Its erect stem is highly branched above, green, and glaberescent to covered with dense cobweb-like hairs. Canada thistle usually occurs in small to large patches with numerous individuals arising from horizontal, lateral roots bearing adventitious shoots. Leaves are simple and placed alternately on the stem. Both leaf surfaces may be glabrous, or the upper surface may be lightly pubescent while the lower surface is densely pubescent. Cauline leaves are reduced in size upwards and less lobed.
Heads are numerous, and occur in terminal corymb-like clusters.
Each head is discoid and unisexual or incompletely dioecious. Each achene has a pappus of numerous white to grayish plumose bristles reaching up to 2.5 cm in length. The four recognized varieties of this species are var. Scop., and var. Canada thistle is a highly competitive, noxious weed.