Scotch thistle, Onopordum acanthiurn L., [ONRAC, acanthe sauvage, Cotton thistle, White thistle] Biennial or sometimes annual, reproducing only by seed. Stems erect, tall, 1 - 2.4 m (3 - 8 ft) high, often much-branched, densely white woolly with broad, spiny, leaf-like wings; leaves of first-year rosette large (to 60 cm, 24 in. long and 30cm, 12 in. wide), lobed or coarsely toothed, densely white woolly (like a felt insole), margins wavy and spiny; stem leaves similar but smaller, alternate (1 per node), the margins of the leaf blades usually continuing down the stem as wings; flower heads at ends of branches and from leaf axils, large, 2.5 - 5 cm (1 - 2 in.) across, nearly spherical, surrounded by numerous, over-lapping, narrow, spine-tipped involucral bracts; ray florets absent; disk florets numerous,  purple, showy; seeds about 5 mm ( 1/5 in.) long, light brownish-gray to dark gray and wrinkled crosswise. Flowers from late June to September.

Scotch thistle occurs in scattered localities throughout southern Ontario in waste places, fence lines, and around old buildings, usually in gravelly soils where it has escaped from cultivation. It is distinguished from all other thistles in Ontario by the very dense, white woolly covering on stems and leaves.

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