Spartina alterniflora is a naturalised alien in Europe first recorded in Britain in 1829 from the river Itchen, Hampshire (Marchant & Goodman 1969b). By the turn of the century, it had spread to occupy extensive areas of Southampton Water and occurred as far east as Chichester Harbour in Sussex (Rankin in Tansley 1911, Marchant & Goodman 1969b). Since then it has declined, at least partly in response to land reclamation (Marchant 1967) though perhaps also as a result of invasion by Spartina anglica, the fertile amphidiploid which arose from hybridisation between S. alterniflora and S. maritima.

S. alterniflora now survives only at Marchwood, Hampshire, and as transplanted clumps in the Spartina Garden in Poole Harbour, Dorset (Marchant & Goodman 1969b). The natural stand comprises a dense cover of S. alterniflora shoots with some Spartina anglica, Puccinellia maritima and Aster tripolium (Beeftink & Géhu 1973, Géhu & Delzenne 1975). The association occurs in scattered localities down the Atlantic coast of western Europe and is characteristic of situations with a lower and more variable salinity than other Spartina-dominated communities (Beeftink & Géhu 1973). The Marchwood stand has been placed in the sub-association asteretosum tripolii which has also been recorded from Brittany and Spain (Beeftink & Géhu 1973, Géhu & Delzenne 1975, Kortekaas et al. 1976).