Perennial, woody below, with slender stems 1-3 m, usually arched and rooting at the tip; lvs opposite or more often in whorls of 3 or 4, short-petiolate, lanceolate, 5-16 × 2-5 cm; fls in dense cymes in the upper axils, the narrow pet pink-purple, 10-15 mm; fr 5 mm thick; 2n=32. Swamps and still water-courses. July-Sept. Var. verticillatus, with the pedicels and lower lf-surface ±tomentulose, occurs mostly near the coast from Me. to Fla. and La., and in the Mississippi Valley to Ind. and Mo. Var. laevigatus Torr. & A. Gray, with glabrous pedicels and lvs, is mostly inland, from N.S. and Que. to Minn., s. to e. Tenn.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
In the mucky or peaty borders of lakes, bogs, and swamps. Infrequent in the lake area, and very rare southward. [Variety laevigatus has glabrous stems and lower leaf surfaces. It grows] on the mucky or peaty borders of lakes, bogs, and swamps. Rather rare and not so frequent as the pubescent form.