Milkweed family (Asclepiadaceae)
Description: This native perennial plant is 2-3' tall and unbranched. The stout central stem is light green and smooth. The thick opposite leaves are up to 6" long and 3½" across. They are broadly oblong, with smooth margins, and hairless. At the base, the leaves are sessile or have short petioles. The central vein of each leaf is pinkish or reddish, from which side veins radiate outward pinnately.
From 1-3 short-stalked umbels of pink flowers emerge from the axils of the upper leaves at the apex of the plant, containing up to 20 flowers each. Each flower is almost ½" across, with 5 strongly reflexed petals that are pink, but whitish near the base. There are also 5 erect pink hoods surrounding a central column that is white. The rather large buds, from which the flowers emerge, are initially green, but become pink. The blooming period is during early to mid-summer and lasts about a month. Afterwards, smooth green follicles develop that are up to 4" long and 1½" across. The upper half of a follicle may contain a few soft prickles, but to a much lesser degree than Asclepias syriaca (Common Milkweed). When mature, a follicle splits along one side and releases the seeds. Each seed has a large tuft of white hairs, and can be dispersed a considerable distance by the wind. The root system is fleshy and rhizomatous, and may occasionally send up vegetative offshoots.
Cultivation: The preference is full sunlight and moist to mesic conditions. A rich loamy soil is best. This plant is far less aggressive than Common Milkweed. During drought, some of the lower leaves may turn yellow and fall off the plant. This also happens when the follicles develop, which is quite normal. Active growth occurs during the late spring and early summer.
Range & Habitat: Prairie Milkweed occurs occasionally in the majority of counties in Illinois, but is uncommon or absent in southern and NW Illinois (see Distribution Map). This is a typical plant of moist to mesic black soil prairies. Elsewhere, it can be found in thickets, moist meadows along rivers or near woodlands, and along roadside ditches.
Faunal Associations: Various insects visit the flowers for nectar, including bumblebees and other long-tongued bees, short-tongued bees, predatory and parasitic wasps, ants, flies, skippers, and butterflies. Among these, long-tongued bees are the most effective in removing the pollinia and promoting cross-pollination. The Ruby-Throated Hummingbird also visits the flowers for nectar. The caterpillars of the butterfly Danaus plexippes (Monarch) feed on the foliage. Various aphids suck juices from the upper stems and leaves, including Aphis nerii (Yellow Milkweed Aphid), Aphis rumicis (Black Aphid), and Myzus persciae (Green Peach Aphid). This is one of the milkweeds that the larvae of Liriomyza asclepiades (Milkweed Leaf-Miner Fly) bore tunnels in the leaves. For more insect species feeding on this and other milkweeds, see Insect Table. Because the foliage contains a white latex that is bitter-tasting and toxic, mammalian herbivores avoid consumption of this plant.
Photographic Location: The photographs were taken at Loda Cemetery Prairie in Iroquois County, Illinois.
Comments: Prairie Milkweed resembles Common Milkweed in overall appearance, but the former has flowers that are slightly larger and usually more pink, the leaves are hairless on the undersides, the follicles are smoother, and it tends not to grow as tall. Individual plants of Prairie Milkweed often fail to form follicles because many insect visitors are not effective in removing the pollinia. Some of them become entrapped on the flowers. Another common name for this plant is Sullivant's Milkweed.