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A Comprehensive Guide to Long-hood Milkweed (Asclepias macrotis)

Long-hood Milkweed (Asclepias macrotis) 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 2 feet tall, this species has white to green flowers that bloom from April to August. It is hardy in zones 8-10.

Taxonomy and Naming of Long-hood Milkweed (Asclepias macrotis)

Herbarium specimen of long-hood milkweed (Asclepias macrotis).
Herbarium Specimen — Asclepias macrotis Torr. collected in United States of America by The New York Botanical Garden (licensed under CC BY 4.0)
Isolectotype specimen of long-hood milkweed (Asclepias macrotis).
Isolectotype Specimen of Long-hood Milkweed — Harvard University Public Domain

Taxonomy

Long-hood Milkweed (Asclepias macrotis) was originally named and described by John Torrey, an American botanist, in 1859. This species has kept the same name since this time 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, macrotis, is Latin for linear, and is in reference to the leaves.

Common Name and Alternative Names

The common name comes the long hoods of the flowers.

Physical Description of Long-hood Milkweed (Asclepias macrotis)

Whitish flowers of long-hood milkweed (Asclepias macrotis).
Flowers of Long-hood Milkweed — Asclepias macrotis Torr. observed in United States of America by Patrick Alexander (licensed under CC0 1.0)
  • Plant Type: This plant is a suffrutescent (semi-shrub) perennial
  • Height: 0.3 to 1 foot
  • Stem: The stem is clustered from the roots (Woodson 1954).
  • Leaves: The leaves are opposite, sessile, simple, revolute, and filiform in shape. The leaves are 1.25 to 3.5 inches long and about 0.02 to 0.16 inches wide.
  • Flower color: greenish-yellow (Woodson 1954), yellowish-white (Singhurst and Hutchins 2015) or pale purple (Baird 1859) or green to purple-tinged (Verrier 2018)
  • Blooming period: This plant blooms from May to October.
  • Fruiting type and period: This plant has follicles that mature in the late summer and fall.

Range of Long-hood Milkweed (Asclepias macrotis) in the United States and Canada

Range map of long-hood milkweed (Asclepias macrotis) 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)]

This milkweed species is native to the south-central and southwestern United States and northern Mexico.

Habitat

Mesa habitat in US desert southwest.
Mesa Habitat — “Brins Mesa No. 119” by Coconino NF Photography is marked with CC0 1.0.

This species grows on dry hills and mesas (Woodson 1954) and on limestone (Singhurst and Hutchins 2015) in scrub grassland (Verrier 2018).

Hosted Insects

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

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, bees, and wasps during the growing season.

Frequently Asked Questions about Long-hood Milkweed (Asclepias macrotis)

Is this plant poisonous?

Like other milkweeds, it has cardiac glycosides (cardenolides) and is considered to be poisonous with ingestion.

Does this plant have any ethnobotanical uses?

The Native American Ethobotanical Database does not specifically mention this species, but milkweeds in general have been used for foods, pharmaceuticals, and fibers.

How is this plant distinguished from other milkweeds?

This species is similar to the slim-pod milkweed (Asclepias quinquedentata), the low milkweed (Asclepias pumila), and the whorled milkweed (Asclepias verticillata). However, long-hood milkweed has hoods that are 3 times as long as the anthers, branched stems, and is pubescent (Wooten and Standley 1915). Neither of the other species have these characteristics. Long-hood milkweed is also similar to pine-needle milkweed (Asclepias linaria), but pine-needle milkweed has alternate leaves and long-hood milkweed has opposite leaves (Kearney and Peebles 1942). The filiform leaves, distinguishes this milkweed from the showy milkweed (Asclepias speciosa). Singhurst and Hutchins state that this species is also close to Sperry’s milkweed (Asclepias sperryi), but can be separated in that A. sperryi has deflexed hoods, whereas A. macrotis has ascending hoods (Singhurst and Hutchins 2015).

Is this plant invasive?

This plant has not been described as invasive in the literature.

Gardening with Long-hood Milkweed (Asclepias macrotis)

Close-up of flowers of long-hood milkweed (Asclepias macrotis).
Greenish-white Flowers of Long-hood Milkweed — Asclepias macrotis Torr. observed in United States of America by Craig Martin (licensed under CC0 1.0)

Hardiness

This species is hardy in zones 8-10. 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 best in places where it can receive full sun and has dry soil.

References

  • Baird, Spencer Fullerton, et al. 1859. Report on the United States and Mexican Boundary Survey Made under the Direction of the Secretary of the Interior. (Washington, DC: US DOI).
  • Kearney, Thomas H. and Robert H. Peebles. 1942. Flowering Plants and Ferns of Arizona. (Washington, DC: USDA) No. 423.
  • Singhurst, Jason and Ben Hutchins. 2015. Identification of Milkweeds in Texas. Lady Bird Johnson Wildflower Center and Texas Parks and Wildlife.
  • Verrier, James T. 2018. Annotated Flora of the Santa Catalina Mountains, Pima and Pinal Counties, Southeastern Arizona. Desert Plants 33(1): 5-291.
  • 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 C. Standley. 1915. Flora of New Mexico. Contributions from the United States National Herbarium v. 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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