRCS PLANT CODE :
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:
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 :
|Charles Weber ©California Academy of Sciences|
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  LIFE FORM:
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 .
Seed production: Stevens  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. . Showy milkweed's persistence along waterways [6,12,17,28,30,34,38,42,44,53,60,61] also suggests dispersal by water .
Seed banking: Seed may be stored in water or in soil, as shown in the following studies. In a study by Bruns  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 :
|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  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 .
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  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 :
Months after test initiated
|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% .
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" .
Asexual regeneration: Showy milkweed spreads clonally from deep rhizomes [10,22], and probably from from sprouting from the root crown.SITE CHARACTERISTICS:
Elevation: Showy milkweed occurs at the following elevations:
|Arizona||6,000-9,000 feet (1,800-2,700 m) |
|California||0-600 feet (0-1,900 m) |
|Colorado||3,500-7,500 feet (1,100-2,300 m) |
|Nevada||2,200-8,500 feet (700-2,600 m) |
|New Mexico||6,000-9,000 feet (1,800-2,700 m) |
|Utah||2,700-8,500 feet (800-2,600 m) |
Soil: Showy milkweed prefers moist soil [6,12,17,28,30,34,38,42,44,53,60,61]. It grows in all soil textures  and tolerates alkaline soils .SUCCESSIONAL STATUS:
Fire regimes: Historically, fire has been an important natural component of the savanna and grassland communities where showy milkweed occurs . 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 , 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.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 [40,47]|
|coastal sagebrush||Artemisia californica||<35 to <100 |
|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 |
|coast live oak||Quercus agrifolia||2-75 |
|blue oak-foothills pine||Quercus douglasii-P. sabiniana||<35 |
|California black oak||Quercus kelloggii||5-30 |
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:
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 .
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) . The following table is a comparison of amino acid composition of showy milkweed residue, alfalfa (Medicago sativa), and corn .
|Amino Acid||Showy milkweed residue
Cover value: Showy milkweed is rated as providing poor cover for small mammals, upland game birds, waterfowl, and small nongame birds .VALUE FOR REHABILITATION OF DISTURBED SITES:
Processed showy milkweed residue is a possible source of a variety of other products. After extraction, showy milkweed produces the following products :
|Tri-terpenoids, esters and related compounds||3.3|
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.
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 .OTHER MANAGEMENT CONSIDERATIONS:
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.]. 
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. 
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. 
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. 
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. 
6. Bare, Janet E. 1979. Wildflowers and weeds of Kansas. Lawrence, KS: The Regents Press of Kansas. 509 p. 
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. 
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. 
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. 
10. Bookman, Susan Stone. 1983. Costs and benefits of flower abscission and fruit abortion in Asclepias speciosa. Ecology. 64(2): 264-273. 
11. Bookman, Susan Stone. 1984. Evidence for selective fruit production in Asclepias. Evolution. 38(1): 72-86. 
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. 
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. 
14. Bruns, V. F. 1965. The effects of fresh water storage on the germination of certain weed seeds. Weeds. 13: 38-39. 
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. 
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. 
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. 
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. 
19. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. 
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. 
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. 
22. Great Plains Flora Association. 1986. Flora of the Great Plains. Lawrence, KS: University Press of Kansas. 1392 p. 
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. 
24. Harrington, H. D. 1964. Manual of the plants of Colorado. 2d ed. Chicago: The Swallow Press, Inc. 666 p. 
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. 
26. Hart, Jeffrey A. 1981. The ethnobotany of the Northern Cheyenne Indians of Montana. Journal of Ethnopharmacology. 4: 1-55. 
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. 
28. Hickman, James C., ed. 1993. The Jepson manual: Higher plants of California. Berkeley, CA: University of California Press. 1400 p. 
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. 
30. Hitchcock, C. Leo; Cronquist, Arthur. 1973. Flora of the Pacific Northwest. Seattle, WA: University of Washington Press. 730 p. 
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. 
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. 
33. Jackson, Laura L. 1999. Establishing tallgrass prairie on grazed permanent pasture in the upper Midwest. Restoration Ecology. 7(2): 127-138. 
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. 
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. 
36. Jones, Stanley D.; Wipff, Joseph K.; Montgomery, Paul M. 1997. Vascular plants of Texas. Austin, TX: University of Texas Press. 404 p. 
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]. 
38. Kartesz, John Thomas. 1988. A flora of Nevada. Reno, NV: University of Nevada. 1729 p. [In 3 volumes]. Dissertation. 
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. 
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. 
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. 
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. 
43. Love, Askell; Love, Doris. 1954. Vegetation of a prairie marsh. Bulletin of the Torrey Botanical Club. 81(1): 16-34. 
44. Martin, William C.; Hutchins, Charles R. 1981. A flora of New Mexico. Volume 2. Germany: J. Cramer. 2589 p. 
45. Mohlenbrock, Robert H. 1986. [Revised edition]. Guide to the vascular flora of Illinois. Carbondale, IL: Southern Illinois University Press. 507 p. 
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. 
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. 
48. Quinnild, Clayton L.; Cosby, Hugh E. 1958. Relicts of climax vegetation on two mesas in western North Dakota. Ecology. 39(1): 29-32. 
49. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. 
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. 
51. Rowe, J. S. 1969. Lightning fires in Saskatchewan grassland. Canadian Field-Naturalist. 83: 317-324. 
52. Shiflet, Thomas N., ed. 1994. Rangeland cover types of the United States. Denver, CO: Society for Range Management. 152 p. 
53. Stevens, O. A. 1945. Asclepias syrica and A. speciosa, distribution and mass collections in North Dakota. American Midland Naturalist. 34(2): 368-374. 
54. Stevens, O. A. 1957. Weights of seeds and numbers per plant. Weeds. 5: 46-55. 
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. 
56. Stomberg, Mark R.; Kephart, Paul; Yadon, Vern. 2001. Composition, invasibility, and diversity in coastal California grasslands. Madrono. 48(4): 236-252. 
57. U.S. Department of Agriculture, Natural Resources Conservation Service. 2006. PLANTS database (2006), [Online]. Available: https://plants.usda.gov /. 
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. 
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. 
60. Weber, William A.; Wittmann, Ronald C. 1996. Colorado flora: eastern slope. 2nd ed. Niwot, CO: University Press of Colorado. 524 p. 
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. 
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. 
63. Woodson, Robert E., Jr. 1954. The North American species of Asclepias L. Annals of the Missouri Botanical Garden. 41(1): 1-211. 
64. Wright, Henry A.; Bailey, Arthur W. 1982. Fire ecology: United States and southern Canada. New York: John Wiley & Sons. 501 p. 
65. Wyatt, Robert; Broyles, Steven B. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics. 25: 423-441.