Polygonum amphibium. © 2001 Eleanor Saulys.ct-botanical-society.org. (Accessed 4/2014).

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. 

Origin: native. 

Habitat: Shallow water or wet soil.