Triangular club-rush  (Schoenoplectus triqueter)

This species was first found in 1650 on the River Thames (4), and it has since been recorded from several sites, all in the south of England, including the River Medway in Kent, the Tamar in Devon and Cornwall, and the Arun in West Sussex. Following a severe decline the species is now known only from the River Tamar in Devon, where it exists as just one or two small clumps (6). In Ireland it has been recorded in three rivers, and although gone from one river, it still has a large and apparently stable population on the Shannon (6).

It is a very widespread species, which is locally abundant in the tropics and becomes rarer further north. It is recorded from west, central and southern Europe through western, central and eastern Asia to Japan and the Philippines, and in north Africa (recorded once on the Suez canal only) and South Africa. It is so prolific in some areas that control with herbicides is required. It occurs from sea-level in many parts of the world to an altitude of at least 2400 m in Kashmir (6).

In Britain and Ireland it is restricted to the upper parts of tidal rivers where there are weakly brackish to fresh water conditions and large fluctuations in water level (at very high tides the plants may be submerged), but it occurs in a variety of wetlands and fresh water ecosystems elsewhere in the world (6). It is even a weed of paddy fields in Asia (6).