Plants perennial, glabrous or indistinctly papillose-pubescent, with distinctly tuberous roots and short rhizomes. Stems usually erect, rarely ascending, branched above middle, 25–90(–100) cm. Leaves: ocrea prominent and persistent at maturity, whitish or silvery white, membranous; blade oblong, oblong-elliptic, or obovate-lanceolate, (5–)8–30 × 2–8(–12) cm, base cuneate or narrowly cuneate, margins entire, flat or indistinctly crisped, apex acute or acuminate, rarely obtuse. Inflorescences terminal, occupying distal 1/2 of stem, narrowly paniculate, rarely simple. Pedicels articulated near middle or in proximal 1/3, filiform, 5–15(–20) mm, articulation indistinct. Flowers 5–20 in whorls; inner tepals oblong-cordate or orbiculate-cordate, 11–16 × 9.5–14 mm, base sinuate or emarginate, margins entire, rarely with few extremely small denticles at base, apex obtuse or subacute; tubercles absent. Achenes brown or reddish brown, 4–5(–7) × 2.5–4.5(–5) mm. 2n = 40.
Phenology: Flowering spring.
Habitat: Sandy and rocky places: plains, slopes, stream beds, alkaline soils
Elevation: 0-1700(-2000) m
Ariz., Calif., Colo., Mont., Nev., N.Mex., Okla., Tex., Utah, Wyo., Mexico (Baja California, Chihuahua).
Rumex hymenosepalus is the only species of subsect. Hymenosepali Rechinger f.
Two varieties have been recognized. The typical variety has achenes 5 mm and ovate-elliptic or oblong-cordate inner tepals with a subacute apex. Variety salinus (A. Nelson) Rechinger f. has larger achenes (to 7 mm) and almost orbiculate inner tepals with an obtuse apex.
Rumex hymenosepalus was reported also from Montana (J. E. Dawson 1979), but no exact localities were given.