Department of Natural Resources
Canada thistle Cirsium arvense
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.
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.
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
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