Small Knotweed is a prostrate, densely branched, annual herb. Stems are branched from base, branches numerous. Lance-like, elliptic, stalkless leaves are 1.0-3.0 x 0.1-0.4 cm, entire. Ochrea (a kind of sheath formed by two stipules united round a stem) are 1-2 mm long, membranous, ovate, tube-shaped. Inflorescence axillary, 3-4-flowered. Flowers sunken between ochrea or hardly coming out, 1-2 mm across, nearly stalkless. Tepals 5, 1.5-3 x 0.5-1.0 mm, oblanceolate - lanceolate, unequal, outer tepals slightly longer and acute, inner too obtuse. Stamens 5, filaments long with broaden base, equal. Ovary small, trigonous with three styles and capitate stigmas. Nuts 1.0-1.75 mm long, circular to ovate, shining, black, glabrous.