An acrid plant of the genus Polygonum (P. Hydropiper), which produces smarting if applied where the skin is tender.

Any of several species of aquatic or semi-aquatic herb of the genus Polygonum. Represented in the Boundary Waters by four species, the Showy Smartweed ( Polygonum amphibium), Scarlet Smartweed ( Polygonum coccineum), Water Smartweed ( Polygonum natans), and Tearthumb ( Polygonum sagittatum).