Whorled milkweed is an herbaceous perennial, usually unbranched but occasionally with a few branches toward the tip. Sap is milky. Flowers in the typical milkweed form, in small umbels arising from upper leaf nodes, white to greenish white, with 6–20 flowers per umbel. Blooms May–September. Leaves threadlike, soft, to 2 inches long, arising from many whorls on the stem, with 3–6 leaves per whorl. Fruits smooth, narrow pods less than 4 inches long, erect, bearing seeds that have a tuft of white hairs.
Similar species: Another Missouri milkweed, fourleaf milkweed (Asclepias quadrifolia) is also called whorled milkweed. It looks much different, with 2 or 4 lance-shaped to ovate leaves per node, and often has pinkish flowers. Also, it lives in open woods instead of upland prairies.
Height: 8–24 inches.
Scattered nearly statewide, but nearly absent from the Bootheel lowlands.
Habitat and Conservation
Occurs in upland prairies, savannas, glades, exposed ledges and tops of bluffs, and sometimes in dry upland forests; also pastures, roadsides, and railroads.
The entire former milkweed family (Asclepiadaceae) has recently been rolled into the dogbane family (Apocynaceae). For many years, botanists have known the two families were closely related. The milkweed group, with its distinct floral structures, is still considered a unique subfamily or tribe of the dogbane family. As you consult various sources, you can expect to see milkweeds grouped in either family.
Milkweeds have a long list of historical medicinal uses, and the milky sap (latex) was once explored as a potential source of rubber. Milkweeds are increasingly popular with native wildflower gardeners, because planting them can aid North America’s declining monarch butterfly populations.
Many bees, butterflies, and skippers drink nectar from the flowers, and crab spiders often hide in the clusters, hunting them. Monarch butterflies use milkweeds as larval food plants, collecting the sap's toxic cardiac gycosides in their bodies and becoming unpalatable to predators.