Canada Thistle (Circium arvense)
Perennial forb with branched stem and spine-tipped leaves, growing to 2m in height. Bloom period is from June to October; flowers are purplish-pink, about 1cm in diameter, and in clusters of 1 to 5 per branch. Flowers have a vanilla scent. Seeds are 2mm long with a pappus of feathery white bristles. Leaves are spiny and lobed. Roots are both horizontal and vertical.
LRISS Category: STRATEGIC CONTROL
Habitat and Ecology
This species can be found in riparian areas, pastures, croplands, roadsides and rangelands.
It is adapted to a wide range of ecosystems, including disturbed areas and native plant communities. It prefers rich, deep soils, and is somewhat shade intolerant.
This plant often forms dense thickets and monocultures in agricultural lands and native plant communities. The plant reproduces through horizontal roots, and typically spreads about 1-2m per year. Thickets can restrict human access to recreational areas.
Hand pulling and cutting can deplete root resources if all root fragments are pulled on a continual, repeated basis. Incomplete mechanical treatment can worsen the infestation. Intensive mechanical treatment along with competitive cropping is an effective long-term strategy. Biocontrol has been released but has not been successful. Burning can be effective if it is conducted in late spring; early spring burns may actually encourage seedling growth. Canada thistle easily invades areas that have been recently burned; seeding with aggressive grass species after burning can prevent the establishment of this species. Several herbicides are registered for use on Canada thistle. Because Canada thistle often grows in wet areas use of herbicides may be restricted. Check with the BC Ministry of Agriculture, Food and Fisheries Crop Production Guides for recommendations.
BC Ministry of Environment and Invasive Plant Council of BC. 2011. Best Management Practices for Invasive Plants in Parks and Protected Areas of British Columbia.
Province of British Columbia. 2002. Guide to Weeds in British Columbia.
USDA Fire Effects Information System http://www.feis-crs.org/beta/
Photo reference: Eric Coombs, Oregon Department of Agriculture, Bugwood.org