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.