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 - 4 mm across, egg-shaped to pyramidal, three-sided with sharply ridged angles, three-chambered, with one seed found in each chamber.
Similar species: Proserpinaca palustris var. amblyogona differs from this species by having fruit with rounded angles.
Flowering: mid June to early September
Habitat and ecology: Frequent on muddy stream and pond borders, especially periodically flooded ones.
Occurence in the Chicago region: native
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. Crebra means "thickly clustered."