Flower:

Tight, rounded, short clusters about 1/3 inch across, at tip of the stem and arising from upper leaf axils, often with 2 or 3 clusters each on a stalk less than 1 inch long, branching at the end of a stem up to 3 inches long. Flowers are white or greenish to pink, about 1/8 inch long with 5 oval elliptic tepals (petals and similar sepals), only 1 to a few open at a time. Stalks are 4-angled with rows of stiff, claw-like, downward curving bristles along each angle.

Leaves and stems:

Leaves are alternate and widely spaced, lance-like or lance-oblong, 1 to 4 inches long to 1 inch wide, with a slightly rounded taper to the tip and deeply lobed at the base, the lobes pointed and directed downward. Lower leaves have stalks up to 1½ inches long, the upper leaves nearly stalkless, the basal lobes often wrapping around the stem.

Leaves and stems are nearly smooth throughout except for short fine hairs along leaf edges and sharp, claw-like bristles on stem angles and the midvein on the underside of leaves. Stems are green to red, 4-angled, weak, erect when young, branching from the base, mostly unbranched in the upper plant, sprawling or vining on surrounding vegetation, often rooting at nodes on the lower plant and creating very dense colonies.

Notes:

Arrow-leaved Tearthumb is relatively common throughout the eastern half of Minnesota. It is quickly identifiable by its sharp, claw-like bristles on stems and leaves that cause it to cling to clothing and even tear tender skin - it has drawn my own blood more than once! In this regard it is similar only to Halberd-leaved Tearthumb (Persicaria arifolia) but the latter can be distinguished by its broader leaves with pointed basal lobes perpendicular to the central blade and its sparse, slender flower cluster.