This perennial wildflower is 1½-3′ tall, consisting of an unbranched stem with several pairs of opposite leaves. The erect central stem is light green to purplish green and more or less pubescent in either patches or lines. The leaves are 3-6″ long and 1-3″ across; they are oblong-ovate to oblong-obovate in shape and smooth along their margins. The upper leaf surface is medium green and glabrous, while the lower surface is pale green. The leaves taper abruptly to oblique tips, while their bases are either rounded or wedge-shaped. The petioles are ¼-1¼” long, light green to purplish green, and appressed-hairy. Leaf venation is pinnate. The central stem terminates in 1-4 umbels of white flowers on peduncles ½-2″ long. In addition to the terminal umbels, 1-2 axillary umbels of flowers may develop from the axils of the upper leaves, although this is uncommon. The peduncles of these umbels are light green to purplish green and short-pubescent. Individual umbels of flowers span about 2-3″ across, consisting of 15-35 flowers; the umbels are dome-shaped to nearly globoid. The flowers are usually densely arranged within the umbels, although in shaded situations the umbels can be more open.
Each flower spans about 8-10 mm. across, consisting of a short light green calyx with 5 teeth, 5 white petals, a corona with 5 white hoods, and a short central column containing the reproductive organs. Each flower usually has a narrow purple ring between the petals and the corona. The petals are obovate in shape and 6-8 mm. long; they are widely spreading to drooping. Each hood (about 4-5 mm. in length) contains an exerted slender horn that bends toward the center of the flower. The pedicels of the flowers are about 1-1½” long, light green to pale purplish green, and finely pubescent. The blooming period occurs from late spring to early summer, lasting about 3-4 weeks. The flowers are fragrant. Cross-pollinated flowers are replaced by lanceoloid seedpods (follicles) about 4-5″ long and ¾” across; their outer surfaces are smooth and downy. During the late summer or fall, each seedpod splits open along one side to release its seeds. The seeds have tufts of white hair and they are distributed by the wind. The rhizomatous root system can produce small colonies of clonal plants.
The preference is partial sun to light shade, dry-mesic conditions, and soil containing clay-loam, loam, or rocky material. Most growth and development occurs during the spring after the danger of hard frost has passed.
The native White Milkweed is occasional in southern Illinois, while in the rest of the state it is absent (see Distribution Map). Illinois lies along the NW range limit of this species. Habitats include upland savannas, barren rocky bluffs, upland rocky woodlands, wooded hillsides, rocky banks of streams, and woodland edges along roadsides. Occasional wildfires and other kinds of disturbance are beneficial if they reduce competition from woody vegetation. This wildflower is found in average to high quality natural areas.
Little is known specifically about the floral-faunal relationships of White Milkweed, although it is probably similar to other milkweeds that are found in wooded areas. The nectar of the showy flowers attracts butterflies, skippers, and possibly moths; other likely floral visitors include various long-tongued bees and wasps. The Insect Table lists the various species that feed on the foliage, stems, plant juices, and other parts of milkweeds. White Milkweed is one of the food plants for the caterpillars of the Monarch butterfly (Danaus plexippus). Because the bitter white sap of the foliage contains toxic cardiac glycosides, it is avoided by mammalian herbivores.