Weed of the Week - Canada Thistle

Canada Thistle is a class C noxious weed native to Europe and Asia. This weed is considered noxious because, once established, it spreads quickly, replacing native plants. Canada Thistle grows in circular patches, spreading vegetatively through roots which can spread 10-12 feet in one season. The numerous seeds from this weed spread through the wind. It poses an economic threat to the agriculture industry by reducing crop yields. Though Canada Thistle is not known to be toxic, it grows 2 to 5 feet tall with slender grooved stems that branch only at the top. Purple Flowerheads are in clusters at tips of branched stems and can be ½ to ¾ inches in diameter and bloom from June to October. Canada Thistle can be found in pastures and croplands. It also invades wet areas with fluctuating water levels such as stream banks, sedge meadows and wet prairies. Canada Thistle is difficult to control once established on a site. One treatment, or a combination of treatments, may need to be used to control populations, depending on the site. Planting competitive crops such as alfalfa and forage grasses can be very effective in controlling an infestation of Canada Thistle.

If you have questions about Canada Thistle, or any other noxious weeds and their identification and control, we can help. Please contact Andy Lea at 360-795-3852.