Taxonomy
- Kingdom: Plantae
- Clade: Angiosperms
- Clade: Eudicots
- Order: Gentianales
- Family: Apocynaceae (milkweed family)
- Subfamily: Asclepiadoideae
- Genus: Asclepias
- Species: A. verticillata (L.)
Common names
- Whorled milkweed
- Horsetail milkweed
Morphology
- Growth form: Herbaceous perennial, 0.5–1.5 m tall.
- Stems: Erect, slender, and often branched, bearing a whorled arrangement of leaves.
- Leaves: Linear to narrowly lanceolate, 5–15 cm long, arranged in whorls of 3–7 around the stem.
- Flowers: Small, greenish‑yellow to orange‑yellow, borne in large, lax terminal umbels; each flower has a typical milkweed corolla with five reflexed lobes and a central corona of hoods and horns.
- Fruit: Pair of slender, cylindrical follicles up to 10 cm long, containing numerous seeds attached to silky comas for wind dispersal.
Phenology
- Flowering period: Late spring to early summer (May–July) in most of its range.
- Seed set follows flowering; follicles mature by late summer to early fall.
Distribution and habitat
- Native range: Central and eastern North America, from the Great Plains westward to the Mississippi River, and eastward to the Atlantic Coast, extending north into southern Canada (Ontario, Quebec).
- Typical habitats: Open, sunny sites such as prairies, glades, savannas, roadsides, and disturbed fields. Prefers well‑drained, sandy or loamy soils.
Ecology
- Pollination: Primarily by insects, especially butterflies (e.g., monarchs, Danaus plexippus) and various bees that seek nectar and pollen.
- Host plant: Serves as a larval food source for monarch butterfly (Danaus plexippus) and queen butterfly (Danaus gilippus) caterpillars, as with other milkweeds.
- Defense: Contains cardiac glycosides (cardenolides) that render tissues toxic to many vertebrate herbivores; these compounds are sequestered by specialist insects.
- Mycorrhizae: Forms associations with arbuscular mycorrhizal fungi, aiding nutrient uptake in nutrient‑poor soils.
Conservation status
- Global assessment: Not listed as threatened; classified as G5 (Secure) by NatureServe.
- Regional concerns: Populations may be locally scarce where habitat loss (agricultural conversion, urban development) has reduced open grassland ecosystems.
Human uses
- Horticulture: Occasionally cultivated in native plant gardens and pollinator‑friendly landscapes for its milky sap and attracting butterflies.
- Ethnobotany: Limited documented traditional uses; some Native American groups reportedly used the latex for medicinal purposes, though specific applications are not well documented.
Potential hazards
- The latex and foliage contain toxic cardiac glycosides; ingestion can cause nausea, vomiting, cardiac irregularities, or more severe poisoning in humans and livestock. Handling may cause skin irritation in sensitive individuals.
References
- USDA, NRCS. “Plants Database: Asclepias verticillata”. https://plants.usda.gov/
- NatureServe Explorer. “Asclepias verticillata”. https://explorer.natureserve.org/
- Butterflies and Moths of North America (BAMONA). Species profile for whorled milkweed.
- Rydberg, P. (1993). Flora of North America (vol. 5). Oxford University Press.
All information presented is based on established botanical and ecological literature.
