Slender perennial with flowers in the form typical of milkweeds, borne in umbels (rounded clusters), usually on stout, single stalks; pale purple to reddish purple to dark purple, with greenish or red tints. Clusters typically begin rather pale and greenish; the purplish color deepens and intensifies with time. Blooms May–July. Leaves opposite, broad, ovate, pointed at both ends, to 6 inches long, with fine down on underside, on very short stalks. Fruit relatively narrow seedpods, to 6 inches long, with numerous, tightly packed seeds, released and windborne on silky floss.
Similar species: Common milkweed (A. syriaca) has paler flower clusters that arise more often from leaf axils along the stalk instead of from the top of the stem; its seed pods are covered with soft prickles instead of being smooth on the outside.