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Canada Thistle

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Canada Thistle Canada Thistle


  • Why should we care?
    • Considered by many to be the most difficult thistle to control, the Canada thistle poses a threat to our economic, social and aesthetic well-being. It  can greatly reduce our crop production, especially corn, wheat and soybeans and also affects forage production. It invades tilled cropland, range, pasture, and natural areas such as prairies, savannas, glades and dunes if some degree of disturbance already exists. It also invades wet areas with fluctuating water levels such as streambanks, sedge meadows and wet prairies. Once it has established itself it spreads quickly replacing/displacing native plants. It grows in circular patches spreading vegetatively through horizontal roots which can spread 10–12 feet in one season. Once it becomes established it can live and reproduce for many years.
  • How to Identify
    • Perennial herbaceous plant, 2–5 feet tall with slender grooved stems that branch only at the top. It has male and female parts. The Leaves are alternate, smooth, oblong, tapering, and directly attached to the stem, deeply divided, with prickly margins. It has numerous small purple flowers that appear on top of the upper branched stems between June and September. The roots consist of a fibrous taproot with wide spreading horizontal roots. Each small section of root can form a new plant enabling the plant to spread vegetatively. 
  • About the Plant and Where it came from
    • Cirsium arvense
    • The Canada thistle occurs in Canada and throughout the northern U.S. from northern California to Maine and southward to Virginia.
    • This plant has small light brown seeds that are tufted for dispersal by the wind. Seeds remain viable in the soil for over 20 years.  
  • How to Control
    • Cultural control: Repeated pulling and moving will weaken roots, mowing when flower buds are just to open. Late spring burns in May/June are most detrimental, but also stimulate seed germination; burn consecutively for 3 years.
    • Chemical control: Spot application with glyphosate or with selective herbicide clopyralid, or metsulfuron.
    • Biological control: Stem weevil, bud Weevil and stem gall fly are commercially available.

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