Persicaria lapathifolia is an annual herb with erect reddish stems with swollen joints, growing to a height of 20 to 80 cm (8 to 31 in). The leaves are alternate with short stalks, often densely hairy underneath. The leaf blades often have a dark-coloured blotch in the centre and are lanceolate or narrowly elliptical and have entire margins. Each leaf base has stipules which are fused into a stem-enclosing sheath that is loose and fringed with few if any hairs at the upper end. The inflorescence is a dense spike, often nodding. The perianth of each tiny pink flower consists of four or five lobes, fused near the base. There are six stamens, two partially fused carpels and two styles. The fruit is a rounded, flattened nut. This plant flowers from July to September in northern temperate regions.