Wolffia columbiana and W. globosa (Araceae) new to Britain

In the course of surveys of ditches in the Pevensey Levels in October 2021, a large population of the non-native Wolffia columbiana H. Karst. was found, new to Britain. Subsequent investigation showed W. columbiana to be widespread on the Pevensey and Gwent Levels and present in other grazing marsh ditch complexes in Somerset, West Sussex and Kent, while W. globosa (Roxb.) Hartog & Plas was also found in the Gwent Levels. This paper describes the known distribution of W. columbiana in Britain, provides information on identification of different Wolffia species and considers the implications of this discovery for conservation of the native W. arrhiza (L.) Horkel ex Wimm.