If you don't know, sedges look a lot like grasses with long green leaves. However, grasses have leaves in two ranks up the stem (i.e. alternate) whereas sedges have leaves in whorls of three. This means that usually the stem of a sedge seems almost triangular. This is a picture of the female part of the flower head. It's a massive, spikly drooping flower head and is fairly unmistakeable, i think. This plant was growing on the side of a ditch at Amberley Wildbrooks.
