Perennial aquatic to semi-aquatic herb 10 to 40 cm long Stem: weak, prostrate or with erect tips. Leaves: alternate, with three different forms. Submersed leaves are stalkless, 1.5 - 6 cm long, with eight to fourteen comb-like (pectinate) divisions. Amphibious leaves stalked, 1.7 - 7 cm long, with incompletely pinnate (pinnatifid) divisions. Emersed leaves are 1.5 - 8.5 cm long, 0.2 - 1.4 cm wide, lance-shaped and widest below, at or above the middle. Flowers: borne in axils of emersed or amphibious leaves, subtended by tiny lance-shaped and toothed bracts, solitary or in clusters of two to five, stalkless, green or with a purple tinge, small, lacking petals but having three sepals, three stamens and three stigmas. Fruit: nut-like, 2.3 - 6 mm across, egg-shaped to pyramidal, three-sided with sharp to rounded angles, three-chambered, with one seed found in each chamber.
Similar species: Proserpinaca palustris is represented by two varieties in the Chicago Region. See links below for further information.
Etymology: Proserpinaca is the name Pliny gave to a Polygonum, meaning "pertaining to Proserpina (the Roman goddess of the underworld)."The name may have been given to this genus because of its adaptation to many habitats. Palustris means marsh-loving.
Stems decumbent and prostrate or rooting at base, often colonial, the floriferous branches erect, 1-4 dm; submersed lvs, if present, 2-4 cm, ovate or broadly oblong, deeply pinnatisect into narrow segments; emersed lvs linear-oblong to linear-oblanceolate, 2-6(-8) cm, serrate, tapering to the base; fr ovoid-pyramidal, 2-5 mm long and wide. Swamps, marshes, wet shores, and shallow water; Que. and N.S. to w. Ont. and Minn., s. to Cuba and Guatemala. July, Aug. The widespread var. crebra Fernald & Griscom has the fr 2-4 mm wide, acutely angled but not winged, the sides nearly flat. Var. palustris (P. platycarpa), on the coastal plain from Mass. to La., has the frs 3.5-5 mm wide, sharply angled, or narrowly winged above or throughout, with concave sides. Var. amblyogona Fernald, chiefly Ozarkian, extending e. to Ind., has frs 2-4 mm wide, plump, obtusely angled with convex sides.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.