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A Comprehensive Guide to Horsetail Milkweed (Asclepias subverticillata)

Horsetail Milkweed (Asclepias subverticillata) is a herbaceous perennial that is native to the south-central and southwestern United States. This plant is a host to the Queen (Danaus gilippus) and Monarch (Danaus plexippus) butterflies. Growing from 0.5 to 4 feet tall, this species has white to greenish-white colored flowers, sometimes with some purple that bloom from June to August. It is hardy in zones 5-8.

Taxonomy and Naming of Horsetail Milkweed (Asclepias subverticillata)

Herbarium specimen of hosrsetail milkweed (Asclepias subverticillata).
Herbarium Specimen — Asclepias subverticillata (Gray) Vail collected in United States of America by The New York Botanical Garden (licensed under CC BY 4.0)

Taxonomy

Horsetail Milkweed (Asclepias subverticillata) was originally named and described by Asa Gray, an American botanist, in 1876 as a variety of whorled milkweed (Asclepias verticillata). Later in 1898, this plant was made its own species by Anna Murrey Vail. This species still recognized as subverticillata and is a member of the Dogbane Family (Apocynaceae).

Meaning of the Scientific and Common Names

Scientific Name

The genus name, Asclepias, is named for the Greek god of healing, Asklepios (Flora of Wisconsin). The species name, subverticillata, is Latin for almost whorled (SW Colorado Wildflowers).

Common Name and Alternative Names

The common name comes from the resemblance of this plant to horsetail (Equisetum spp). Other common names include poison milkweed, whorled milkweed, western whorled milkweed, and silkweed.

Physical Description of Horsetail Milkweed (Asclepias subverticillata)

Plants of horsetail milkweed (Asclepias subverticillata).
Horsetail Milkweed (Asclepias subverticillata) in a field — Patrick Alexander from Las Cruces, NM, CC0, via Wikimedia Commons

Description

  • Plant Type: This plant is a herbaceous perennial
  • Height: up to 4 feet
  • Stem: The stem is simple or branched and slightly pubescent at the nodes.
  • Leaves: The leaves are whorled (lower) and opposite (upper), short-petiolate, entire, and linear (Singhurst and Hutchins 2015). They range from 1 to 5 inches long and 0.1 to 0.2 inches wide.
  • Flower color: white, sometimes with purple (Woodson 1954) or greenish-white (Vail 1898)
  • Blooming period: This plant blooms from May to September.
  • Fruiting type and period: This plant has follicles that mature in the late summer and fall.

Range of Horsetail Milkweed (Asclepias subverticillata) in the United States and Canada

Range map of horsetail milkweed (Asclepias subverticillata) in the United States and Canada.
Range Map Credit: Kartesz, J.T., The Biota of North America Program (BONAP). 2023.(website https://bonap.org/). Chapel Hill, N.C. [maps generated from Kartesz, J.T. 2023. Floristic Synthesis of North America, Version 1.0. Biota of North America Program (BONAP). (in press)]

Horsetail milkweed species is native to the south-central and southwestern United States. Its range extends south into Mexico and Guatemala.

Habitat

Prairie habitat in United States.
Prairie Habitat — USFWS Mountain-Prairie, Public domain, via Wikimedia Commons

Horsetail milkweed grows in rocky plains and flats (Woodson 1954), prairies (Singhurst and Hutchins 2015), sunny canyon sides (Holiday 2000), roadsides (Kindscher 2020), waste places (Welsh and Erdman 1964), and marshes and wet areas near streams and ditches (Correll and Correll 1972).

Hosted Insects

Queen Butterfly on Twig.
Queen Butterfly (Danaus gilippus) — Korall, CC BY 3.0, via Wikimedia Commons

This species is a host for the Monarch Butterfly (Danaus plexippus), the Queen Butterfly (Danaus gilippus). This milkweed also hosts a species of cuckoo bee (Triepeolus grandis) (Rozen 1989) and three wasps (Cerceris finitima vierecki), (Cerceris macrosticta), and (Cerceris mimica) (Scullen 1965) and possibly a beetle (Tetraopes linsleyi) (Hovore and Giesbert 1976).

Other Supported Wildlife

Bumblebee on pink flower.
Bumblebee on Flower — Weerlicht, CC0, via Wikimedia Commons

This species is a nectar source to other butterflies, skippers, panurgine bees (Shinn 1967), and wasps (Heil 2013) during the growing season.

Frequently Asked Questions about Horsetail Milkweed (Asclepias subverticillata)

Is this plant poisonous?

Like other milkweeds, it has cardiac glycosides (cardenolides) and is considered to be poisonous with ingestion. This species has been noted as being very poisonous to livestock (Woodson 1954) and is said to contain the toxin, galitoxin (Plants of Texas Rangelands).

Does this plant have any ethnobotanical uses?

The Native American Ethobotanical Database shows that this species has been used for foods, fibers, and as a drug.

How is this plant distinguished from other milkweeds?

This species was first described as a variety of the whorled milkweed (Asclepias verticillata) by Asa Gray and is very similar to it. However, the whorled milkweed has ascending leaves and horsetail milkweed (Asclepias subverticillata) has leaves that droop or lean down (Singhurst, et al 2015) and the not all of the leaves are whorled as the upper leaves are often opposite (Heil 2013).

Is this plant invasive?

This plant is has been described as being weedy (Sage Notes 2012 and Gullion 1962).

Gardening with Horsetail Milkweed (Asclepias subverticillata)

Add Horsetail Milkweed to Your Garden

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Greenish-white flowers of horsetail milkweed (Asclepias subverticillata).
Flowers of Horsetail Milkweed (Asclepias subverticillata) — Patrick Alexander from Las Cruces, NM, CC0, via Wikimedia Commons

Hardiness

This species is hardy in zones 5-8. If your garden is within these zones and you have the right growing conditions (soil, moisture and exposure), you may well be able to grow this plant. However, if planted outside of its range, the hosted species may not recognize the plant or be harmed by ingesting a different species with an unfamiliar chemical composition.

Optimal Conditions

This species grows in a variety of moisture conditions, but likes full sun.

References

  • Correll, Donovan Stewart and Helen B. Correll. 1972. Aquatic and Wetland Plants of southwestern United States. (Washington, DC: EPA).
  • Gullion, Gordon W. 1962. Notable Nevada Plants. Leaflets of Western Botany 9 (15): 225-233.
  • Heil, Kenneth D., Steve L. O’Kane, Jr., Linda Mary Reeves, and Arnold Clifford. 2013. Flora of the four corners region: vascular plants of the San Juan River drainage, Arizona, Colorado, New Mexico, and Utah. (St. Louis: Missouri Botanical Garden).
  • Holiday, Susan. 2000. A Floristic Study of Tsegi Canyon, Arizona. Madrono 47: 29-42.
  • Hovore, Frank T. and Edmund F. Giesbert. 1976. Notes on the Ecology and Distribution of Western Cerambycidae (Coleoptera). The Coleopterists Bulletin 30(4): 348-360.
  • Kindscher, Kelly. 2020. The Vascular Plants of Lake Roberts, Gila National Forest, Grant County, New Mexico. The New Mexico Botanist 68: 1-22.
  • Rozen, Jerome G. 1989. Two New Species and the Redescription of Another Species of the Cleptoparasitic Bee Genus Triepeolus with Notes on Their Immature Stages (Anthophoridae: Nomadinae) (New York: American Museum of Natural History).
  • Sage Notes. 2012. Six Asclepias native to Idaho 34 (2): 5.
  • Scullen, Herman A. 1965. Review of the genus Cerceris in America north of Mexico (Hymenoptera: Sphecidae). Proceedings of the United States National Museum 116 (3506): 333-548.
  • Shinn, Alvin E. 1967. A Revision of the Bee Genus Calliopsis and the Biology and Ecology of C. andreniformis (Hymenoptera, Andrenidae). The University of Kansas Science Bulletin 61 (21): 753-936.
  • Singhurst, Jason, Ben Hutchins, and Walter Holmes. 2015. Identification of Milkweeds of Texas. Texas Parks and Wildlife Department.
  • Vail, Anna Murray. 1898. Studies in the Asclepidaceae – III. Descriptions of New Species. Bulletin of the Torrey Botanical Club 25 (4): 171-182.
  • Welsh, Stanley L. and James A. Erdman. 1964. Annotated Checklist of the Plants of Mesa Verde, Colorado. Brigham Young University Science Bulletin 4 (2): 1-32.
  • Woodson, Robert E. 1954. The North American Species of Asclepias L. Annals of the Missouri Botanical Garden 41: 1-211.
  • Wooten, E.O. and Paul Carpenter Standley. 1915. Flora of New Mexico. Contributions from the United States National Herbarium 19.
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Robert Coxe

Robert Coxe

Robert Coxe is a professional ecologist and botanist who has worked as the State Ecologist of Delaware and as an ecologist for the Western Pennsylvania Conservancy. He is also a former Past-President of the Pennsylvania Academy of Science. He currently is an innkeeper at McMullen House Bed & Breakfast LLC and a web designer and owner for Silphium Design LLC.

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