Proserpinaca palustris is a perennial from a rhizome, reclining, rooted in mud under shallow water, often colonial, submerged stem prostrate, to 40 cm tall, erect above, with tiny black spines in leaf axils.
Leaves alternate, stalkless (sessile), submerged leaves to 4 cm long, pinnately divided, 8-14 pairs of linear divisions to 3 cm long, midrib wider than leaflets, fern frond-like; emergent leaves narrow, to 6 cm long, finely toothed, tapered to base.
Flowers green, in axils of erect, emergent stem tips; blooming June to Sept. (Hough 1983).
Fruit nut-like, to 0.5 cm long and wide, 3-angled; fruiting July-Oct.
Wetland status: OBL.
Frequency in NYC: Infrequent.
Habitat: Shallow water of freshwater marshes or vernal ponds (Hough 1983).