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Canada thistle (Cirsium arvense)
ODA rating: B 
USDA Symbol: CIAR4
Oregon Canada thistle distribution
Other common names
Californian thistle, Canadian thistle, creeping thistle, field thistle, corn thistle, perennial thistle
Click on photo below to view larger image
Image courtesy of Glen Miller, ODA 
Images courtesy of Eric Coombes, ODA
Image courtesy of Glen Miller, ODA
If images are downloaded and used from the ODA web site please be sure to credit the photographer.
Perennial; blooms midsummer. Leaves wavy margined to lobed, up to 6 inches long and armed with yellowish spines. Small purple to white flowers in clusters. Extensive deep seated root system spreading horizontally. Large seed production but low percentage of viable seeds.
Canada thistle can be found in cultivated fields, riparian areas, pastures, rangeland, forests, lawns, gardens, roadsides, and waste areas. A lack of control will result in a reduction in crop production of up to 25% in heavily infested ground. New infestation can be spread from seeds, but are more often caused by redistribution of roots by tillage practices.
Canada thistle is a native of southeastern Eurasia and was introduced to the US by accident in the early 1600s.
Distribution in Oregon
Occurs in every county in Oregon.

Biological controls
Four approved biocontrol agents, a stem weevil, a seed head weevil, a crown weevil, and a stem gall fly are established in Oregon.
Altica carduorum
Ceutorhynchus litura
Rhinocyllus conicus
Urophora cardui