More than 50 varieties of thistle exist, and most are pernicious pests. These weeds are easily identified by their large purple blossoms and sharp thorns that scratch passersby. Some people are even allergic to the acid that these plants emit when they become embedded in skin. And most who encounter it want to kill it. But this is not as easy as cutting back their foliage. Thistle is an aggressive grower with deep roots that must be cut to kill the plant.
Cut back the tops of thistle plants before they flower. Smaller varieties can be mowed with a lawnmower. Larger thistle like Russian thistle should be cut down with a sharp hoe.
Use a rototiller to cut through the root of the thistle in late fall. The depth that you will have to till depends on the variety of thistle. Canadian thistle sends down roots as deep as 1 foot. To ascertain the depth of your thistle's roots, dig up one plant. Then till to the depth that those roots reach. Cutting the thistles' roots this way will kill most annual varieties of thistle. Perennial varieties are cut back by 20 percent. And if root cutting is the only method of control, it will have to be repeated periodically.
Gather any large sections of thistle root, bag them and throw them away.
Spray any new thistle growth with a broad leaf suppressant herbicide prescribed for use on thistle. Combining root cutting and herbicide application is the best way to kill thistle and keep it from coming back.
Plant competing ground cover in place of the removed thistle. This step is most advisable where perennial varieties of thistle like Canadian thistle grow. It is impossible to remove every fiber of thistle root and, if conditions are right, cut root sections as short as a half-inch can generate a new plant. To prevent an even more virulent outbreak of thistle after root cutting, plant ground cover that grows rapidly in your area.