Invasive Plants, Pacific Northwest
Canada thistle is a perennial plant that is invasive in the Pacific Northwest. It is considered a noxious weed under the BC Weed Control Act.
This thistle is commonly found on edge of roads, stream banks, horse pastures, planted fields, logged over areas and other disturbed areas. It is a major concern on the coast and is a widespread problem throughout the province.
It has purple or white flowers, with short, spiny, dark green leaves, the plant grows up to 2 meters in height at maturity. Canada thistle spreads rapidly through horizontal roots that are hard to control and give rise to large infestation that can crowd out competing native plants. Canada thistle develops many seeds and may produce up to 1,500 seeds per plant. Seeds can be dispersed by wind, water, animals and by hitching a ride on the clothing of passing hikers.
Best adapted to a rich heavy loam, a clay loam or a sandy loam, it grows poorly in shaded conditions but can tolerate wet saline areas and dry soils, but does not tolerate waterlogged or compacted soils.