Stems erect or ascending, often rooting at proximal nodes, glabrous or sparsely pilose. Roots not thickened basally, glabrous. Proximal cauline leaf blades ovate to lanceolate or narrowly elliptic, 1.5-5.7 × 0.4-2.4 cm, base cordate to acute, margins finely denticulate or entire, apex broadly rounded to acuminate. Inflorescences: bracts linear to lanceolate or oblanceolate. Flowers: receptacle glabrous; sepals 4-5, spreading or reflexed from base, 2-3 × 1.5-3 mm, glabrous or pubescent; petals 4-6, 2-6 × 1-2 mm; nectary scales glabrous. Heads of achenes hemispheric to ovoid, 2-4 × 2-3 mm; achenes 0.8-1 × 0.8 mm, glabrous; beak deciduous, leaving stump 0.1-0.2 mm. Flowering late winter-summer (Mar-Jul). Around ponds and ditches, in meadows, roadsides, and open woods; 0-100 m; Ala., Ark., Del., Ga., Ill., Ind., Kans., Ky., La., Md., Miss., Mo., N.J., N.C., Okla., S.C., Tenn., Tex., Va.
Annual; stems weak and slender, 2-8 dm, sometimes reclining, sometimes rooting at the lower nodes and even becoming colonial, much branched at least distally; lvs petioled, the basal cordate to cuneate, the principal cauline ones linear-elliptic to lanceolate, 5-15 mm wide, obtuse, entire or sparsely dentate; pet 5.5-9 mm, much exceeding the sep; stamens ca 25; achenes 0.7-1.1 mm, minutely pitted, very minutely beaked. Marshes and muddy ground on the coastal plain; s. Del. to Tex., n. in the interior to s. Ill. and Ind. May, June. (R. laxicaulis; R. subcordatus; R. oblongifolius, misapplied)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.