Crassula helmsii
Crassula helmsii

Taxonomy

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

Synonyms:

  • Crassula recurva (Hook. f.) Ostenf. non N.E. Br.
  • Tillaea recurva (J.D. Hook.) J.D. Hook.
  • Tillaea helmsii Kirk

    • Chromosome No. 2n = 36.

    Distribution

    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 345m (Llyn Pendam, Cardiganshire).

    Ecology

    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

    Origin: neophyte. A native of Australia & 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.

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    Visit the Maps Scheme map

     

    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.

    Crassula helmsii flowers late into the winter and may provide a useful source of nectar; studies of its value to bees and other invertebrates might therefore prove interesting and topical.

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

    References

    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.

    Citation

    Lockton, A.J. (date accessed). Species account: Crassula helmsii. Botanical Society of the British Isles. www.bsbi.org.uk.

     

    Case Study: Brown Moss, ShropshireSpraying some of the Crassula

    Brown Moss was once famous for its rare plants and heathland habitats, which earned it every conservation designation going, including SSSI, Ramsar and cSAC status. But since the 1960s it has been deteriorating slowly, and a host of characteristic species have become extinct there, including Pilularia globulifera, Sparganium natans, Baldellia ranunculoides, and Luronium natans. The cause is very obviously a lack of grazing, leading to succession to woodland.

    In the 1990s Crassula helmsii became very abundant around one of the pools at Brown Moss and efforts have been made to control it. The photograph here (top, right) shows a half-hearted attempt to spray it out, which coincided with the diappearance of Lythrum portula and Hypericum elodes from the same areas - but did little to harm the Crassula.

    In 2003 a more serious effort was made to dig up and remove all the Crassula and the ‘contaminated’ mud. The photograph here (bottom) shows the outcome of that effort a year later. In many places only Persicaria amphibia and Crassula helmsii survived. This effort coincided with the brief reappearance of both Baldellia ranunculoides and Luronium natans - almost certainly from buried seed that had been uncovered by these operatiCrassula and Persicaria amphibiaons. Unfortunately, only three plants germinated, and none of them set seed.

    This case study shows how difficult it is to eradicate Crassula helmsii without also damaging rare and more vulnerable native species. It also shows the value of active site management which can restore habitat and facilitate the return of lost species by uncovering the buried seedbank.

    Unfortunately, it shows that focusing too much on alien species can distract from essential conservation work, and even harm the native species that are supposedly being protected. The current status of Brown Moss is that almost all the features of conservation interest are now absent or damaged, and it arguably no longer warrants cSAC or SSSI status.

    If you have experience of Crassula helmsii control - whether successful or not - they can usefully be contributed to Conservation Evidence.

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