Canada thistle is a hardy weed that, as its name suggests, is found throughout the Canadian provinces, as well as across the northern United States. The thistle is notorious for its fast growth, with a single shoot spreading 20 feet per season, and roots that grow at a rate of 20 feet per year, according to Dow AgroSciences. Control the weed to keep it from taking over your field or yard.
Mow the Canada thistle down to 1 to 2 inches in height. Use a lawn mower for flat surfaces or a weed eater for trimming along pathways, fences or soil with irregular heights.
Spray the mowed Canada thistle with a picloram-based herbicide. This is a systemic chemical that moves down through the foliage and kills the thistle's underground root network to prevent regrowth. Apply according to the herbicide's label, as toxicity varies by product formulation.
Reapply the herbicide nine to 12 months after the initial application. This controls and kills any thistle vegetation that didn't succumb to the first herbicide spraying.
Replant the area with desirable vegetation. Canada thistle is highly susceptible to shading due to its need for lots of sunshine. Sow fields with a crop like alfalfa or grass. Ornamental areas like garden beds or lawns should be reseeded with grass or ornamental plants.