Canada thistles (Cirsium arvense) are prickly nuisances. If left undisturbed, they grow to a height of 2 to 4 feet and quickly spread. Canada thistles are considered weeds and are often a problem for farmers. They are also an issue in lawns and home gardens. Since Canada thistle's roots can run 10 feet deep, getting rid of them takes extra effort. Applying an herbicide once in the early summer and again in the fall works well to eradicate Canada thistles completely.
Put on rubber gloves, goggles, a long sleeve shirt and long pants. This will protect your eyes and skin from getting in contact with the herbicide.
Dip a sponge in the herbicide. It should not be dripping. Using a sponge gives you better control and helps prevent the unintentional spraying other vegetation.
Squeeze three or four drops of herbicide onto the thistle's foliage. Do not get any of it on the ground, other plants, grass or trees in the process. The herbicide will work its way down to the roots and kill the entire plant.
Reapply the herbicide in the fall to any new thistles that have grown since the first application. Apply in the same manner as you did in the early summer.