A recent arrival in Northern Ireland - first noticed in a pool at Gosford in 1984 and at the flooded Glastry clay-pits in Co. Down in about 1985. Now a nuisance at the latter place. A native of New Zealand which has spread acosss Great Britain in recent years and has a deleterious effect on other aquatic flora. This plant is introduced for garden ponds and escapes into watercourses, or is thrown out by gardeners (compare Azolla).
All names: Crassula helmsii (Kirk) Cockayne; Crassula recurva (Hook.f.) Ostenf. non N.E.Br.; Tillaea recurva (Hook.f.) Hook.f.