The Lady Slipper newsletter of the Kentucky Native Plant Society has been published since the Society’s founding in 1986. We occasionally feature an article from a past issue. This one, about Kentucky’s most widespread milkweed species, Common Milkweed (Asclepias syriaca), first appeared in the fall of 2011, Vol. 26, No. 3. If you would like to see other past issues, visit the Lady Slipper Archives, where all issues from Vol. 1, No. 1, February 1986 to Vol. 34, No. 1, Winter/Spring 2019 (after which we moved to this blog format) can be found.
Kentucky’s Common Milkweed (Asclepias syriaca L.)
By David Taylor, US Forest Service
Common milkweed is a perennial forb that spreads by means of rhizomes and seed. It is one of about 115 species that occur in the Americas. Most species are tropical or arid land species. Plants may occur as a few individuals, but once established, form small to large colonies. Individual plants range from 1 to 2 m (~ 3 to 6 ft) tall. Leaves are elliptic to ovate to oblong and somewhat thick. Mature leaves are 15-20 cm (6-8 in) long and 5 to 9 cm (~ 2 to 3.6 in) wide, with a prominent midvein. The underside of the leaf is frequently finely pubescent. The stem is stout, usually simple, and green to black (see below) in color. When broken, the leaves, as well as stem and fruit, exude milky latex. Flowers are purplish to rosy pink to mostly white or even greenish and about 2 cm (0.75 in) long and 1 cm (0.4 in) wide. They occur in semi-spherical umbels (umbrella-like clusters) in the upper leaf axils. Flowers are somewhat complex in their structure, with structures not found in the average flower. The flowers are strongly and sweetly scented.
The fruits (pods), known as follicles, are formed from the union of multiple flowers. They are green, covered in soft spiky projections and are finely pubescent. When the seeds are mature, the follicle splits exposing the seeds. Each seed is equipped with a coma, a soft group of hairs. As the newly exposed seeds dry, the hairs of the coma expand allowing the seed to catch a ride on the wind. When broken, the leaves, as well as stem and fruit, exude milky latex.
Common milkweed is a widespread and somewhat weedy species. It is known from most of the eastern U.S 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.
The genus name, Asclepias, commemorates Asklepios, the Greek god of medicine. Some of the species have a history of medicinal use including common milkweed (wart removal and lung diseases) and butterfly weed (aka pleurisy root— pleurisy and other lung disease). The specific epithet, syriaca, means ‘of Syria’ and is a misnomer: Linnaeus thought the species was native to Syria. This species is some times eaten as a salad herb, requiring multiple boilings of the young shoots before it is palatable. The reason for the boiling is to rid the shoots of various cardiac glucosides and other bitter principles. Milk weeds contain various levels of these compounds which render the plants toxic to most insects and animals. For some insects, the cardiac glucosides 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 species and are in fact toxic.
The stems contain a bast (inner ‘bark’) fiber used by Native Americans to produce twine and rope. The concentration and quality of the fiber make it potentially useful as a commercial fiber plant. Fiber quality is that of flax.
Common milkweed is an important pollinator and food plant for a large number of insects (more than 450 documented). It could be said that common milkweed is Nature’s mega food market for insects. Numerous butterflies, flies, bees, wasps, and beetles feed on the nectar and pollen produced by the flowers. Even hummingbirds will try, apparently unsuccessfully, to extract nectar. Aphids, especially the yellow-orange oleander or milkweed aphids (Aphis nerii), are commonly found on milkweeds including common milkweed. Large infestations of aphids can lead to formation of sooty mold on the plants which can turn the stems and leaves from green to gray to black. Two true bugs, the large milkweed bug (Oncopeltus fasciatus) and the small milkweed bug (Lygaeus kalmia) feed on the seeds, but the large milkweed bug is more often encountered. Large populations of either species can reduce the seed production potential of a colony of common milkweed by as much as 80-90%. The colorful (red with black dots) red milkweed beetle (Tetraopes tetraophthalmus) feeds on the leaves. The milkweed leaf beetle (Labidomera clivicollis), another orange-red and black beetle may feed on common milkweed but has a preference for swamp milkweed (A. incarnata). At least two caterpillars, the milkweed tussock caterpillar (Euchaetes egle) and the monarch butterfly (Danaus plexippus) feed on this plant. 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. Other palatable species mimic the toxic species and gain some protection as a result. A well known example is the viceroy butterfly (Limenitis archippus) which mimics the monarch .
For monarchs, common milkweed is among the most important food plants. It is the primary food plant for northern U.S. and southern Canada monarchs and is a major food plant for monarchs in the central and southern U.S. Monarchs migrating from the mountains of Mexico lay eggs on milkweed species in northern Mexico and the southern U.S. The butterflies that result from these eggs move further north in stages, with a change in species of milkweeds utilized as they move north. Common milkweed is the usual northern species. Monarchs can be helped by encouraging existing common patches of common milkweed 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.