AppearanceCommon 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 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.
DistributionDeforestation 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, in the last three decades mostly in the United States, milkweed populations have diminished dramatically due to factors such as increased habitat loss due to development, roadside median mowing, and herbicide use. Subsequently, this has played a significant part in the monarch butterfly's 90% population decline since the 1990 peak of 1B migrators as the milkweed can no longer survive among the corn and soybean rows.
HabitatMany kinds of insects visit ''A. syriaca'' flowers, and some kinds pollinate them, including ''Apis mellifera'' and ''Bombus'' spp. . 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. 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 , large milkweed bug , small milkweed bug , milkweed aphid , milkweed leaf beetle , milkweed stem weevil , milkweed tiger moth and monarch butterfly . Monarch larvae consume only milkweeds, and monarch populations may decline when milkweeds are eliminated with herbicides.
UsesThe plant's latex contains large quantities of glycosides, making the leaves and follicles toxic to sheep and other large mammals, and potentially humans. The young shoots, young leaves, flower buds and immature fruits are all edible raw.
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