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Asclepias (speciosa)

Dave Skinner
PMC Farm Manager
USDA NRCS - Pullman Plant Materials Center
Room 211A Hulbert Hall WSU
Pullman, Washington 99164-6211
509-335-2940 (fax)

Family Scientific Name: Asclepiadaceae
Family Common Name: milkweed
Scientific Name: Asclepias speciosa Torr.
Common Name: showy milkweed
Species Code: ASSP
Ecotype: Paradise Creek drainage near Pullman, Washington.
General Distribution: Native to mesic places in western North America from British Columbia to Manitoba and south to Texas. In the Palouse of eastern Washington and northern Idaho it is not common and most frequently found along roadcuts and in road ditches.
Mean annual precipitation range is from 16-30 inches (USDA NRCS 2008).
Wetland indicator status is FAC+ for the northwestern US (US Fish and Wildlife Service 1988).
Known Invasiveness: Considered weedy by some (Whitson et al 1996), possibly because it can be toxic to livestock when consumed in large quantities. It is not common on the Palouse and does not appear to be invasive.
Propagation Goal: plants
Propagation Method: seed
ProductType: Container (plug)
Stock Type: 10 cu. in.
Time To Grow: 4 Months
Target Specifications: Tight root plug in container.
Propagule Collection: Fruit is a follicle and seed is reddish brown in color when mature. Seed is collected by hand when the follicles begin to split in September or October. Seed is attached to a long white coma which aids on wind dispersal. It must be harvested before becoming wind borne. Collected material is stored in paper bags or envelopes at room temperature until cleaned.
Propagule Processing: Seeds with the attached coma can be collected by hand removal from the follicle in the field or the follicles can be collected and later opened by hand to extract the seed. The coma can be removed by hand or by rubbing over a 14/64 hand screen. If necessary, seed can be cleaned using a air column separator.
72,000 seeds/lb (USDA, NRCS 2008).
Pre-Planting Treatments: Buhler & Hoffman (1999) state fresh seeds planted in autumn germinate the following spring and summer. We found seed of this ecotype to germinate readily without pretreatment. Unpublished data from trials conducted at the Pullman Plant Materials Center comparing untreated seed with seed treated by cold moist stratification for periods of 45, 90, or 120 days showed no increase in total emergence following stratification. Untreated seed emerged at 85%. Stratified seed emerged at the same time as untreated seed, suggesting that germination does not begin until temperatures warm.
Growing Area Preparation/
Annual Practices for Perennial Crops:
In January seed is sown in the greenhouse in 10 cu. in. Ray Leach Super cell conetainers filled with Sunshine #4 and covered lightly. Head space of ¬ to « inch is maintained in conetainers to allow deep watering. A thin layer of coarse grit is applied to the top of the planting soil to prevent seeds from floating during watering. Conetainers are watered deeply.
Establishment Phase: Medium is kept moist until germination occurs. Germination usually begins in 6 days and is complete in 14 days.
Length of Establishment Phase: 2 weeks
Active Growth Phase: Plants are watered deeply every other day and fertilized once per week with a complete, water soluble fertilizer containing micro-nutrients.
Length of Active Growth Phase: 10-12 weeks
Hardening Phase: Plants are moved to the cold frame in late March or early April, depending on weather conditions. They are watered every other day if the weather is cool, and every day during hot, dry spells.
Length of Hardening Phase: 2-4 weeks
Harvesting, Storage and Shipping: Plants can be stored in the lath house over winter. They must be afforded some protection from extreme cold temperatures. Containerized material of A. speciosa is much more sensitive to winter damage than many of the other native forbs of the Palouse. Mulch or foam sheets provide sufficient protection. The protection should be removed in spring as temperatures begin to rise.
Other Comments: Established plants of A. speciosa are among the last local native forbs to resume growth in the spring, apparently requiring warm temperatures to break winter dormancy.
Plants are strongly rhizomatous and can be propagated by division. This method should only be used for plants growing in cultivation. Plants should not be dug up from stands in the wild.
Viable seed production in wild plants is highly variable between different years.
Flowers are insect pollinated (Bookman 1983a).
Bumblebees are the most common pollinator (Finer 2004).
97% of ovaries fail to develop into mature pods (Bookman 1983b).
High levels of geitonogamy result in high levels of fruit abortion (Finer & Morgan 2003, Finer 2003).
Larva of the monarch butterfly (Danaus plexippus) are obligate Asclepias feeders (Pyle 2002).
Adults of the western milkweed long-horned beetle (Tetraopes femoratus) feed on the leaves, buds, and flowers of A. speciosa. The larva feed on the roots.
References: Bookman, Susan S. 1983a. Costs and Benefits of Flower Abscission and Fruit Abortion in Asclepias speciosa. Ecology 64:264-273.
Bookman, Susan S. 1983b. Effects of Pollination Timing on Fruiting in Asclepias speciosa Torr. (Asclepiadaceae). American Journal of Botany. 70:897-905.
Buhler, Douglas D., and Melinda L. Hoffman. 1999. Andersen's Guide to Practical Methods of Propagating Weeds and Other Plants. Weed Science Society of America. Lawrence, Kansas. 248 pp.
Craighead, John J., Frank C. Craighead, and Ray J. Davis. 1963. A Field Guide to Rocky Mountain Wildflowers. Houghton Mifflin Co. Boston, MA. 277 pp.
Faust, Ralph and Peggy Faust. 1999. Wildflowers of the Inland Northwest. Museum of North Idaho. Coeur d'Alene, ID. 141 pp.
Finer, Matthew. 2003. Effects of Geitonogamy, Habitat Fragmentation, and Population Size on Plant Reproductive Success: Ecological and Evolutionary Studies. PhD thesis, Washington State University (Biological Sciences).
Finer, Matt. 2004. Insight into the Pollination Crisis: Effects of Population Size on Pollinator Diversity for Showy Milkweed. Douglasia 28(4):7-8.
Finer, Matthew S. and Martin T. Morgan. 2003. Effects of Natural Rates of Geitonogamy on Fruit Set in Ascepias speciosa (Apocynaceae): Evidence Favoring the Plant's Dilemma. American Journal of Botany 90:1746-1750.
Hitchcock, C. Leo, and Arthur Cronquist. 1973. Flora of the Pacific Northwest. University of Washington Press. Seattle, WA. 730 pp.
Larrison, Earl J., Grace W. Patrick, William H. Baker, and James A. Yaich. 1974. Washington Wildflowers. The Seattle Audubon Society. Seattle, WA. 376 pp.
Lyons, C.P. 1997. Wildflowers of Washington. Lone Pine Publishing, Renton, WA. 192 pp.
Piper, C.V., and R.K. Beattie. 1914. The Flora of Southeastern Washington and AdjacentIdaho. Lancaster, PA. Press of the New Era Printing Company. 296 p.
Pyle, Robert M. 2002. The Butterflies of Cascadia. The Seattle Audubon Society, Seattle, Washington. 420 pp.
Rickett, Harold W. 1973. Wildflowers of the United States: The Central Mountains and Plains. Vol. 6. (3 parts). McGraw Hill, New York.
St. John, Harold. 1963. Flora of Southeastern Washington and of Adjacent Idaho. 3rd edition. Outdoor Pictures. Escondido, CA. 583 pp.
Strickler, Dee. 1993. Wayside Wildflowers of the Pacific Northwest. The Flower Press, Columbia Falls, MT. 272 pp.
Taylor, Ronald J. 1992. Sagebrush Country. Mountain Press Publishing Co. Missoula, MT. 211 pp.
USDA ARS National Genetic Resources Program. Germplasm Resources Information Network - (GRIN) [Online Database]. National Germplasm Resources Laboratory, Beltsville, Maryland. URL: http://www.ars-grin.gov2/cgi-bin/npgs/html/taxon.pl?448068 (11 February 2008).
USDA, NRCS. 2008. The PLANTS Database (http://plants.usda.gov, 11 February 2008). National Plant Data Center, Baton Rouge, LA 70874-4490 USA.
US Fish and Wildlife Service. 1988. National list of vascular plant species that occur in wetlands. US Fish & Wildlife Service Biological Report 88 (24).
Whitson, Tom D., Larry C. Burrill, Steven A. Dewey, David W. Cudney, B.E. Nelson, Richard D. Lee, and Robert Parker. 1996. Weeds of the West. 5th ed. Western Society of Weed Science. Newark, CA. 630 pp.


Skinner, David M,. 2008. Propagation protocol for production of Container (plug) Asclepias speciosa Torr. plants 10 cu. in.; USDA NRCS - Pullman Plant Materials Center Pullman, Washington. In: Native Plant Network. URL: http://NativePlantNetwork.org (accessed 2021/05/11). US Department of Agriculture, Forest Service, National Center for Reforestation, Nurseries, and Genetic Resources.

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