Canada thistle is a perennial weed that can reach anywhere between 1 and 5 feet in height. It can easily be identified by its branched stem, leathery spine-tipped leaves and rounded lavender blossoms. Each plant can produce between 1,500 and 5,000 seeds per year, each of which can remain viable for anywhere from six to 21 years. However, Canada thistle also reproduces through root regeneration, making it a tough weed to kill. To permanently eradicate Canada thistle, you must apply a variety of control methods.
Physically remove the top-growth of the thistle plants by mowing or hand tilling. The best time to do this is just before the flower bud breaks. This will prevent the weed from supplying nutrients to its regenerative root system. If physical removal is the only means of control applied, then you will likely have to kill the Canada thistle’s foliage several times before its root stores are depleted and it stops coming back for good.
Kill the Canada thistle’s foliage with an herbicide prescribed for the plant like Amitrol 420 (amitrole). Spray herbicides on the Canada thistle before flower bud break. As with the physical removal method, you will likely have to apply the herbicide several times before the Canada thistle's root stores are depleted and it stops growing back.
Plant ground cover, suitable for your area, over the area in which the Canada thistle was growing. Grow competitive forages, particularly alfalfa or sweet clover, in the crop rotation. According to the Government of Saskatchewan, planting ground cover for three years can permanently eradicate the Canada thistle population. After that time, other, slower-growing crops can be planted.