Information Last Reviewed June 2007
Family: Asteraceae
Symbol: CIAR4
Home Range/U.S. Introduction:
Canada thistle is native to Eurasia. It has become established in all but the southeastern part of the United States.
U.S. Range Map:
Species Descriptions:
Canada thistle is a dioecious perennial that spreads from deep horizontal lateral roots bearing adventitious shoots. Stems may reach up to 10 dm tall, branching near the tup. The leaves are alternate, oblong to oblanceolate, usually lobed with the margins bearing fine to strong spines. The leaf bases are sessile, clasping to short decurrent. The blades are glabrous to pubescent. The inflorescence is a head of unisexual flowers occurring in loose clusters terminating the branches. The flowers are pink to purple, rarely white. The involucral bracts are 1-2 cm, at most with a weak spine tip ca. 1 mm long. The pappus is composed of plumose bristles united in a ring at the base.
Growth characteristics:
Canada thistle is dioecious with male and female flowers on separate plants. It flowers from Jun-Aug. It occurs in pastures, ditches, bottomlands and other moist areas.
Problems:
The plant is difficult to control because it spreads aggressively from horizontal roots. Plowing also breaks the root into fragments each of which can develop into a new plant.