Canada thistle (Cirsium arvense) begins growing each year as a cluster of leaves close to the ground (rosette) and has the typical prickly leaves characteristic of thistles. If the plant is allowed to mature, it reaches a height of about 2 to 4 feet and has lavender blossoms similar in appearance to dandelion blossoms but half as large.
Canada thistle is a deep-rooted perennial, which means it lives for several years. Mowing or pulling of this weed is not effective, and cultivation in gardens and flowerbeds will not help but can even worsen the problem when roots are cut into pieces. Pieces only one-half-inch long can produce new plants.
To manage Canada thistle, use a herbicide that will not only kill foliage but will also move down into the root system to kill the root buds (a systemic, non-selective herbicide). When using a pesticide always remember to read and follow label directions.