Common milkweed (Asclepias syriaca)

Description:

This native perennial plant is 2-6′ tall and unbranched. The leaves are dull green, up to 8″ long and 3½” wide, oblong or oval in shape, and mostly oppositely arranged on the central stem. There is a prominent central vein along the length of [Common Milkweed at Prairie Edge] each leaf, and finer side veins that radiate outward toward the smooth margins. When either the central stem or leaves are torn, a milky sap oozes out that has variable toxicity in the form of cardiac glycosides. Umbels of flowers, each about 3-4″ across, emerge from the axils of the upper leaves. These flowers are quite fragrant, with a scent resembling violets or pansies, and they range in color from faded light pink to reddish purple. Each flower is about ¼” across, with 5 reflexed petals that occasionally entrap the legs of insects, and 5 raised hoods with a horn arising in the middle. The blooming period lasts about a month from early to mid-summer. The seedpods are 3-4″ long, rather fat and covered with soft prickles, and they split along one side when mature to release numerous seeds that are individually equipped with large white tufts of hair. Dispersion of seed is by wind. The root system has long creeping rhizomes, promoting the vegetative spread of this plant.

Cultivation:

The preference is full sun, rich loamy soil, and mesic conditions, but this robust plant can tolerate a variety of situations, including partial sun and a high clay or sand content in the soil. Under ideal conditions, Common Milkweed can become 6′ tall and spread aggressively, but more typically it is about 3-4′ tall. This plant is very easy to grow once it becomes established.

Distribution Map:

Range & Habitat:

Common Milkweed occurs in every county of Illinois and is quite common (see Distribution Map). Habitats include moist to dry black soil prairies, sand prairies, sand dunes along lake shores, thickets, woodland borders, fields and pastures, abandoned fields, vacant lots, fence rows, and areas along railroads and roadsides. This plant is a colonizer of disturbed areas in both natural and developed habitats.

Faunal Associations:

The flowers are very popular with many kinds of insects, especially long-tongued bees, wasps, flies, skippers, and butterflies, which seek nectar. Other insect visitors include short-tongued bees, various Milkweed plant [Seedpods of Common Milkweed] bugs, and moths, including Sphinx moths. Among these, the larger butterflies, predatory wasps, and long-tongued bees are more likely to remove the pollinia from the flowers. Some of the smaller insects can have their legs entrapped by the flowers and die. Common Milkweed doesn’t produce fertile seeds without cross-pollination. The caterpillars of Danaus plexippes (Monarch Butterfly) feed on the foliage, as well as the caterpillars of a few moths, including Enchaetes egle (Milkweed Tiger Moth), Cycnia inopinatus (Unexpected Cycnia), and Cycnia tenera (Delicate Cycnia). Less common insects feeding on this plant include Neacoryphus bicrucis (Seed Bug sp.) and Gymnetron tetrum (Weevil sp.); see Insect Table for more common insect feeders). Many of these insects are brightly colored – a warning to potential predators of the toxicity that they acquired from feeding on milkweed. Mammalian herbivores don’t eat this plant because of the bitterness of the leaves and their toxic properties.

Comments:

Depending on the local ecotype, Common Milkweed is highly variable in appearance. The color of the flowers may be attractive, or faded and dingy-looking. This plant is often regarded as a weed to be destroyed, but its flowers and foliage provide food to many kinds of insects. Common Milkweed can be distinguished from other milkweeds by its prickly follicles (seedpods) – other Asclepias spp. have follicles that are smooth, or nearly so.