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.
In the low sedge borders of lakes, in swamps, dried-up ponds and sloughs, and cypress swamps. Infrequent to rare. [Variety amblyogona] is a form with the angles of the fruit rounded. My Kosciusko County specimen might be referred to this variety although it is not well marked. Fernald cited 0. E. Lansing's no. 2509 from a ditch, Roby, Lake County, Indiana as the type.