General: Perennial from strong spreading rhizomes, armed with stinging hairs, otherwise hairless to hairy; stems leafy, upright, 1-3 m tall.
Leaves: Opposite, narrowly lance-shaped to oval or heart-shaped, coarsely saw-toothed; stipules prominent, 5-15 mm long.
Flowers: Greenish, tiny; numerous in dense, drooping clusters or spikes in the leaf axils and at the stem tips; male and female flowers in separate spikes with female spikes usually outermost.
Fruits: Flattened, lens-shaped achenes.
Ecology: Meadows, thickets, stream banks, open forest, often growing en masse in disturbed habitats such as avalanche tracks, middens, slash piles, barnyards, roadsides, always in moist rich soil; common, locally abundant, from lowlands to subalpine elevations.