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Noxious Weed!Canada Thistle (Cirsium arvense)
Sunflower Family (Asteraceae)


Noxious Weed: Canada Thistle
University of Idaho


Description:  Dioecious perennial from deep creeping rootstocks. Stems 1 to 4 feet, rigid, smooth to slightly hairy, branched at top. Leaves variable light to dark green, sessile, oblong or lance shaped, divided into spiny tipped lobes. Either male or female flowers present, light lavender to purple, 3/8 to 3/4 inch in diameter, in several to many headed clusters. Phyllaries spineless. Seeds 1/8 inch long, brownish, with a protrusion at one end, and a tuft of haris at the top.

Habitat:  Native to Europe. Invades cropland, meadows, pastures, roadsides, clearcuts, and waste areas. The extensive underground root system can penetrate the soil as deep as 10 feet and grow laterally 12 to 15 feet a year. Randomly spaced root buds can develop new shoots under the right environmental conditions.

Distribution:  Canada thistle is fairly widespread throughout northeastern California and can be seen along most waterways. The largest infestations occur on the Modoc National Wildlife Refuge (Modoc County), Lower Klamath National Wildlife Refuge (Siskiyou County), and Honey Lake Refuge (Lassen County). The moist conditions at these refuges are ideal habitat for this species.

Flowering Period:  July to August.


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