Description: This perennial herbaceous plant is 2-3' tall and unbranched. The erect central stem is relatively stout, terete, light green, and glabrous. Pairs of opposite leaves occur along the entire length of the stem; they are widely spreading to ascending (usually the latter). These leaves are 3-6" long and 1½–3" across; they are broadly oblong to ovate-oblong in shape, entire (toothless) and sometimes wavy along their margins, and either sessile or short-pedicellate. The leaf bases are rounded, while the tips of leaves are round or obtuse, tapering abruptly to a short narrow point. The upper and lower surfaces of the leaves are light-medium green and glabrous; the leaf texture is slightly fleshy. Leaf venation is pinnate; the prominent central veins are light green, pink, or purple. Each fertile plant develops 1-3 umbels of flowers (rarely more) from the apex of the central stem and the axils of the uppermost leaves. These umbels span 2–3½" across and they are slightly dome-shaped, consisting of 15-25 flowers each (rarely more).
Each flower is about ½" across and ¾" long, or slightly smaller in size, consisting of 5 sepals, 5 petals, 5 hoods with horns, and a central reproductive column that is whitish. The sepals are light green to pinkish purple, lanceolate-ovate in shape, and glabrous. The petals are mostly pink to purplish pink (although white at their bases), lanceolate in shape, and declined (bent downward). The hoods are erect, open-tubular in shape, and pink to purplish pink. Each hood has an exserted horn that is sickle-shaped and incurved. The pedicels of the flowers are 1–1½" long, light green to reddish purple, and glabrous. The peduncles of the umbels are ½–3" long, light green, and terete. The blooming period occurs during early to mid-summer, lasting about 1 month. The flowers have a pleasant fragrance. Afterwards, cross-fertilized flowers (if any) are replaced by erect to ascending follicles (seedpods that open along one side) that are 3-4" long and 1–1¼" across at maturity. These follicles are light green (while immature), lanceoloid in shape, and smooth to bluntly warty. At maturity, the follicles split open to release their seeds to the wind. The seeds are brown, flattened-ovate in shape, and narrowly winged along their margins; they have large tufts of white hair at their apices. The root system is fleshy and rhizomatous, occasionally producing clonal offsets.
Cultivation: The preference is full sunlight, moist to mesic conditions, and a rich loamy soil. While it can spread by means of its rhizomes, this plant is far less aggressive than Common Milkweed (Asclepias syriaca). During dry weather, some of the lower leaves may turn yellow and fall off the plant. This also happens when the seedpods develop, which is quite normal. Active growth occurs during the late spring and early summer.
Range & Habitat: The native Prairie Milkweed is uncommon to occasional in the majority of counties in Illinois, but it is rare or absent in many counties of southern and NW Illinois (see Distribution Map). Habitats include black soil prairies, cemetery prairies, prairie remnants along railroads, moist meadows along rivers or near woodlands, thickets, and roadside ditches. Prairie Milkweed is an indicator plant of average to high quality prairies.
Faunal Associations: Various insects visit the flowers of Prairie Milkweed for nectar, including bumblebees, cuckoo bees (Epeolus spp.), leaf-cutting bees (Megachile spp.), Halictid bees (including green metallic bees), Halictid cuckoo bees (Sphecodes spp.), sand wasps (Bembix spp.), Sphecid wasps, Ichneumonid wasps, thick-headed flies (Conopidae), Tachinid flies, flesh flies (Sarcophagidae), butterflies, skippers, moths, and ants (Robertson, 1929). The Ruby-throated Hummingbird also visits the flowers for nectar. Among the various visitors, bumblebees and other long-tongued bees are the most effective in cross-pollinating the flowers. Other insects feed on the roots, stems, leaves, flowers, and seedpods of Prairie Milkweed and other milkweeds (Asclepias spp.). The larvae of one species, the Milkweed Leaf-miner Fly (Liriomyza asclepiadis), tunnels through the leaves of Prairie Milkweed (Betz et al., 1997). Other insects that feed on milkweeds include long-horned beetles (Tetraopes spp.), the Swamp Milkweed Leaf Beetle (Labidomera clivicollis), the Milkweed Stem Weevil (Rhyssomatus lineaticollis), the Small Milkweed Bug (Lygaeus kalmii), the Large Milkweed Bug (Oncopeltus fasciatus), the Yellow Milkweed Aphid (Aphis nerii) and other aphids, caterpillars of a moth, the Delicate Cycnia (Cycnia tenera), and caterpillars of a butterfly, the Monarch (Danaus plexippus). The Insect Table provides additional information about these species. Because the foliage of Prairie Milkweed contains a white latex that is bitter-tasting and toxic, mammalian herbivores avoid consumption of this plant.
Photographic Location: The photographs were taken at the Loda Cemetery Prairie in Iroquois County, Illinois.
Comments: Prairie Milkweed (Asclepias sullivantii) somewhat resembles Common Milkweed (Asclepias syriaca) in appearance, but the former species has flowers that are slightly larger in size and its leaves are hairless on their undersides. Prairie Milkweed is usually a shorter plant than Common Milkweed, and it produces fewer umbels of flowers from the axils of its leaves. Another similar species is Purple Milkweed (Asclepias purpurascens). This latter species differs by having seedpods that are always smooth (rather than bluntly warty) and it has short hairs on the undersides of its leaves. In addition, the flowers of Purple Milkweed are slightly smaller in size than those of Prairie Milkweed, and they are usually more purple. Sometimes Prairie Milkweed has difficulty in forming seedpods because many flower-visiting insects are not very effective in removing and transferring pollinia from one plant to another. In addition, it is not uncommon for some of these insects to become entrapped on the flowers and unable to escape. Another common name of this plant is Sullivant's Milkweed.