South Wales is something of a hotspot for invasive plant species, with Himalayan (Indian) Balsam and Japanese Knotweed rightly getting a lot of the publicity due to their ubiquity. However there is another invasive non-native plant species that is possibly even worse than those due to how difficult it is to remove; Crassula helmsii, sometimes known as New Zealand Pigmyweed or Australian Swamp Stonecrop. As the common names suggest, it is a native of New Zealand and Australia and was introduced to Britain as an “oxygenating plant” in the 1920s. It was subsequently recorded in the wild in 1956 and has since spread across the UK, though most commonly in the south of England.
Crassula is a yellowish-green perennial with small white flowers. It favours still water up to 3m deep and forms a dense mat of vegetation which grows rapidly and for most of the year, without the winter die-back which is common amongst most flowering plants. It can completely engulf a pond or lake, shading out the waterbody and out-competing all native vegetation. Beneath the dense mat there is often severe oxygen depletion as well as removal of light which makes the pond unsuitable for the majority of invertebrates and fish that would normally inhabit it. The mat can be so dense that waterbirds can walk across the pond or lake without getting their feet wet and pets have been known to drown after mistaking it for a solid surface. Not only is it disastrous for native wildlife but it can also prevent recreational and commercial activities on affected waterbodies.
The plant hasn’t been recorded producing viable seed in this country but it can colonise areas from fragments as small as 5mm in length so is often spread by waterbirds or people who inadvertently cause the plant to break up and then such small sections can easily be accidentally transferred to previously unaffected water. Once Crassula is established it can be incredibly difficult to eradicate as it is dangerous to mechanically remove it due to the likelihood of causing further spread and it is frost-tolerant, dessication-tolerant and has no natural control in this country. It is possible to remove small areas by shading it from sunlight but even this needs to be done for as long as 6 months for it to be effectively killed. It is vulnerable to some pesticides but their use in and around water is not always viewed favourably and needs to be done carefully to avoid affecting non-target species. Often the only way to successfully stop an infestation is to drain the waterbody completely, which is obviously a major decision, particularly when that waterbody is on a nature reserve.
The plant has recently completely dominated a pond at Parc Slip near Bridgend and we are in the process of deciding how best to deal with the infestation. This really highlights how important it is not to transfer plants or animals between ponds, even if it seems to be for good reasons as this is often how the plant spreads; for example, we have had goldfish, terrapins and amphibians dumped at the reserve by well-meaning but ill-advised members of the public. It is also essential to maintain good biosecurity when working (or doing any activity) around water; washing pets, boots and equipment and allowing to dry thoroughly before entering another waterbody can help prevent the spread of invasive non-native species, such as Crassula, across the country.
The plant was formally available to buy from garden centres but it is now illegal to sell it. If anyone sees it for sale anywhere then please let us know.