Large-seed Milkweed
Asclepias macrosperma Eastw.
A sprawling Colorado Plateau milkweed of loose sand and dune country, only a few inches high but throwing up to twelve stems from a running rootstock, with crinkle-edged leaves, dense heads of small green and red flowers, and seeds large enough that the name refers to them.
Also called Eastwood’s Milkweed.
- Height
- 2 in–6 in
- Spread
- 8 in–1.7 ft
- Flowers
- green, red, yellow
- Bloom
- April to June
- Light
- Full sun
- Soil moisture
- Dry
- USDA zones
- 5–8
- Leaves
- Opposite, lanceolate
- Sap
- Milky latex
- Roots
- Rhizomatous, spreads by root
- Stratification
- 30 days cold-moist
- Range
- 4 states
When it flowers
Large-seed Milkweed blooms April to June, peaking in May.
How to identify it
- Low and decumbent, 6 to 15 cm, with up to twelve stems from one rootstock, densely covered in short curved hairs.
- Leaves lanceolate to lance-ovate, 2.5 to 7 cm, short-stalked, with distinctly crisped or wavy margins.
- Dense stalkless terminal clusters of 12 to 40 small flowers; green corolla tinged red, especially underneath.
- Hoods uniformly yellow to buff, not cream with a dark stripe down the back. That stripe is what separates dwarf milkweed.
- Seeds 8 to 12 mm, very large for a milkweed, in rough ovoid pods. The name macrosperma means large-seeded.
Unfamiliar words are in the glossary, and the identification key will narrow a plant down from what you can see.
The parts that identify it
Ecology and wildlife value
Confined to the Colorado Plateau, and unlike its close relatives it keeps to loose, often unstabilised sand. Flowering in April to June makes it available to monarchs moving north early, well before the summer species open.
Monarchs are not the only thing that will find it. Expect large milkweed bugs on ripening pods, oleander aphids on the stems, red milkweed beetles at the flowers and, in late summer, milkweed tussock moths. Everything that lives on milkweed.
Growing large-seed milkweed
Not in cultivation. It needs deep moving sand and will not take garden soil.
Seed needs about 30 days of cold-moist stratification. Rhizomatous, spreads by root. See how to plant milkweed seed for the full method.
Cautions
Of conservation concern in Colorado, where it is known from Montezuma County only, and in New Mexico, where it is known from San Juan County only. Secure in Utah and on Navajo land in Arizona. Do not collect. More on milkweed toxicity.
Where large-seed milkweed grows
Recorded in 4 states: Arizona, Colorado, New Mexico, Utah.
USDA PLANTS holds no county-level records for this species, so the map above is generalized to whole states. For what county data exists see USDA PLANTS and BONAP.
Frequently asked questions
How is large-seed milkweed different from dwarf milkweed?
They were long treated as one species and intermediates occur where the two ranges touch. Large-seed milkweed has broader leaves with crinkled margins, and its hoods are evenly yellow; dwarf milkweed (Asclepias involucrata) has cream hoods with a dark stripe down the back. Habitat helps too: large-seed milkweed grows on loose sand, dwarf milkweed on stable rocky or clay ground.
References and further reading
Names, ranges and conservation status here are drawn from the databases below. Where those disagree with a regional flora, trust the flora.
- Plants of the World Online 21236-2. Royal Botanic Gardens, Kew. Accepted name, authorship and global distribution.
- GBIF 3170244. Global Biodiversity Information Facility. Aggregated occurrence records.
- iNaturalist 148083. Community observations, photographs and phenology.
- Wikipedia. General-audience article on this species.
- BONAP county distribution map. Biota of North America Program. County-level range map.
- NatureServe Explorer. Conservation status ranks at global and state level.
- World Flora Online. Nomenclature and synonymy.
- Woodson, R. E. (1954). The North American species of Asclepias L.. Annals of the Missouri Botanical Garden 41(1): 1–211. The monograph the modern taxonomy of the genus still rests on. View
- Flora of North America Editorial Committee (eds.) (2026). Flora of North America North of Mexico. Oxford University Press, New York and Oxford. Accepted names, keys and descriptions for the North American flora. View
- Royal Botanic Gardens, Kew (2026). Plants of the World Online. Facilitated by the Royal Botanic Gardens, Kew. Nomenclature and global distribution, keyed to IPNI name identifiers. View
- Kartesz, J. T. (Biota of North America Program) (2026). North American Plant Atlas. BONAP, Chapel Hill, North Carolina. County-level distribution maps, the basis for the ranges generalized here. View
- USDA, NRCS (2026). The PLANTS Database. National Plant Data Team, Greensboro, North Carolina. State-level distribution, wetland status and nomenclature. View
- Agrawal, A. A. (2017). Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution. Princeton University Press. The standard general account of the milkweed and monarch relationship. View
Cite this page
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Milkweed USA. (2026). Large-seed Milkweed (Asclepias macrosperma). Retrieved DATE, from https://milkweedusa.com/species/large-seed-milkweed/
"Large-seed Milkweed (Asclepias macrosperma)." Milkweed USA, 2026, https://milkweedusa.com/species/large-seed-milkweed/. Accessed DATE.
Milkweed USA. "Large-seed Milkweed (Asclepias macrosperma)." Last modified 2026-08-01. Accessed DATE. https://milkweedusa.com/species/large-seed-milkweed/.
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title = {Large-seed Milkweed (Asclepias macrosperma)},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/species/large-seed-milkweed/},
note = {Accessed DATE}
}