A true American native, milkweed is indigenous to North America (and parts of north Mexico and the American tropics). Named for its milky sap, milkweed belongs to the genus Asclepias. In 1753, Swedish botanist Carl Linnaeus named the genus after Asclepius, the Greek god of healing, due to the many folk-medicinal uses for this plant. There are over 140 species of milkweed in the United States. Each species has its own unique leaf shape and flower characteristics. What they all have in common is their importance as the sole larval food source for the monarch butterfly.
Milkweeds share a unique pollen structure with the orchid family. Instead of bearing small grains of pollen, milkweed pollen is grouped into wishbone-shaped structures called pollinia (or "pollen sacs"). When a butterfly or pollinator slips its foot or mouthparts into one of five pockets in a flower (called an anther slit), the base of the pollen sac attaches to the insect. (Note the pollinia attached to the bee's foot, below). As the insect flies away, the pollen sac is pulled free from the flower. Smaller insects who snare a pollen sac can get trapped on the milkweed plant and subsequently starve to death.
Milkweeds produce their seeds in pod-like fruits called follicles. Seeds are arranged in overlapping rows, each bearing a cluster of white, silky, filament-like hairs known as the coma (also called pappus, floss, plume, or silk). When the follicles ripen and split open, the seeds, each carried by its coma, are blown by the wind (much like a dandelion seed).
In addition to nourishing all life stages of the monarch butterfly, milkweed feeds a variety of other insects including beetles, moths, and true bugs specialized to feed on the plant despite its chemical defenses. Milkweed flowers are an important nectar source for native bees, wasps, and other nectar-seeking insects. Milkweed is beneficial to nearby plants, repelling some pests, especially wireworms. However, milkweed is not just for insects. People through the ages have found uses for it.
Photo credits: Common milkweed flowers by Flickr user Fritz Flohr Reynolds under CC BY-ND 2.0 (cropped); bee with pollinia on feet by Flickr user wplynn under CC BY-ND 2.0; milkweed pollinia (three sets) on bee's leg by Flickr user USGS Bee Inventory and Monitoring Lab under Public Domain; milkweed follicle showing seeds by Natalie Gagnon by permission; milkweed follicle with seeds and floss by Flickr user USFWSmidwest under CC BY 2.0; milkweed fibers & cordage by Flickr user FreedomoftheHills by permission.