Stems erect or ascending, rooting at proximal nodes, glabrous or sparsely hirsute. Roots not thickened basally, glabrous or somewhat canescent proximally. Proximal cauline leaf blades lanceolate, 5.9-12.2 × 1.1-2.4 cm, base rounded-obtuse to acuminate, margins denticulate, apex acuminate. Inflorescences: bracts linear or lanceolate. Flowers: receptacle glabrous; sepals 5, spreading or sometimes reflexed from base, 3-5 × 2-3 mm, glabrous; petals 5, 5-8 × 2-3 mm; nectary scales glabrous. Heads of achenes short-ovoid to depressed-globose, 5-7 × 4-8 mm; achenes 1.8 × 1.2-1.4 mm, glabrous; beak lanceolate, straight, 0.6-1.2 mm.
Stems hollow, rooting from the lower nodes, to 1 m, ascending; cauline lvs with dilated, sheathing petiole and well defined, lanceolate blade to 15 נ3 cm, usually denticulate; pet 5-10 mm, equaling or a little exceeding the sep; achenes 1.5-2 mm, the horizontal beak 1-1.3 mm. Swamps, marshes, and shores, mostly in heavy soil; Me. to Minn., s. to Va., Tenn., and La. June-Sept. (R. obtusiusculus Raf., probably misapplied)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Very local in swamps and ponds throughout the state although there are no reports from the northwestern part. The most northern report is that from Noble County by Van Gorder. This species grows in swamps, ponds, and ditches that are full of water in the spring and become dry in summer when it flowers and fruits.