Fourleaf, or whorled milkweed, is a slender, single-stemmed perennial with round clusters of usually pink flowers. Flowers are technically in loose umbels, either upright or drooping, from 1 to 3 umbels per plant, light pink or cream-colored, nicely fragrant. Blooms May–July. Leaves opposite or whorled. There are 3 or 4 sets of leaves, of which 1 or 2 of the upper sets has 4 leaves in a whorl, the other sets with 2 leaves. The leaves are broadly lanceolate, pointed at both ends. Sap is milky white.
Similar species: Another Missouri milkweed, Asclepias verticillata, is also called whorled milkweed. It looks much different, bearing 3–6 soft, threadlike leaves per whorl and has white or greenish-white flowers. It grows in upland prairies, fields, glades, and is not commonly found in the woods.
Height: normally 12–18 inches, but occasionally taller.
Scattered nearly statewide, but apparently absent from the Mississippi Lowlands of the Bootheel and from the western portion of the Glaciated Plains (northwestern Missouri).
Habitat and Conservation
Occurs in open, dry, or rocky woods, usually on upland slopes. Most of our more familiar milkweed species are more robust plants that favor prairies, pastures, and other more open places.
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. They are increasingly popular in native plant gardens, because people are wanting to help the declining populations of monarch butterflies, which use milkweeds as their larval food plants.
The cardiac glycosides and other chemicals in the milky sap (latex) are unpalatable and toxic, so few herbivores eat milkweeds. The larvae of monarch butterflies, however, use milkweeds as a food plant. They store the toxins in their bodies, rendering them unpalatable to predators.