Walla Walla County Extension

Weed of the Month

Canada Thistle

Canada thistle (Cirsium arvense) is an aggressive, creeping perennial weed with a vigorous, spreading root system. New shoots and roots can form almost anywhere along the root system of established plants. One plant can colonize an area 3 to 6 feet in diameter in one or two years. Infestations often start on disturbed ground including ditch banks, overgrazed pastures, tilled fields, or abandoned sites.

Canada thistle grows from 1 to 4 feet tall, with many branches growing from a heavily ridged, often slightly hairy, stem. Prickly, lance-shaped, irregularly lobed leaves with toothed margins grow directly from the stalks without any stems. Rose-purple, lavender, or sometimes white flower heads appear from June through August. Flowers are small (about 1 inch long and 0.5 inches wide) and occur in rounded, umbrella-shaped clusters. The petals turn tan to white in color as the seed heads mature. Seeds are about 1/8 inch long, somewhat flattened, light brown, with feathery hairs at the top. Seed can remain viable in the soil up to 20 years.

Management of Canada thistle can be achieved through hand-cutting, mowing, biological control, chemical control, or a combination of methods. Tillage is not recommended as it divides roots and stimulates new plants to develop. Cutting or mowing needs to be done prior to seed set and must be repeated until the root reserves are exhausted, usually over several growing seasons. 2,4-D and triclopyr should be applied prebloom, glyphosate is effective at the bloom stage, and dichlobenil is useful in midwinter. Be sure to follow label instructions and precautions.

Biological controls must be used in combination with other methods to be effective.

• Ceutorhyncus litura is a weevil that bores into the main leaf vein and then down into the plant’s crown.
• Urophora cardui larvae burrow into shoots, causing large galls to form that stress the plant, sometimes killing it. Galls that form where flowers develop keep the weed from flowering and reduce seed set.
• Larinus planus and Rhinocyllus conicus larvae eat seed-producing tissue of developing buds. The adult beetles feed on young thistle foliage and buds. Although they decrease the amount of seed available, they are not effective in reducing established stands of the weed.