Herbs , perennial, rhizomatous, 5-30 dm. Stems simple or branched, erect or sprawling. Leaf blades elliptic, lanceolate, or narrowly to broadly ovate, 6-20 × 2-13 cm, base rounded to cordate, margins coarsely serrate, sometimes doubly serrate, apex acute or acuminate; cystoliths rounded. Inflorescences paniculate, pedunculate, elongate. Flowers unisexual, staminate and pistillate on same or different plants, staminate ascending, the pistillate lax or recurved. Pistillate flowers: outer tepals linear to narrowly spatulate or lanceolate, 0.8-1.2 mm, inner tepals ovate to broadly ovate, 1.4-1.8 × 1.1-1.3 mm. Achenes ovoid to broadly ovoid, 1-1.3(-1.4) × 0.7-0.9 mm.
Erect, rhizomatous perennial, usually simple, to 2 m; lvs serrate, acute or acuminate, 5-15 cm; stipules lance-linear, 5-15 mm; infls branched, many-fld, commonly exceeding the subtending petioles; fruiting cal 2 mm, pubescent; achene ovate, 1.5 mm. Nearly cosmopolitan, either as a native or an intr. sp., and highly variable. June-Sept. Var. dioica, native to Europe but well established in our range, is dioecious, weak-stemmed, and rather densely hairy, with mostly broadly ovate, cordate-based lvs with stinging hairs on both sides, and with the teeth commonly 5-6 mm deep; 2n=52. Var. procera (Muhl.) Wedd., the native phase, is usually monoecious, stouter and more sparsely hairy, with ovate to lance-oblong lvs that are seldom cordate at the base, with the stinging hairs commonly confined to the lower surface, and with the teeth averaging 2-3.5 mm deep; 2n=26 in ours. (U. gracilis; U. viridis)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.