Poison Milkweed
Asclepias subverticillata (A. Gray) Vail
The whorled milkweed of the Southwest and southern Rockies: thin, wiry, colony-forming, and the species behind most milkweed poisoning cases in the region. Ecologically useful on wild ground, but not a plant to introduce deliberately.
Also called Western Whorled Milkweed, Horsetail Milkweed.
- Height
- 1 ft–3 ft
- Spread
- 2 ft–8 ft
- Flowers
- white, green
- Bloom
- June to September
- Light
- Full sun
- Soil moisture
- Dry
- USDA zones
- 4–9
- Leaves
- Whorled, threadlike
- Sap
- Milky latex
- Roots
- Rhizomatous, spreads by root
- Stratification
- 30 days cold-moist
- Range
- 10 states
When it flowers
Poison Milkweed blooms June to September, peaking in July and August.
How to identify it
- Very narrow leaves in whorls, with smaller leaves clustered in the axils giving a bushy, feathery look.
- Forms extensive rhizomatous colonies in disturbed sandy ground, roadsides and overgrazed pasture.
- Small greenish-white flowers in clusters along the upper stem.
Ecology and wildlife value
Monarchs and queens do use it, and in the arid Southwest it is often the most abundant milkweed available. Its dense colonies support numerous milkweed specialists including large and small milkweed bugs and milkweed tussock moths.
Growing poison milkweed
Not recommended for planting. It colonizes aggressively, is difficult to remove, and creates a genuine hazard where horses or livestock are kept.
Seed needs about 30 days of cold-moist stratification. Rhizomatous, spreads by root. See how to plant milkweed seed for the full method.
Cautions
The most dangerous milkweed for livestock in the western United States. Poisoning of horses, cattle and sheep is well documented, most often through baled hay. Do not plant near paddocks or hay meadows. More on milkweed toxicity.
Where poison milkweed grows
Recorded in 10 states: Arizona, Colorado, Kansas, Nebraska, New Mexico, Nevada, Oklahoma, Texas, Utah, Wyoming.
Distribution is generalized to state level. For county-level maps see USDA PLANTS and BONAP.
Frequently asked questions
Is poison milkweed dangerous to horses?
Yes. Asclepias subverticillata is highly toxic to horses, and because it dries without losing potency, contaminated hay is the usual route. Symptoms include tremors, staggering and cardiac arrhythmia. Remove it from hay ground and call a vet immediately if you suspect ingestion.
References and further reading
Names, ranges and conservation status here are drawn from the databases below. Where those disagree with a regional flora, trust the flora.
- Plants of the World Online 21298-2. Royal Botanic Gardens, Kew. Accepted name, authorship and global distribution.
- GBIF 3170284. Global Biodiversity Information Facility. Aggregated occurrence records.
- iNaturalist 127159. Community observations, photographs and phenology.
- Wikipedia. General-audience article on this species.
- BONAP county distribution map. Biota of North America Program. County-level range map.
- NatureServe Explorer. Conservation status ranks at global and state level.
- World Flora Online. Nomenclature and synonymy.
- Woodson, R. E. (1954). The North American species of Asclepias L.. Annals of the Missouri Botanical Garden 41(1): 1–211. The monograph the modern taxonomy of the genus still rests on. View
- Flora of North America Editorial Committee (eds.) (2026). Flora of North America North of Mexico. Oxford University Press, New York and Oxford. Accepted names, keys and descriptions for the North American flora. View
- Royal Botanic Gardens, Kew (2026). Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Nomenclature and global distribution, keyed to IPNI name identifiers. View
- Kartesz, J. T. (Biota of North America Program) (2026). North American Plant Atlas. BONAP, Chapel Hill, North Carolina. County-level distribution maps, the basis for the ranges generalized here. View
- USDA, NRCS (2026). The PLANTS Database. National Plant Data Team, Greensboro, North Carolina. State-level distribution, wetland status and nomenclature. View
- Agrawal, A. A. (2017). Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution. Princeton University Press. The standard general account of the milkweed and monarch relationship. View
Cite this page
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Milkweed USA. (2026). Poison Milkweed (Asclepias subverticillata). Retrieved DATE, from https://milkweedusa.com/species/poison-milkweed/
"Poison Milkweed (Asclepias subverticillata)." Milkweed USA, 2026, https://milkweedusa.com/species/poison-milkweed/. Accessed DATE.
Milkweed USA. "Poison Milkweed (Asclepias subverticillata)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/species/poison-milkweed/.
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title = {Poison Milkweed (Asclepias subverticillata)},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/species/poison-milkweed/},
note = {Accessed DATE}
}