There are about 25 species of Polygonum known for Wisconsin. The key to the genus Polygonum in Michigan Flora vol II (Voss 1984) is highly recommended.

Polygonum hydropiper is a common species of wet, sunny sites, and is found to a lesser degree under partial canopy in swamps. Important characters in the key include the bristles along the edge of the ochrea (a sheath at each node) and glands on the tepals.