Common milkweed, Asclepias syriaca L., [ASCSY, asclépiade de Syrie, cotonnier, petit-cochon] Perennial, reproducing by seed and by horizontally spreading underground roots which produce new leafy stems. Stems erect, 1 - 2 m (3 - 6½ ft) high, stout, unbranched or sometimes with 1 or 2 branches near the top, usually several stems close together from the underground root system; leaves opposite (2 per node) or whorled (3 or more per node), oblong with a rounded or tapered base and a rounded to somewhat pointed tip, without teeth, underside covered with fine velvety hair, upper surface usually without hair and deeper green; flowers in dense, nearly spherical clusters or umbels at tip of stem and from axils of upper leaves, each flower 8 -10 mm (1/4 2/5 in.) across, greenish to purplish or whitish, with 5 thin sepals and 5 larger petal lobes bent back along the flower stalk and an unusual arrangement of 5 hoods, and horns forming a crown or "corona" around the top of each flower. The flowers are  uniquely adapted for insect pollination, having waxy pollen in tiny wishbone-shaped structures which hook onto an insect's leg but come off when transferred to the flower of a different plant. Fruits at first green, fleshy, 7 - 10cm (2½ - 4 in.) long and ¼ to ½ as wide, covered with soft, warty protuberances, later turning brown, splitting lengthwise along a single opening and releasing numerous seeds: usually only 1 or 2 (rarely up to 5) seedpods develop from the many flowers of a single flower cluster: seed flat, oval, with a tuft of long silky hair at one end. The whole plant, root, stem, leaves. flowers and fruit, contain abundant, thick. white, milky juice.  Flowers from mid-June to August, and matures seed from August to October.

Common milkweed occurs throughout southern Ontario in pastures, meadows, waste places, roadsides and cultivated land. It is especially common in the Manitoulin Islands and the east-central portions of southern Ontario, but it seems to be increasing in most other portions of the province as well. Other species of milkweed have been found to be highly toxic to livestock, and circumstantial evidence suggests Common milkweed may, under some circumstances, also be toxic.

It is distinguished by its pairs of broad, oval, softly hairy leaves, umbels of purplish to whitish flowers with their peculiar arrangement of parts, and the large, thick, softly warty seedpods.

(Source: Ontario Ministry of Agriculture and Food, Publication 505, Ontario Weeds)