Polygonum aviculare has a wide distribution as an arable weed and plant of fields, shingle, sand, roadsides, yards and waste places.

There is much morphological variation among different populations and several different sub-species are recognized:[1][2][3][9] Widespread and common in Great Britain,[14] Ireland,[14][15] and Scandinavia.

Its Ellenberg values in Britain are L = 7, F = 5, R = 6, N = 7, and S = 0, [17] which shows that it prefers average light, moisture and fertility, neutral pH and non-saline conditions.

Many species of insects feed on knotgrass, including the eponymous Knot Grass moth, Acronicta rumicis, whose larvae are phytophagous on the leaves.

[20] One fossil fruit of Polygonum aviculare has been extracted from borehole samples of the Middle Miocene fresh water deposits in Nowy Sacz Basin, West Carpathians, Poland.

[21] It formed a traditional ingredient in porridge consumed by Germanic peoples of western Europe, and has been found in numerous autopsies of peat bodies, including the Tollund Man.

In Vietnam, where it is called rau đắng, it is widely used to prepare soup and hot pot, particularly in the southern region.