Continual cultivation reduces the root reserves that will weaken the plant and make it more susceptible to herbicides. Crop competition can be an effective tool in Canada thistle control if a vigorous crop is established. Alfalfa and forage grasses have been the best competitors due to the repetitive cutting associated with these crops. The greatest success has occurred when Canada thistle infestations are weakened by treating with a short residual herbicide before the land is prepared for seeding. Alfalfa and forage grass seeding should be delayed until the herbicide has degraded in the soil.
Forage grasses differ in their ability to suppress Canada thistle.
Intermediate wheat grass and tall fescue reduced Canada thistle density 60 percent when the competitive grasses were combined with mowing. Several native North American insects can cause damage to Canada thistle, but none has been found to cause sufficient damage to control the plant. The insect feeds within the thistle stem from April through June. The adults feed through the summer and overwinter in the soil. Eggs are laid in the spring on early bolting plants.