Taxonomy

Crassula helmsii (Kirk) Cockayne is correctly called, in English, New Zealand Pigmyweed but it is also often known as Australian Swamp Stonecrop.

Synonyms:

2n = 36.

Distribution

Crassula helmsii 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 has spread to most parts of England & Wales since it was introduced to Britain in the early 20th century. It is rare in Scotland & Ireland.

It is a lowland plant: maximum recorded altitude is 345m (Llyn Pendam, Cardiganshire).

Ecology

This is 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.

Status

In the 1980s and ‘90s it was believed to be spreading very rapidly in the British Isles, and it was alleged that it would outcompete rare native species (Preston & Croft 1997, Defra 2003). In consequence it has often been branded an invasive alien weed, and many organisations attempt to eradicate it.

More recent authors (e.g. Crawley 2005) have observed how the more dire predictions have failed to come true, and analysis of Maps Scheme data shows that it is no longer among the top 100 most rapidly spreading species in Britain.

Photographs on this page show it coexisting quite readily with native plants.

Future work

The issue of invasive alien species is an emotive one, and even within the field of scientific research it is difficult to find neutral studies of species such as this. Owing to the high cost of eradicating such plants, it would make sense to have a clearer understanding of their ecology and the effects - if any - that they really have on native species.

There have apparently been no studies yet of its phytosociology or its interaction with animal species.

Preston & Croft (1997) suggested that its seeds may not be viable in this country, which may be worthy of further investigation.

No evidence has yet emerged for it outcompeting rare native plants.