Clump- or patch-forming perennial to 1 m tall, with extensive creeping roots and small unisexual flower heads. Plants are male or female (dioecious), and dense patches of a single sex often occur. Canada thistle is widespread in cool temperate regions of Eurasia and North America. In the U.S. it is listed as a noxious weed in 28 states. Some taxonomists recognize 4 varieties or biotypes that differ in growth habit, phenology, seed germination, and leaf characteristics. Introduced from Europe.

Mature Plants:Rosette leaves few or lacking. All leaves oblong to lanceolate, ~5-20 cm long. Margins nearly entire to shallowly lobed and toothed. Main prickles ~3-6 mm long. Upper surfaces +/- glabrous, green. Lower surfaces sometimes sparsely woolly. Leaf bases sometimes extend briefly down stems as inconspicuous prickly wings to ~ 1 cm long. Stems slender, +/- glabrous, leafy, several to numerous from creeping roots.

Seedlings: Cotyledons ~ 5-14 mm long, 3-6 mm wide, lower surface midvein shiny. First leaf margins +/- wavy to unevenly toothed. Surfaces covered with stiff hairs. Lower surfaces +/- covered with soft cobwebby hairs. Seedlings initially develop a deep taproot. Creeping roots develop in ~ 2-4 months. Seedlings sometimes initiate stems early and have poorly developed rosettes.

Root and Underground Parts: Root systems consist of an extensive network of vertical and creeping horizontal roots. Most roots occur in the top 45 cm of soil, but vertical roots 2-3 m deep are common. Horizontal roots may spread several meters in all directions. Individual roots survive up to ~ 2 years. Roots are brittle and fragment easily. Some roots survive in frozen soil.

Flowers: June-October. Heads unisexual, numerous, often clustered, +/- cylindrical or ovoid to bell-shaped. Involucres 1-2(2.5) cm long, 0.5-2 cm diameter, often purplish, glabrous or with white woolly hair. Outer phyllaries ovate, close-laying (appressed) with +/- spreading tips. Spines ~ 1 mm long, fine, +/- dark. Corollas pink, purple, or white, male ~ 10-14 mm long with a shorter pappus, female ~ 14-20 mm long with a longer pappus. Corolla lobes 2-3 mm long.

By vegetative propagation, a single seedling can establish a large patch of stems. However no seeds will be produced because seed production requires the presence of both sexes. More than one introduction is thus needed. Plant competitors vary in their effectiveness. In one study, the percentage of thistles increased by 192 percent in four years in a continuous cropped spring wheat, but decreased to one percent in alfalfa grown for hay.