Milkweeds are various species of perennial plants in the family Asclepiadaceae, a mostly tropical group that contains more than 1,800 species. Most species in this family are herbaceous, but others are woody climbers, shrubs, or small trees. The foliage and stems of milkweeds are often succulent, and when
they are broken they weep a white, milky latex from which the common name of these plants is derived.
The most common milkweeds in North America are species in the genus Asclepias such as A. syriaca, the common milkweed. This species is the principal food of one of North America’s best-known butterflies, the monarch or milkweed butterfly (Danaus plexippus). The monarch lays its eggs on A. syriaca and other species of milkweeds, and its larvae feed on their foliage. The monarch is a brightly orange-colored butterfly that flies slowly and might seem to be an easy prey for insect-eating birds and other predators. However, the adult and larval monarchs taste terrible because they contain chemicals obtained from the milkweeds while feeding. As a result, the monarch is avoided as food by most predators. An unrelated butterfly, the viceroy (Limenitis archippus), tastes fine but it is rarely eaten by most predators because it closely resembles the monarch in shape, color, and behavior. The milkweed-tainted monarch is the model in this system, while the viceroy is the mimic.
Some milkweeds have very attractive flowers and may be grown as ornamentals. The orange-colored flowers of the butterfly weed (A. tuberosa) are a beautiful aspect of the taller-grass prairies of North America, and this species is sometimes grown in gardens.
The latex of milkweeds and other species in the family Asclepiadaceae can be used to make a natural rubber. During World War II (1939–1945), when supplies of rubber from Asia were not readily available in North America, research was undertaken to see whether this strategically important material could be obtained from milkweeds under cultivation. However, the yields of rubber were too small to make this enterprise worthwhile.
Some people like to gather the young, partially developed shoots or the seed pods of milkweeds in the late springtime before they have developed to the stage that they contain much of the milky latex. The shoots or pods are steamed, dressed with butter, and eaten as a tasty and nutritious vegetable.