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.