Illinois Exotic Species:
Canada thistle Cirsium arvense

Canada thistle, which is native to Europe, is common in the northern half of Illinois and rare in the southern half. This plant is a noxious weed under Illinois law. Canada thistle may attain a height of five feet. It has deep, wide-spreading, horizontal roots. Its grooved, slender stems branch only at the top and are slightly hairy when young, becoming covered with "hair" as the plant grows. The oblong, tapering, sessile leaves are deeply divided, with prickly margins. Leaves are green on both sides with a smooth or slightly downy lower surface. Numerous small, compact, rose-purple or white flowers appear on the upper stems from June to September. Seeds are small, light brown, smooth and slightly tapered, with a tuft of tan hair loosely attached to the tip.

Canada thistle thrives in disturbed areas such as overgrazed pastures, old fields, waste places, fence rows and along roadsides. It sometimes occurs in wet areas where water levels fluctuate (along stream banks and ditches). It can invade sedge meadows and wet prairies from adjacent disturbed sites. This thistle does not do well in undisturbed prairies, good pastures or in woodlands. Plants grows in patches. Introduction to new areas occurs mostly by seed. It spreads rapidly by rhizomes (underground stems) or root segments. Lateral roots three or more feet deep spread from a fibrous taproot. Aerial shoots are sent up at two- to six-inch intervals. Basal leaves are produced the first year, flowering stems the next. Seeds remain viable in soil up to 20 years in some cases.

Canada thistle is an alien species capable of crowding out and replacing native grasses and forbs. It is detrimental to natural areas where it occurs, and it can change the natural structure and species composition where it becomes well-established.

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