WSU Whatcom County Extension

Integrated Pest Management for Blueberries

Canada Thistle

(Cirsium arvense)

Perennial Broadleaves

 

Canada Thistle

Habitat Characteristics

Canada thistle flourishes where over-grazing, cultivation or mulching has created open habitat for it. The plant does not do well in tight and healthy ecosystems. Canada thistle is a weed of many crops but is most troublesome in perennial crops and areas where reduced tillage is practiced. It can grow on a wide variety of soil types; clay loam, sandy loam, sandy clay, and sand dunes.

 

Identification

A spreading rhizomatous perennial, Canada thistle grows up to 4 feet tall and has leaves with spines on the tips of irregular lobes. Leaves are alternate along the stem. Flowers are purple or white, about ¾ inch in diameter and are in clusters at the top of the plant. Spines are not present at the base of this flower as they are on bull thistle. Perennial roots are deep and extensive. Young plants develop as a basal rosette with thick leaves covered with short, bristly hairs.

Canada Thistle

Canada Thistle

Life History

Canada thistle spreads quickly by creeping rhizomes, colonizing an area in a short period of time. Reproduction is also by wind-blown seed. Flowers are either male or female – both are needed to produce seed. Flowering commences in early summer and continues for several months. Seeds germinate in late spring or early autumn. The top part of the plant often dies back in the winter and new growth comes in the spring from the over-wintering roots. Vegetative growth occurs in the spring and again in the autumn.

 

Management

Cultural Techniques

     - Cultivation breaks up roots and spreads this weed. This is not recommended as a control option.

     - Burning in late spring (May – June) is an effective tool for control, but burning earlier in the spring increases sprouting and reproduction.

Chemical Techniques

     - See Herbicide Efficacy Table (pdf)

 

Web References

B.C. Ministry of Agriculture and Lands, Canada Thistle
http://www.agf.gov.bc.ca/cropprot/weedguid/canthist.htm

University of California IPM, Weed Gallery, Canada Thistle
http://www.ipm.ucdavis.edu/PMG/WEEDS/canada_thistle.html

Invasive Species, The Source for Information and Images of Invasive & Exotic Species, Canada Thistle
http://www.invasive.org/browse/subject.cfm?sub=2792

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WSU Whatcom County Extension • 1000 N. Forest St., Bellingham, WA 98225 • (360) 778-5800 • whatcom@wsu.edu