# Poison Milkweed (Asclepias subverticillata)

> 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.

Source: https://milkweedusa.com/species/poison-milkweed/  
Licence: CC BY 4.0 (Milkweed USA)

## Summary

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.

## Key facts

| Field | Value |
| --- | --- |
| Scientific name | Asclepias subverticillata (A. Gray) Vail |
| Genus | Asclepias |
| Family | Apocynaceae |
| Common names | Poison Milkweed, Western Whorled Milkweed, Horsetail Milkweed |
| Status | Native |
| Height | 1 ft–3 ft |
| Spread | 2 ft–8 ft |
| Flower color | white, green |
| Bloom period | June to September |
| Light | Full sun |
| Soil moisture | Dry |
| USDA zones | 4-9 |
| Leaf arrangement | whorled |
| Leaf shape | threadlike |
| Sap | Milky latex |
| Root system | Rhizomatous, spreads by root |
| Seed stratification | 30 days cold-moist |
| Monarch value (0-5) | 3 |
| Garden value (0-5) | 1 |
| Nectar value (0-5) | 3 |
| US states recorded | Arizona, Colorado, Kansas, Nebraska, New Mexico, Nevada, Oklahoma, Texas, Utah, Wyoming |

## Identification

- 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

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

Not recommended for planting. It colonizes aggressively, is difficult to remove, and creates a genuine hazard where horses or livestock are kept.

## 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.

## 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

- Plants of the World Online: https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:21298-2
- GBIF: https://www.gbif.org/species/3170284
- iNaturalist: https://www.inaturalist.org/taxa/127159
- Wikipedia: https://en.wikipedia.org/wiki/Asclepias_subverticillata
- BONAP county map: https://bonap.net/MapGallery/County/Asclepias%20subverticillata.png
- NatureServe: https://explorer.natureserve.org/Search?searchTerm=Asclepias%20subverticillata

## Photographs

- Asclepias subverticillata (SALLY KING, Public domain): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata.jpg
- Asclepias subverticillata kz02 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz02.jpg
- Asclepias subverticillata kz01 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz01.jpg
- Asclepias subverticillata kz03 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz03.jpg
- Asclepias subverticillata kz05 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz05.jpg
- Asclepias subverticillata kz04 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz04.jpg
- Asclepias subverticillata kz06 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz06.jpg
- Asclepias subverticillata kz07 (Kenraiz, CC BY-SA 4.0): https://commons.wikimedia.org/wiki/File:Asclepias_subverticillata_kz07.jpg

## How to cite

Milkweed USA. (2026). Poison Milkweed (Asclepias subverticillata). https://milkweedusa.com/species/poison-milkweed/