The other common knotgrass species, polygonum aviculare, has leaves up to three times smaller on the side branches; for polygonum arenastrum the leaves are all equal in size.
Main flower color:
Most of the British Isles; least common in north Scotland
Usually around 20 cm or less - forms low clumps
Disturbed ground, fields, roadsides, beach margins
Small, with five green tepals, resembling a calyx, and five petaloid lobes, white to pale pink, which are fused for half or more of their length. Flowers are produced singly or in small clusters, from the leaf nodes
Ovate, equal in size on both main and side branches
June to October