Canada Thistle (Cirsium arvense)
Lookout for this weed and don’t let it get established, because it’s difficult to get rid of.
Canada thistle is a perennial that spreads rapidly. It can reproduce from root fragments as well as by seed. To eliminate Canada Thistle, you must exhaust its root system. This requires multiple seasons and treatments within a season.
The flowers are purple, less than an inch in diameter, and grow in clusters from 1-5. The leaves are bright green, lance shaped, and irregularly lobed with spiny toothed margins.
Methods of Control:
If you’re diligent, this method may be effective on small infestations. Mechanical methods can reduce the seed bank in the soil if done before flowering. Mowing, unless done regularly, can stimulate new growth of Canada thistle. Hand pulling & tilling creates root fragments that grow into new plants. Combining mechanical methods with herbicides will enhance Canada thistle control.
Herbicide treatments alone may be necessary for 1-3 years because of root nutrient stores. Always read the label and follow safety instructions. Please refer to the PNW Weed Management Handbook for information on herbicides.
Cattle, goats, and sheep will graze on Canada thistle when plants are young. Follow up grazing with a fall herbicide application. Insects provide limited control and not eradication. One insect is currently approved for use in Washington state and that is the Canada thistle stem gall fly. More information can be found at: http://invasives.wsu.edu/biological/urophoracardui.htm
Canada Thistle is a Class “C” Noxious Weed in Benton County