Canada thistle is an upright perennial forb often found in dense infestations. Plants grow 1–4 ft (0.3–1.2 m) tall with taproots and extensive, creeping lateral roots. Leaves are lobed with very prickly, ruffled margins and a slightly downy lower surface. Leaves grow alternately along the slender, grooved stems which can be finely haired but are typically not spiny. Upper branches are topped by clusters of small, compact flower heads with non-spiny bracts and florets that vary from white to deep lavender. Male and female flowers occur on different plants. Each female floret produces one small brown seed topped by a tuft of fine hairs.