Plant of the Week
Range map of Asclepias syriaca. States are colored green where the species may be found.
Whole plant with flowers. Photo by David Taylor.
Asclepias syriaca pods. Photo by David Taylor.
Monarch caterpillar on leaf. Photo by David Taylor.
Large milkweed bug adults and nymphs. Photo by David Taylor.
Tussock moth caterpillars. Photo by David Taylor.
Predated seeds. Photo by David Taylor.
Common Milkweed (Asclepias syriaca L.)
By David Taylor
Common milkweed is a member of the Asclepiadaceae (milkweed) family. It is one of about 115 species that occur in the Americas. Most species are tropical or arid land species. The genus name, Asclepias, commemorates Asklepios, the Greek god of medicine. Some of the milkweed species have a history of medicinal use including common milkweed (wart removal and lung diseases), and butterfly weed, A. tuberosa (also known as pleurisy root, used for pleurisy and other lung disease). The specific epithet, syriaca, means ‘of Syria’ in reference to Linnaeus's mistaken belief it was from Syria. It is a widespread and somewhat weedy species known from most of the eastern United States and the eastern most prairie states as well as southern Canada from New Brunswick to Saskatchewan. It is frequently found in fence rows, on roadsides, in fields, and in prairies and pastures. Given the opportunity, it will establish in gardens and even thin lawns. It is tolerant of light shade, but generally is a full sun species.
This milkweed grows to about 1.5 meters(5 feet) tall, usually occurring in clusters of stout stems. It has rhizomes and quickly forms colonies. Leaves are 15-20 centimeters (6-8 inches) long and 5-9 centimeters (2-3.6 inches) wide. They are somewhat thick with a prominent midrib beneath. The upper surface is light to dark green while the lower surface is lighter, almost white at times. Broken leaves and stems exude a milky latex. Flowers are borne in nearly spherical clusters (umbels) at the top of the plant, usually with 2-5 clusters per plant. Each flower is about 2 centimeters (0.75 inches) long and 1 centimeters (0.4 inches) wide. Flowers are greenish-pink to rosy pink to purplish-pink and very strongly and sweetly scented. Fruits (pods) are about 10 centimeters (4 inches) long, inflated and covered in little finger-like projections. They are green initially, turning brown as they mature. They split open revealing 50-100 seeds each with a white, fluffy coma ("parachute") that allows wind dispersal.
Common milkweed is Nature's mega food market for insects. Over 450 insects are known to feed on some portion of the plant. Numerous insects are attracted to the nectar-laden flowers and it is not at all uncommon to see flies, beetles, ants, bees, wasps, and butterflies on the flowers at the same time. Occasionally hummingbirds will try, unsuccessfully, to extract nectar. Its sap, leaves and flowers also provide food. In the northeast and midwest, it is among the most important food plants for monarch caterpillars (Danaus plexippus). Other common feeders are the colorful (red with black dots) red milkweed beetle (Tetraopes tetraophthalmus), the milkweed tussock caterpillar (Euchaetes egle) and the large (Oncopeltus fasciatus) and the small (Lygaeus kalmia) red and black milkweed bugs. The latter two are particularly destructive as both the adults and nymphs are seed predators. They can destroy 80 to 90 percent of a colony's seed crop. The red (or orange-red) and black coloration of most of these insects is known as aposematic coloration; that is, the colors advertise the fact that the organism is not good to eat.
Milkweeds contain various levels of cardiac glycoside compounds which render the plants toxic to most insects and animals. For some insects, the cardiac glycosides become a defense. They can store them in their tissue which renders them inedible or toxic to other animals. Monarch butterflies use this defense and birds leave them and the caterpillars alone. What the birds do not know is that northern monarchs feeding on common milkweed accumulate relatively little of the toxic compounds and probably would be edible. The more southern butterflies accumulate large amounts of the compounds from other milkweed species and are in fact toxic. Monarchs can be helped by encouraging existing patches and planting new ones. The plant grows readily from seed and spreads quickly by deep rhizomes. Because common milkweed can be weedy and difficult to remove, care should be used to establish the plant only in places where spread can be tolerated.