Crassula helmsii, New Zealand Pigmyweed

Should we control Crassula helmsii?

New Zealand Pigmyweed, sometimes known as Australian Swamp Stonecrop, is a small wetland plant from the southern hemisphere which was introduced to Britain in the 1950s and has since spread across much of the country.

It is often described as an aggressive invader and is said to swamp native vegetation and eliminate rare native plants. But there is little evidence for this and it seems more likely that eutrophication and mismanagement of wetlands are the main cause of the loss of rare species, whereas Crassula is merely a convenient scapegoat.

As a hardy amphibious plant, able to grow equally well in deep water as on damp shores, it does not necessarily have a negative effect on an ecosystem:

  • It is an oxygenating plant, contributing to the water quality in ponds that might otherwise become stagnant.
  • It flowers late into winter, attracting many insects at a time when few other nectar sources are available (it has a very distinctive smell, reminiscent of honey).
  • It often creates a damp mat of vegetation that is protected from desiccation and characteristically abounds with young frogs.

Although Crassula does sometimes become abundant at some sites, it can be less damaging that it at first seems. Other plants are capable of growing though it - even small or rare species - and after a few years it usually becomes much less abundant. In ponds it does not fare well if nutrient levels are low and if the margins are kept free from leaf litter.

Eradication of Crassula is expensive and often damaging to an ecosystem, and it is rarely if ever effective. Tiny fragments of plants remain and recolonise bare ground again rapidly afterwards. Techniques include spraying with herbicides and covering the ground with opaque plastic sheets. But this of course eliminates all plants and it can be highly detrimental to rare species. On SSSIs it is illegal to do this without permission from Natural England.

Advice from SEDN

Our advice, if you have a pond or lake with Crassula helmsii, is not to worry or take any drastic action. It is best to continue to manage the habitat as a whole rather than concentrate on any one species. Control scrub that shades the margins of the pool and thus reduce the build-up of leaf litter around the shore. Try to prevent nutrient enrichment by runoff from fields or contamination from underground sources such as septic tanks. Wave action on the shore will break up a dense sward of Crassula and help to restore an open shoreline, so activities such as boating and water-skiing have been found to be beneficial to nature conservation in places like Bomere Pool, Cole Mere and White Mere. Livestock grazing and horse riding are generally compatible with good management of such sites.

 

Written by Alex Lockton, September 2015

 

Crassula helmsii 1

Crassula helmsii has thick, fleshy leaves about 5 mm long and tiny 4-petalled flowers that smell of honey.

Crassula sward 1

Other plants grow easily though a carpet of Crassula and the moist conditions it creates on the margins of ponds can be advantageous to a range of species.

Luronium and Crassula

The rare plant Luronium natans (Floating Water-plantain) grows intermixed with Crassula at Bomere Pool, where there are few other aquatic plants.

 

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