Flowers: Unisexual, male and female flowers in separate clusters in upper leaf axils and at ends of branches. Flowers mostly summer and autumn.
Annual herb to 2.5 (rarely to 4) m high. Stems rough to touch with short stout upward directed hairs, green and usually blotched or streaked purple. Leaves dark green above, paler below, covered with small bristles and glandular hairs, margins coarsely toothed, with 3 prominent veins, and with veins and leaf stalks often reddish. Brown burrs each contain two brown, grey or black seeds.
Distinguished by spineless stems; ovate or triangular leaves 5–15 cm long with 3 or 5 lobes, rough to touch; burrs 7–25 mm long, covered with hooked spines and ending in 2 diverging stout straight spines.