Canada Thistle

 California thistle, creeping thistle, field thistle 

Canada thistle is a perennial plant that can reach heights of four feet tall and forms a very deep and extensive root system, which makes it difficult to control. The creeping horizontal roots sprout new plants and can reach more than nineteen feet in one season and roots can also go as deep as twenty two feet. Rosettes are smooth, have irregular lobes and have spine tipped edges. Leaves are lance-shaped and the edges have yellowish spines making hand pulling the thorny problem. The stems grow tall and are often branched, slightly hairy, and lack spines.

Flowers resemble Spotted knapweed in that they are pink to purple and ray-like. Each plant produces both female and male flowers. One plant is capable of producing over 3,000 seeds annually. This plant reproduces by both seed and the roots, but primarily by the roots.

Have roots that are creeping, extensive and deep, have spines on the tips of the leaves and pink flowers that resemble Spotted knapweed. 

Canada thistle is found in open areas with moderate amounts of moisture, but does not do well in wet soils due to the lack in sufficient oxygen. This plant can grow on many different soil types, but does not grow well in shady areas and is usually not found in wooded areas except where logging or clearing has been done. Canada thistle is commonly found in abandoned fields, lots, gravel pits, pastures, right-of-ways, railways right-of-ways, lawns, gardens and agricultural fields. It will also invade wet areas that have fluctuating water levels such as streambanks, irrigation ditches or sloughs. 

Herbicides that control Canada thistle are Milestone, Banvel and 2, 4-D, Escort, Tordon, and others. For more information and mixing instructions on herbicides read the labels or contact the Crook County Weed and Pest office.

Plants can be hand pulled with roots prior to seed production. Mowing can often stimulate growth and spread of Canada thistle.

Bio-control agents may be available at the Weed and Pest. Gall midges (mosquito-like insects) attack Canada thistle stems and form large “galls” when they lay their eggs. These galls reduce seed production and overall plant health.

The name Canada thistle did not come from Canada, but was introduced in the 17th century from Mediterranean region and southeast Europe. Tea made from Canada thistle leave has been used as a diuretic as well as treatment for tuberculosis.