Table of Contents for Rusby’s Milkweed (Asclepias rusbyi)
Rusby’s Milkweed (Asclepias rusbyi) is a herbaceous perennial that is native to the southwestern United States, except California and extends south into Mexico. This plant is a host to the Queen (Danaus gilippus) and Monarch (Danaus plexippus) butterflies. Growing from 0.5 to 3.5 feet tall, this species grows on rocky hills, oak woodlands, and fields. The greenish-purple tinged flowers bloom from June to August and the plant is hardy in zones 6-10.
Taxonomy and Naming of Rusby’s Milkweed (Asclepias rusbyi)
Taxonomy
Rusby’s Milkweed (Asclepias rusbyi) was originally named and described by Anna Murray Vail, an American botanist in 1898 as Acerates rusbyi. Later in 1941, Robert Woodson, moved the Acerates genus into Asclepias. In 1949, Asclepias rusbyi, was placed as a variety of Asclepias engelmanniana by Thomas Kearney (Rintz 2014). However in 1954, Robert Woodson raised it back to the species level. It has kept this same name since 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, rusbyi, is in honor of Henry Hurd Rusby, another American botanist, who collected the original specimen (Heil 2013).
Common Name and Alternative Names
The common name comes from the last name of Henry Hurd Rusby.
Physical Description
- Plant Type: This plant is a herbaceous perennial.
- Height: 0.5 to 3.5 feet tall
- Stem: The stems are slender to caespitose (Woodson 1954).
- Leaves: The leaves are approximate, sessile, linear, and are 3.5 to 6 inches long and 0.1 to 0.3 inches wide. The leaves are somewhat succulent, glaucous, glabrous (Woodson 1954) and reflexed (Heil 2013).
- Flower color: pale green with a purplish tint (Woodson 1954) or yellowish-green (Heil 2013)
- Blooming period: This plant blooms from May to July.
- Fruiting type and period: This plant has follicles that mature in the late summer and fall.
Range of Rusby’s Milkweed in the United States and Canada
This milkweed species is native to the southwestern United States, except for California. Its range extends south into Mexico.
Habitat
This species grows in open oak forests (Woodson 1954) and pinyon-juniper communities (Heil 2013).
Hosted Insects
This species is a host for the Monarch Butterfly (Danaus plexippus), the Queen Butterfly (Danaus gilippus).
Other Supported Wildlife
This species is a nectar source to other butterflies, skippers, bees, and wasps during the growing season.
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 cite this species specifically, but milkweeds in general have been used for pharmaceuticals, fibers, and foods.
How is this plant distinguished from other milkweeds?
This species as the taxonomy treatment shows is most similar to Engelmann’s milkweed (Asclepias engelmanniana). However, Engelmann’s milkweed has flattened hoods and Rusby’s milkweed hoods are more spreading. There are other differences between the two, but the above is the easiest to determine. The anthers have been described as conic in A. rusbyi and spheriodal in A. engelmanniana (Heil 2013).
Is this plant invasive?
This plant has not been noted as being weedy.
Gardening with Rusby’s Milkweed
Hardiness
This species is hardy in zones 6-10. 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 can grow in full sun to partial-shade in well-drained soil.
References
- Heil, Kenneth D., Steve L. O’Kane, Linda Mary Reeves, and Arnold Clifford. 2013. Flora of the four corners region: vascular plant of the San Juan River drainage, Arizona, Colorado, New Mexico, and Utah. Monographs in Systematic Botany v. 124.
- Rintz, R.E. 2014. A closer look at Asclepias engelmanniana Woodson and Asclepias rusbyi (Vail) Woodson (Apocynaceae, Asclepiadoideae). Phytologia 96: 241-246.
- Woodson, Robert. 1954. The North American Species of Asclepias L. Annals of the Missouri Botanical Garden 41: 1-211.
- Wooten, E.O. and Paul Standley Carpenter. 1915. Flora of New Mexico. Contributions from the United States National Herbarium v. 19.
