This is Common Reed, Phragmites australis, which is a grass. It forms large beds on suitable substrate and has been of great economic importance historically, primarily as a roofing material.
Ecologically reed beds provide habitat for a wide range of species. Three birds dependent on reed beds are the Bittern, Botaurus stellarius, the Reed Warbler, Acrocephalus scirpaceus, and the Bearded Reedling (aka the Bearded Tit), Panurus biarmicus. Many others are associated with them, e.g. Reed Bunting and Sedge Warbler.
I was down on the RSPB reserve doing a slightly overdue WeBS count and picked up a few species amenable to photographic capture. This was one: Water Mint or Mentha aquatica. It smells minty (being a mint).
