Lemna gibba vegetations which remain flat throughout the summer season have not been observed in the field. However, a flat “winterform” could be found above a hydrosoil of clay until May and above a hydrosoil of sand or peat until July/August. It has been concluded that flat Lemna vegetations in The Netherlands consist either of pure L. minor (mostly above sand), a mixture of L. minor and (temporarily) flat L. gibba, or exceptionally, (temporarily) flat L. gibba only.