Polygonum. Polygonum densiflorum . Polygonum hydropiperoides . Both from Institute of Food and Agricultural Sciences.

Most of Polygonum is referred to as “smartweed.” Diggs et al. (1999) cite a comment by Kirkpatrick (1992) suggesting that the name was derived from the tendency of the sap to “smart” (burn) when it touched the skin. However, historical data support a slight modification of that view. “Smartweed” was applied to P. hydropiper by 1787. In that year, W.H.Marshall, writing on the rural economy of Norfolk, said, “Smartweed, biting and pale-flowered persicarias; arsmart.” Indeed, the OED lists “arsesmart” as the common name for “smartweed.” So, Kirkpatrick had the right idea, but the wrong part of the body.