This adventive perennial plant is 1-3' tall, branching occasionally in the upper half. The light green stems are slightly ridged. The alternate leaves are up to 8" long and 1" across, but usually about 3-4" long and 2/3" (17 mm.) across. They are lanceolate or elliptic in shape, pinnatifid and/or dentate along their margins, and either sessile or clasping the stems. Some of the lower leaves may taper gradually to a petiole-like base. The leaf margins have numerous pale yellow thorns, and tend to be rather crinkly. The stems and leaf undersides can be hairless or somewhat hairy, depending on the variety. The leaf uppersides are hairless. The upper stems terminate into one or more flowerheads. Each flowerhead is about ½–¾" across, consisting of numerous disk florets and no ray florets. The corollas of disk florets are pale pink or lavender. At the base of each flowerhead, there are overlapping green to purple bracts (phyllaries) in several series; these bracts are appressed together. The bracts have sharp pointed tips. The blooming period usually occurs from early to mid-summer, although plants may bloom later in the year if their growth is disrupted by disturbance. The flowerheads have a sweet fragrance. Canada Thistle is occasionally dioecious, with male and female flowerheads occurring on separate plants; otherwise they are perfect. The brown achenes of female plants are slender and slightly ridged at their apices, where they have tufts of light brown hair. These achenes are distributed by the wind. The root system is rhizomatous. Canada Thistle often forms clonal colonies as a result of these rhizomes; these colonies can extend several feet across.