Controlling Crassula helmsii – impact and options

New Zealand Pigmyweed Crassula helmsiithe status, spread, impact and options for control for this non-native invasive plant

Freshwater Habitats Trust has recently prepared a report on behalf of the New Forest Non-native Plants Project (Hampshire and Isle of Wight Wildlife Trust), as part of the RINSE (Reducing the Impacts of Non-native Species in Europe) project. It brings together work which has been ongoing since 2009 investigating New Zealand Pigmyweed Crassula helmsii in the New Forest, Hampshire.

The report is divided into three sections:

  • Part A describes the history, current distribution and spread of Crassula helmsii in the New Forest
  • Part B summarises our current understanding of the potential impacts of Crassula helmsii on the flora and fauna of the New Forest
  • Part C present the results of plant and invertebrate surveys following two years of trials to investigate the potential for treatments to control Crassula helmsii in New Forest ponds.

Part A: History, current distribution and spread of Crassula helmsii in the New Forest

Crassula helmsii was first recorded in the New Forest in 1976 in a roadside pond adjacent to houses. Sporadic records were made at various other sites within the New Forest Special Area for Conservation (SAC) thereafter, but no comprehensive data existed on its distribution. In 2000, 194 ponds (including some ditches) in the New Forest were surveyed by volunteers and staff as part of a Hampshire and Isle of Wight Wildlife Trust project (Crutchley and Wicks, 2001). Of these, 76 (39%) were found to contain Crassula helmsii and concerns were raised that Crassula helmsiimay pose a significant and increasing risk to these important freshwater habitats.

These waterbodies and an additional 385 sites (total 579) were surveyed by Dr Naomi Ewald and trained volunteers from the University of Sussex in 2009/2010. In total 116 (20%) were found to contain Crassula helmsii; including spread to 13 ponds which were not infected in 2000. These new investigations have shown that this distribution, whilst widespread, is concentrated in areas around car parks, dwellings and lay-bys, strongly suggesting that the main route of introduction is via accidental or deliberate release by people. Spread from these points of introduction, in the majority of cases, is via running water along ditches or surface flooding into adjacent ponds.

Since 2010, new sites for Crassula helmsii continue to be identified each year. These follow the same pattern as suggested by the previous survey, i.e. introduced by people or spread via water to an adjacent site. The vast majority of sites visited which are not easily accessed from a car park or dwelling do not currently have Crassula helmsii. The only exception to this rule is where vehicles and/ or equipment have been used which are likely to have come from an infected site. To date approximately 700 waterbodies have now been visited, of which 18% are known to contain Crassula helmsii.

Part B: Potential impacts of Crassula helmsii on the flora and fauna of the New Forest

The New Forest ponds are of exceptional quality for wildlife, identified as an important feature of the New Forest SAC and supporting many species which have significantly declined elsewhere in the UK and Europe. In 2009 and 2010 further investigations were made to understand the potential impacts of Crassula helmsii on native flora and fauna in New Forest ponds.

On average over two thirds of ponds surveyed had less than 70% cover of Crassula helmsii and over a third of ponds had less than 30% Crassula helmsii cover. Therefore, although widely distributed less than a third of ponds could be described as heavily infested (>75%).

The degree to which Crassula helmsii dominated a site was shown to be correlated with a number of different factors, including pH and availability of nutrients. At low pH, the extent of Crassula helmsii was apparently limited, and at nutrient rich sites the extent of Crassula helmsii at a pond was greater. However, confounding factors, such as time since introduction, the complexity and structure of the native plant community, and the presence of other competitively dominant native plant species, mean that it is not possible to attribute any one factor to the limited extent of Crassula helmsii at some sites. Further research to answer these questions would be of value.

Dominance of Crassula helmsii was shown to have a significant effect on the availability of bare ground and the abundance of native plant species. For every 10% increase in the amount of Crassula helmsii the amount of bare ground decreased by 6% and the amount of native vegetation by 5%. We could find no evidence that dominance of Crassula helmsii alone had caused the extinction of any plant species. There was anecdotal evidence at one site where native plant species of conservation importance were no longer recorded, but this could not be attributed solely to the presence of Crassula helmsii, but to an overall deterioration in conditions at the site from poor water quality.

Many ponds in the New Forest are grazed by commoners’ livestock, which is an essential element of the management of these ponds for rare and threatened native species. The intensity of this grazing pressure at some sites maintains an open sward in the pond margin and creates patches of bare ground essential for the germination and growth of native plants. Without this grazing pressure Crassula helmsii is likely to become dominant but the quality of the ponds would decline regardless because other dominant native species would also increase. There was no significant difference between pond macro-invertebrate communities in ponds with and without Crassula helmsii.

Part C: Investigating the potential for treatments to control Crassula helmsii in New Forest ponds

In 2011 the New Forest Non-Native plants project began trials of three different control techniques to determine whether it would be possible to eliminate Crassula helmsii from the New Forest. These methods were (i) a traditional herbicide technique and two novel approaches – (ii) hot foam and (iii) dye treatments. Treatments were applied in the summer/winter 2011 and 2012. In the following February of each year the ponds were re-visited to record the percentage cover of Crassula helmsii, bare ground and cover of native plant species. From May– July a full survey of percentage cover of Crassula helmsii, plant species and pond macro-invertebrate communities was undertaken.

  • Within 6 months of the first treatment, the cover of Crassula helmsii had been significantly reduced but not eliminated by the herbicide treatment (on average 84% reduction). The effect of the other techniques was not significant (hot foam: reduced by 12%, aquatic dye: reduced by 14%, control group: increased by 8%).
  • Within 1 year of the first treatment, the cover of Crassula helmsii had returned to pre-treatment levels, for all treatments. We also found that there was no difference in the composition or abundance of native plant and invertebrate communities.
  • In late summer/winter 2012, treatment was incomplete due to adverse weather conditions. Only the aquatic dye treatment was successfully applied because the ponds did not dry out enough for hot foam or herbicide treatments.
  • An interim survey was conducted in spring 2013, following the incomplete treatments in 2012. This showed that there was no difference in the cover of Crassula helmsii from the previous survey. A slight, but non-significant increase in Crassula helmsii cover may have been caused by the very wet conditions during the summer of 2012, favouring the growth of Crassula helmsii.
  • A full survey in summer 2013 showed a significant reduction in Crassula helmsii in some ponds. This difference cannot be attributed to a treatment type, because no treatments were undertaken in 2012 on these ponds. However, the reduction may be linked with increased grazing pressure due to very dry conditions in spring/early summer of 2013.
  • Full treatments using herbicide, hot foam and aquatic dye were completed in the summer/winter of 2013. At present it appears as though the most successful treatment option for reducing the cover of Crassula helmsii is the use of herbicide, followed by hot foam. Aquatic dye was not successful in this trial.

In summary, no treatment can be considered to be fully successful, because eradication has not been confirmed in any of the ponds in the trial. Results have shown that fragments will recolonise a pond to the same or greater extent than before. Treatments did not negatively impact upon cover of native plants in this trial, but results suggest that ongoing unsuccessful treatment could negatively impact on native species over time, if Crassula helmsii returns to the same or greater extent following treatment. The cover of native plant species and the amount of bare ground fluctuates naturally between seasons and between years. The presence of Crassula helmsii if not controlled by grazing to maintain an open sward, could disrupt the delicate balance of these ponds, by displacing species in years when the cover of bare ground is high, and the cover of native species is naturally low.


The distribution and spread of Crassula helmsii in the New Forest is now well understood. As a priority, biosecurity protocols for staff working within the New Forest SAC should be updated and adhered to, to prevent spread to uninfected/ isolated ponds. On-going public awareness campaigns for visitors to the New Forest National Park and home-owners within the park should focus on preventing introduction to currently uncontaminated sites.

Our understanding of the impact of Crassula helmsiion native flora and fauna in the New Forest is increasing. Crassula helmsii has wide tolerance limits, but is clearly less dominant at the edge of these limits and its dominance may be determined by the composition and structure of the existing native plant community. Grazing pressure is critical in limiting the dominance of Crassula helmsii at some ponds particularly where these ponds naturally dry out during the summer months; wet summers unsurprisingly encourage greater growth, whilst a reduction in growth follows a dry summer.

Therefore, whilst many ponds with Crassula helmsii maintain diverse flora and fauna factors, such as increasing nutrients from pollution, climate change and a reduction in grazing pressure due to changes in the socio-economy of the New Forest are likely to result in greater dominance of Crassula helmsii and corresponding reduction in the cover of native plant species. Whilst not observed during this study this could weaken the integrity of native plant communities and ultimately result in the decline and extinction of native plants. Even without the presence of Crassula helmsii, these factors would have a significant detrimental impact on the quality of these ponds for wildlife.

A significant reduction in Crassula helmsii was observed following treatment with herbicide and hot foam, but to date none of the treatments have been effective in eradicating Crassula helmsii at a site. Crassula helmsii was able to re-grow to the same greater extent following treatment which is highly undesirable in ponds which contain species with high conservation value. In permanent ponds, fragments of Crassula helmsii from deeper water recolonised the pond margin, therefore, both hot foam and herbicide treatments could only be used to eradicate Crassula helmsii in ponds which dry out completely. Eradication was not possible following a single treatment, therefore successful treatment may only be possible with repeat treatments in a single year and over a number of years. The unpredictability of the UK climate and re-colonisation of Crassula helmsii from adjacent sites means that planning a successful eradication programme is unfeasible in the New Forest at this time.

Further research and other control options should be explored, but in the interim, prevention of spread and monitoring to ensure that grazing is sufficient to maintain an open sward for native plant communities will be needed to maintain the quality of these ponds for biodiversity until an effective control treatment is found.



Crassula helmsii can become dominant at some sites – understanding the ponds which may be vulnerable can help to target areas which need to be safeguarded.


Making out the extent of Crassula helmsii in one of the trial ponds due for treatment with hot foam


Species rich pond margins in the New Forest which need to be protected from non-native species – raising awareness and better bio-security measures will be key.

You can read the full project report here – RINSE_Partner_Annex_report_by FHT and HWT