Asclepias syriaca

Asclepias syriaca, commonly called common milkweed, butterfly flower, silkweed, silky swallow-wort, and Virginia silkweed, is a species of flowering plant. It is in the genus Asclepias, the milkweeds. This species is native to southern Canada and of much of the conterminous eastern U.S., east of the Rocky Mountains, excluding the drier parts of the prairies. It grows in sandy soils and other kinds of soils in sunny areas. It was one of the earliest North American species described in Cornut's 1635 work Canadensium Plantarum Historia. The specific name was reused by Linnaeus due to Cornut's confusion with a species from Asia Minor.

Common milkweed is a clonal perennial herb growing up to 2.6 m tall. Its ramets grow from rhizomes. All parts of common milkweed plants produce white latex when broken. The simple leaves are opposite or sometimes whorled; broad ovate-lanceolate; up to 25 cm long and 12 cm broad, usually with entire, undulate margins and reddish main veins. They have very short petioles and velvety undersides.


The highly fragrant, nectariferous flowers vary from white (rarely) through pinkish and purplish and occur in umbellate cymes. Individual flowers are about 1 cm in diameter, each with five cornate hoods and five pollinia. The seeds, each with long, white flossy hairs, occur in large follicles. Fruit production from selfing is rare. In three study plots, outcrossed flowers had an average of about 11% fruit set.

Ecology
Many kinds of insects visit A. syriaca flowers, and some kinds pollinate them, including Apis mellifera (Western honey bees) and Bombus spp. (bumble bees).[3][4] In the U.S. Mid-Atlantic Region, the introduced A. mellifera was the most effective and most important diurnal pollinator with regard to both pollen removal and pollen deposition.[5] However, when considering the self-incompatibility of A. syriaca, A. mellifera was not the most important pollinator because of its high self-pollination rate compared to Bombus spp. Additionally, the rate of self-pollination increased more rapidly with the number of flowers per inflorescence in A. mellifera than in native Bombus spp.

Many insect species feed on common milkweed, including the red milkweed beetle (Tetraopes tetrophtalmus), large milkweed bug (Oncopeltus fasciatus), small milkweed bug (Lygaeus kalmii), milkweed aphid (Aphis nerii), milkweed leaf beetle (Labidomera clivicollis), milkweed stem weevil (Rhyssomatus lineaticollis), milkweed tiger moth (Euchaetes egle) and monarch butterfly (Danaus plexippus). Monarch larvae consume only milkweeds, and monarch populations may decline when milkweeds are eliminated with herbicides.[6]

Efforts to increase monarch butterfly populations by establishing butterfly gardens require particular attention to the butterfly's food preferences and population cycles, as well to the conditions needed to propagate milkweed. For example, in the Washington, D.C., area and in the northeastern United States, monarchs prefer to reproduce on Asclepias syriaca, especially on young, soft leaves.[7] Common milkweed new, tender leaves are preferred monarch oviposition sites and larval food.[7] In the Washington, D.C., area, one can have such leaves in July, August, and early September during the main oviposition period in three ways. First, one can grow seedlings.[7] Second, one can cut large shoots to about half their height in June and July before or after they bloom.[7] Third, one can cut large shoots to the ground in June and July. Cut plants often produce new shoots from their rhizomes.[7] It is advisable to let some large, mature shoots remain in summer and fall because large monarch larvae, milkweed tiger moth larvae, and other native species feed on mature leaves. Milkweed bugs commonly feed on follicles. Monarch larvae can consume small seedlings to the ground. To save seedlings, one can transfer larvae from seedlings to larger shoots.


Deforestation due to European settlement may have expanded the range and density of common milkweed. This plant can become invasive; it is naturalized in several areas outside of its native range, including Oregon and parts of Europe. However, recently in the United States, milkweed populations have diminished dramatically due to factors such as development and the use of herbicides, which has played a significant part in the monarch butterfly's dramatic population decline.