U. urens: annual, with stiff, stinging hairs; stems erect, branching, 1/2-1 foot high, glabrous; leaves ovate, deeply toothed; flowers in small, loose, almost sessile, axillary clusters. - Cultivated and waste places. Fl. June to October.
U. dioica: stems erect, 2-3 feet high, with copious stinging bristles; leaves cordate-ovate, coarsely-toothed; flowers dioecious, both males and females clustered in axillary, branched, spreading spikes. - Hedges and waste places. Fl. July, August.