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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 perennial herb growing up to 2.6 m tall from a rhizome. All parts of common milkweed plants produce white latex when broken. The leaves are opposite or sometimes whorled; simple, broad ovate-lanceolate; up to 25 cm long and 12 cm broad, usually with undulate margins and reddish main veins. They have very short petioles and velvety undersides.
The fragrant, nectariferous flowers 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.
The plant's latex contains large quantities of glycosides, making the leaves and seed pods toxic to sheep and other large mammals, and potentially humans (though large quantities of the foul-tasting parts would need to be eaten). The young shoots, young leaves, flower buds and immature fruits are all edible raw.
Concerns about milkweed bitterness and toxicity can be traced back to Euell Gibbons, author of Stalking the Wild Asparagus (1962). It is theorized that Gibbons inadvertently prepared dogbane (Apocynum cannabinum), a poisonous look-alike instead. He devised a method to remove the bitterness and toxicity by plunging the young shoots into boiling water (not cold) and cooking for one minute, repeating the procedure at least three times to make the plant safe to eat. Gibbons' method was copied from book to book, dominating edible plants literature for forty years. Most modern foragers consider the bitterness/toxicity issue a myth. The plants have no bitterness when tasted raw, and can be cooked like asparagus, with no special processing.
Failed attempts have been made to exploit rubber (from the latex) and fiber (from the seed's "floss") production from the plant industrially. The fluffy seed hairs have been used as the traditional background for mounted butterflies. The compressed floss has a beautiful silk-like sheen. The plant has also been explored for commercial use of its bast (inner bark) fiber which is both strong and soft. U. S. Department of Agriculture studies in the 1890s and 1940s found that Milkweed has more potential for commercial processing than any other indigenous bast fiber plant, with estimated yields as high as hemp and quality as good as flax. Both the bast fiber and the floss were used historically by Native Americans for cordage and textiles. Milkweed oil from the seeds can be easily converted into cinnamic acid, which is a very potent sunscreen when used at a 1-5% concentration.
Many insects live on the plant, including the red milkweed beetle (Tetraopes tetrophtalmus), the large milkweed bug (Oncopeltus fasciatus), the small milkweed bug (Lygaeus kalmii), the milkweed leaf beetle (Labidomera clivicollis), the weevil species Rhyssomatus lineaticollis, and the monarch butterfly (Danaus plexippus). The larva of the monarch butterfly specializes on milkweeds, and its populations may fall when milkweeds are eliminated with herbicides.
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 elsewhere in the northeastern United States, monarchs prefer to reproduce on Asclepias syriaca, especially when its foliage is soft and fresh. As monarch reproduction in that area peaks in late summer when Asclepias syriaca leaves are old and tough, the plant needs to be cut back at least by half in June to assure that it will be regrowing rapidly when monarch reproduction reaches its peak. In addition, Asclepias syriaca seed needs a period of cold treatment before it will germinate.
Deforestation due to European settlement may have expanded the range and density of milkweed. The plant can become invasive; it is naturalized in several areas outside of its native range, including Oregon and parts of Europe. However, more recently, 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.
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