Northern Ontario Plant Database
Asclepias syriaca L.
En: common milkweed, silkweed, silky swallowwort
Apocynaceae (Dogbane Family)
General: A perennial herb with erect stout stems, to 2 m tall; spreading by rhizomes. Stems, petioles, and pedicels are covered with short downy hairs. Warning: All parts of the plant have a milky-white sap that contains poisonous glycosides. The sticky, bitter sap deters most insect predators, but larvae of monarch butterflies are apparently immune and feed exclusively on milkweed, retaining the poisonous qualities of the sap as adult monarchs, thereby making them unpalatable to predators.
Leaves: Opposite, simple, large, and firm, measuring about 10—25 cm long and 5—12 cm wide, with a short stout petiole and many pinnately-arranged secondary veins. Leaf blades are oval to oblong; the base is rounded or slightly tapering to the petiole; the apex is blunt (obtuse) to rounded and narrows abruptly to a short mucronate tip; margins are entire. Blades are green and smooth (glabrous) to slightly hairy above, grayish-woolly below. The prominent midrib is usually reddish, at least on the upper surface.
Flowers: Numerous fragrant flowers are arranged in large, spherical, axillary inflorescences (umbels); each flower is attached to the top of the main flower stalk (peduncle) by a thin downy pedicel. The greenish-tinged, pink, or purplish flowers have 5 narrow sepals and petals, 6—9 mm long, valvate in bud, reflexed in flower, and downy on the outer surface. The pistil has 2 superior ovaries that share a common style and stigma, form an enlarged stylar head that is surrounded by 5 highly modified stamens. The filaments of the stamens are fused together to form a fleshy star-shaped corona divided into 5 segments, each consisting of a hood, 3—4 mm tall, with an incurved horn emerging from within the hood and arching over the sylar head; nectar collects at the base of the corona. Each of the 5 anthers has 2 waxy pollen masses (pollinia). Pollination occurs when an insect accidentally pushes one of the pollinia into the stigmatic area of the flower while sipping nectar.
Fruit: The dry pods (follicles) are 7—10 cm long, narrowly ovoid, and taper to a blunt tip. Developing pods turn from green to brown; the downy surface is covered with vertical rows of soft spine-like bumps (protuberances). When mature, the pod splits open (dehisces) along one side to release numerous flat seeds, each bearing a tuft of long silky hairs (the coma) that aid in wind dispersal.
Habitat and Range: Disturbed areas, such as roadsides, dry fields, rights-of-way, and railway margins. Common milkweed is native throughout most of eastern and central North American. In Ontario, it occurs throughout the Algoma District, and extends north to Timmins in the Cochrane District and west to southern portions of northwestern Ontario.
Similar Species: The only other milkweed species that occurs commonly in northern Ontario is swamp milkweed (Asclepias incarnata L.), which is easily identified by its narrower glabrous leaves, deep pink to reddish flowers, and narrower glabrous pods. As its name implies, swamp milkweed is found in wet habitats, particularly along stream or pond margins.
In northeastern Ontario, green milkweed (Asclepias viridiflora Raf.) has been collected only on the LaCloche Peninsula, near Manitoulin Island. Green milkweed grows on basic soils and can be recognized by its green flowers and narrow pods; its hairy leaves are similar to, but are narrower and more pointed at the apex, than those of common milkweed.Back to species list