Crassula helmsii (Kirk) Cockayne is correctly called, in English, New Zealand Pigmyweed, but it is also often known as Australian Swamp Stonecrop.
- Crassula recurva (Hook. f.) Ostenf. non N.E. Br.
- Tillaea recurva (J.D. Hook.) J.D. Hook.
- Tillaea helmsii Kirk
Chromosome No. 2n = 36 (Stace 2010).
It is native to Australia and New Zealand. The GBIF map shows it to have a restricted world distribution in south Australia and parts of NW Europe. It is a lowland plant: maximum recorded altitude is 345 m (Llyn Pendam, Cardiganshire).
- Origin: neophyte. A native of Australia and New Zealand that was introduced to Britain in the early 20th century and was first recorded in the wild in 1927 (in Essex).
- Rarity: common in England & Wales; local in Scotland; rare in Ireland.
- Threat: in the 1980s and ‘90s it was believed to be spreading very rapidly in the British Isles, but more recent authors (e.g. Crawley 2005) have observed that the increase was not so dramatic; and the Maps Scheme data shows that it is no longer among the top 100 most rapidly spreading species in Britain.
- Conservation: it has been said that Crassula helmsii would outcompete rare native species (e.g. Defra 2003) and, in consequence, many organisations attempt to eradicate it. There is, however, no published evidence for this point of view.
A perennial semi-aquatic plant that grows on the margins of ponds and lakes. It is notorious for regrowing from small fragments, which makes it difficult to eradicate. Like many non-native species it can become very abundant for short periods, especially in sites that have become eutrophic and where the native flora is already under stress. In less enriched conditions it is not so vigorous, and it tends to form sparse stands in shallow water. It favours light shade and is easily damaged by wave action, which explains why it is most commonly found in small ponds.
- Crawley, M.J. 2005. The Flora of Berkshire. Brambleby Books, Harpenden.
- Defra. 2003. Review of non-native species policy. Department of Environment, Food & Rural Affairs, London.
- Preston, C.D. & Croft, J.M. 1997. Aquatic Plants in Britain and Ireland. Harley Books, Colchester.