Plants perennial, glabrous, with vertical rootstock. Stems erect, rarely ascending, usually pro-ducing axillary shoots below 1st-order inflorescence or at proximal nodes, 50-90(-120) cm. Leaf blades ovate-lanceolate, elliptic-lanceolate, or lanceolate, 10-15 × 3-5.5 cm, usually ca. 2.5-4 times as long as wide, widest in proximal 2, thick, often subcoriaceous, base broadly cuneate, rarely almost rounded, margins entire, flat, apex acute or attenuate. Inflorescences terminal and axillary, terminal usually occupying distal 1/5- 3 of stem, rather dense, normally broadly paniculate. Pedicels articulated in proximal 1/ 3, sometimes almost near base, thick, (2-)3-7(-8) mm, usually approximately as long as inner tepals, occasionally slightly longer or shorter, articulation swollen. Flowers 12-20 in whorls; inner tepals with broadly triangular, ovate-triangular, or broadly ovate-deltoid, 4.5-6 × 3-4.5(-5) mm, base truncate or indistinctly cordate, margins entire, apex acute; tubercles (2-)3, equal or subequal, glabrous or minutely rugose. Achenes brown or dark reddish brown, 2.5-3.5 × 1.8-2.3 mm. 2n = 20. Flowering late spring-summer. Swamps, marshes, wet shores, alluvial woods, other wet habitats; 0-1800 m; Ont.; Ala., Ariz., Ark., Colo., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis., Wyo.; n Mexico; Europe (introduced in Denmark, Sweden, United Kingdom. The name Rumex britannica Linnaeus was misapplied to this species by C. F. Meisner (1856) and some North American authors.

Some records of Rumex altissimus from Arizona and New Mexico may refer to R. ellipticus. Two reports from New Mexico were based on misidentification of R. hymenosepalus.