Polygonum amphibium is a perennial herb from rhizomes, to 1 m tall, erect or floating aquatic, ocreae often fringed, those above water often forming a flared collar .
Leaves alternate, those of aquatic form elliptic, blunt or rounded, dock-like, leaves of terrestrial form narrow, hairy, long-pointed.
Flowers pink or red, to 0.5 cm, to 0.7 cm in fruit, male and female flowers on different plants (dioecious); inflorescence spikes, 1-2, dense, blunt, stout, to 15 cm long, 2 cm wide, at ends of branches; terrestrial form has glandular-hairy inflorescence stalks.
Fruit consists of developed seed (achene) enclosed in old flower parts.
Wetland status: OBL.
Frequency in NYC: Very infrequent.
Habitat: Shallow water or wet soil.