Canada thistle, Cirsium arvense, is a member of the sunflower family. This 1-4-foot tall perennial has spiny alternate leaves that are lance-shaped and lobed. The flowers form clusters near the top of the plant, and the blossoms are light purple fading to white. The seeds have long white hairs that blow in the wind. The roots grow horizontally and vertically, with new shoots coming up from the horizontal roots.

Canada Thistle Prevention and Treatment

Some people choose to ignore Canada thistle if it is lightly intermixed with a healthy component of native vegetation. Like other noxious weeds, it is one of the first plants to establish a foothold in disturbed areas and burned ground. Canada thistle does well during drought when many native plants are under stress. Pulling causes the plant to send up more shoots. Clipping and disposing of the seed heads will help reduce the spread but won't be enough to wipe out the plant. Canada thistle can be eradicated by repeated mowing. Individual plants will eventually die if every few weeks they are sliced off with a shovel at ground level. Herbicides can be used when the plant is vigorously growing in late spring or early summer.

Any effective treatment should include re-vegetation with native plants other desirable vegetation.

For more information visit Missoula County Weed District.