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A Comprehensive Guide to Serpentine Milkweed (Asclepias solanoana)

Serpentine Milkweed (Asclepias solanoana) is a herbaceous perennial that is native to the state of California. This plant is a host to the Queen (Danaus gilippus) and Monarch (Danaus plexippus) butterflies. Growing from 0.3 to 1 feet tall, this species has greenish-white, pink to purple flowers that bloom from May to June. It is hardy in zones 8-9.

Taxonomy and Naming of Serpentine Milkweed (Asclepias solanoana)

Herbarium specimen of serpentine milkweed (Asclepias solanoana).
Herbarium Specimen — Asclepias solanoana Woodson collected in United States of America by The New York Botanical Garden (licensed under CC BY 4.0)

Taxonomy

Serpentine Milkweed (Asclepias solanoana) was originally named and described by Asa Gray, an American botanist, as Gomphocarpus purpurascens in 1874. Through time it has had a number of different names, but in 1941, Robert Woodson, determined the species to its current name (Woodson 1941). Since 1941, it has kept the same name 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, solanoana,

Common Name and Alternative Names

The common name of the plant comes from the type of rocks that this plant grows on. Other common names include prostrate milkweed and solanoa, from the species name.

Physical Description

Pink flower cluster of serpentine milkweed (Asclepias solanoana).
Flowers of Serpentine Milkweed (Asclepias solanoana) — Shasta-Trinity National Forest USDA Forest Service., CC0, via Wikimedia Commons
  • Plant Type: This plant is a herbaceous perennial.
  • Height: 0.3 to 1 foot (Woodson 1954)
  • Stem: The pubescent stem is prostrate on the ground.
  • Leaves: The leaves are opposite, simple, entire, and ovate to oval in shape. The leaves are 1 to 1.5 inches long and 1 to 1.2 inches wide (Woodson 1954).
  • Flower color: red, purple, or white
  • Blooming period: This plant blooms in June.
  • Fruiting type and period: This plant has follicles that mature in the late summer and fall.

Range of Serpentine Milkweed (Asclepias solanoana) in the United States and Canada

Range map of serpentine milkweed (Asclepias solanoana) 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 and endemic to the state of California, where it is considered to be rare.

Habitat

Serpentine outcrop in California.
Outcrop of Serpentine in California — Mkauffmann, CC BY 3.0, via Wikimedia Commons

This species grows in open woodlands and rocky outcrops with serpentine.

Hosted Insects

Purple coneflower (Echinacea purpurea) with Monarch butterfly.
Monarch Butterfly on Purple Coneflower — Robert Coxe, Image

Other Supported Wildlife

Bumblebee on pink flower.
Pink flower with Bumblebee — Joaquim Alves Gaspar, Public domain, via Wikimedia Commons

This species is a nectar source to other butterflies, skippers, bees, and wasps during the growing season. Carpenter bees (Xylocarpa californica) were found to be the most frequent pollinator of this plant, followed by a bumblebee (Bombus vosnesenskii) (Lynch 1977).

Frequently Asked Questions

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 list this species but, milkweeds in general have been used for pharmaceutical uses, fibers, and foods.

How is this plant distinguished from other milkweeds?

This milkweed is similar to the California milkweed (Asclepias californica) and the Pallid milkweed (Asclepias cryptoceras), but can be separated from both by the much smaller corolla lobes that are 5 mm. In addition, serpentine milkweed grows in a very distinct habitat.

Is this plant invasive?

This plant is quite rare in the state of California and has very specific habitat requirements. It is not invasive.

Gardening with Serpentine Milkweed

Plants of serpentine milkweed (Asclepias solanoana) in rocks.
Plants of Serpentine Milkweed in rocks — Asclepias solanoana Woodson observed in United States of America by Scott Yarger (licensed under CC0 1.0)

Hardiness

This species is hardy in zones 7-9. 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 places of full sun and a specific geology, of ultramafic rock called serpentine.

A Note about this Plant

This plant is rare and endemic to California. Even though it is a native milkweed to California, it is best to leave it to its own habitat and out of cultivation.

References

  • Greene, Edward Lee. 1896. On the Classification of the Asclepiads. Pittonia (Berkley, CA: Doxey and Co.).
  • Lynch, Steven P. 1977. The Floral Ecology of Asclepias solanoana Woods. Madrono 24: 159-177.
  • Woodson, Robert E. 1941. The North American Asclepiadaceae. I. Perspective of the Genera. Annuals of the Missouri Botanical Garden 28: 193-244.
  • Woodson, Robert E. 1954. The North American Species of Asclepias L. Annals of the Missouri Botanical Garden 41: 1-211.
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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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