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United States Department of Agriculture

Agricultural Research Service

Title: Biology and Control of Canada Thistle

Author
item Boydston, Rick

Submitted to: Washington State Weed Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 1, 1996
Publication Date: N/A

Interpretive Summary: Canada thistle (cirsium arvense) is a perennial thistle that was introduced to North America as a contaminant in crop seed in the late 18th century. It is a native of Eurasia and has infested millions of acres throughout the world, including rangeland, roadsides, and cultivated fields. In this article, the biology and control of Canada thistle are discussed. Tillage, ,crop competition, biological controls, and herbicides all contribute to Canada thistle suppression and are discussed.

Technical Abstract: Canada thistle (cirsium arvense) is a perennial thistle that was introduced to North America as a contaminant in crop seed in the late 18th century. It is a native of Eurasia and has infested millions of acres throughout the world, including rangeland, roadsides, and cultivated fields. Canada thistle grows from 1 to 4 feet tall and has spiny, lobed, alternate leaves. .Canada thistle blooms in late June to August forming rose to purple colore flowers located on the tips of branches. Flowers are 1/2 to 3/4 inch in diameter. Canada thistle is a dioecious plant, meaning that male and female flowers are located on separate plants. Seed are about 1/8 inch long and have a feathery pappus attached. They are easily dispersed by wind and irrigation water, and are a food source for several birds. Canada thistle can spread by seed and by creeping roots. New shoots emerge from adventitious buds on roots. Some fairly large patches will not produce seed if the patch originates from a plant with male flowers. However, these patches can continue to spread from roots and can extend up to 20 feet in a single season. Most roots are located in the upper two feet of the soil profile, but some roots can extend as deep as 20 feet. When attempting to control Canada thistle one should use a integrated approach of several methods. Tillage, crop competition, biological controls, and herbicides all contribute to Canada thistle suppression and are discussed.

Last Modified: 8/29/2014
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