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.
Many kinds of insects visit A. syriaca flowers, and some kinds pollinate them, including Apis mellifera (Western honey bees) and Bombus spp. (bumble bees). 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 (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.
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. Common milkweed new, tender leaves are preferred monarch oviposition sites and larval food. 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. Second, one can cut large shoots to about half their height in June and July before or after they bloom. Third, one can cut large shoots to the ground in June and July. Cut plants often produce new shoots from their rhizomes. 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[when?] 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.
Growing A. syriaca in a garden
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Plant A. syriaca in a sunny location with good drainage and moist soil. In a few years after one plants A. syriaca, it will start to spread by rhizomes. Simply remove any shoots that are in the wrong place in the garden, leaving at least a few in the garden. This species moves around in its habitat by cloning and sprouting from seeds. In one garden of about 3000 square feet in the Washington, D.C., area, this plant started as a small patch in the garden's southwest corner. In about 20 years, new shoots appeared off and on throughout the garden. Its manager pulled out hundreds of unwanted shoots from 2005–2015,[when?] leaving small ones in as monarch food in places where he did not want large shoots. Female monarchs frequently lay their eggs on small, tender shoots, and larvae favor such shoots as food. It is valuable to leave at least a few mature shoots in one's garden until early autumn because shoot leaves produce food used by rhizomes which produce shoots in the next year. The nonnative Aphis nerii (oleander aphid) can become abundant on milkweed shoots and stunt them. One can control this aphid by spraying soapy water on aphid groups, then immediately rinsing the groups with plain water. It might take more than one application to the same shoot on different days to control the aphids. The soapy water also kills aphid predators and parasites. However, the aphid outbreaks can be so large, that they can markedly harm milkweeds before their natural enemies control them. To make the soapy water, use 1 teaspoon of a dish detergent, used for hand-washing dishes, per quart of water. Asclepias syriaca seeds require a period of cold treatment before they germinate. Monarch Watch provides much information on rearing monarchs and their host plants.
The plant's latex contains large quantities of glycosides, making the leaves and follicles (often called "seed pods", although they are not botanical 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 common dogbane (Apocynum cannabinum), a poisonous somewhat similar-looking plant 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 and 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 seeds' "floss") production from the plant industrially. The fluffy seed hairs have been used as the traditional background for mounted butterflies and other insects. 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 common 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.
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