Constant species

Poa annua, Polygonum aviculare, Polygonum lapathifolium, Polygonum persicaria.

Physiognomy

The Polygonum-Poa community includes open or closed stands of vegetation in which various knotweeds are the most characteristic feature. P. persicaria, P. lapathifolium and P. aviculare are all very common here and each can be abundant, often in locally dense, even more or less monodominant patches. Poa annua is also constant but not usually of extensive cover and there is often some Stellaria media, Chenopodium album, Plantago major and Chamomilla suaveolens. More bulky occasionals include Elymus repens, Lolium perenne, Urtica dioica and Cirsium arvense but none of these is more than locally prominent. Smaller herbaceous associates found at low frequency are Viola arvensis, Anagallis arvensis, Ranunculus repens, Trifolium repens, Spergula arvensis and Veronica persica, and there can be occasional scattered individuals of Rumex crispus, R. obtusifolius, Sonchus asper and Euphorbia helioscopa.

Habitat

This Polygonum-Poa community is characteristic of damp eutrophic soils in disturbed places such as gateways, tracks, farmyards and ill-managed leys, on dumped topsoil and made ground around building sites.

Compared with the Rorippa-Filaginella community, this vegetation is less dependent on long winter flooding to maintain extensive areas of ground then laid bare by a drop in the water table. Indeed, local impedence of drainage of rainwater, as on heavy or trampled ground, is sufficient to create suitable conditions for the rapid appearance of the annual knotweeds, Poa annua and weeds like Stellaria media, Chenopodium album and Chamomilla suaveolens.