Constant speces

Bidens tripartita, Filaginella uliginosa, Phalaris arundinacea, Polygonum amphibium, Polygonum hydropiper.

Physiognomy

The Polygono-Bidentetum comprises stands of open or closed vegetation variously dominated by Bidens tripartita, Polygonum amphibium or P. hydropiper. Other knotweeds are typically scarce but P. persicaria sometimes occurs. Filaginella uliginosa is constant but generally at low cover and there are very often some sparse shoots of Phalaris arundinacea. Alopecurus geniculatus and Agrostis stolonifera are quite common, occasionally forming dense mats of shoots and Callitriche hamulata occurs frequently. Other occasionals include Alisma plantagoaquatica, Myosotis laxa ssp. cespitosa, Juncus bufonius, Plantago major, Potentilia anserina and Stellaria alsine.

Habitat

The Polygono-Bidentetum is characteristic of periodically-flooded, eutrophic silts and clays towards the limit of inundation around fluctuating ponds, lakes and reservoirs.

It is the combination of periodic inundation and high levels of nitrogen in waters and/or substrates favoured by the ephemeral species of this assemblage that invade as flooding subsides. Typically, inundation occurs in winter or at least more extensively then, so that exposure of a damp, fertile substrate coincides with the warmer temperatures of spring for germination of the annual plants. In fact, some frequent species in this community are perennials, like Polygonum amphibium and Phalaris arundinacea, occurring at generally low covers but maintaining themselves towards the upper limits of flooding.