Crassula helmsii in the New Forest. Final report on the status, spread and impact of this non-native invasive plant, and the efficacy of control techniques following a 3 year trial

Published source details Ewald N.C. (2014) Crassula helmsii in the New Forest. Final report on the status, spread and impact of this non-native invasive plant, and the efficacy of control techniques following a 3 year trial. Freshwater Habitats Trust (Oxford) report.
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This study is summarised as evidence for the following.

	Action	Category
	Crassula helmsii: Chemical control using herbicides Action Link
	Crassula helmsii: Use hot foam to control plants Action Link
	Crassula helmsii: Use dyes to reduce light levels Action Link
	Crassula helmsii: Use grazing to control plants Action Link

Crassula helmsii: Chemical control using herbicides

A replicated, controlled study in 2011-2014 at waterbodies in the New Forest, UK (Ewald 2014) reported that treatment with herbicide reduced cover of C. helmsii, although this was not statistically tested. Average coverage of C. helmsii fell from 41% before to 9% after herbicide treatment, and it was eliminated from two out of five sites. Coverage of C. helmsii at control sites increased from 63% to 70%. The study also found that coverage of native plant species fell from 33% to 20% at treatment sites and from 17% to 14% at control sites during the trial. The glyphosate-based herbicide Roundup was used at five ponds at rate of 0.3 l/ha, once in autumn 2011 and twice during autumn 2013. No treatment occurred in 2012, and some ponds were only partially treated in 2013, because of high rainfall. The authors also reported that some C. helmsii patches were missed from treatment. C. helmsii coverage was assessed in five random 0.25 m² quadrats within each treatment area in winter and summer 2011-2014, and also in seven control ponds.
Crassula helmsii: Use hot foam to control plants

A replicated, controlled study in 2011-2014 at waterbodies in the New Forest, UK (Ewald 2014) reported that treatment with hot foam did not reduce cover of C. helmsii, although this was not tested statistically. Average coverage of C. helmsii was 56% before and 60% at the end of the hot foam treatment, compared to 63% and 70% respectively at control sites. The study also found that coverage of native plant species fell from 31% to 14% at treatment sites and from 17% to 14% at control sites over the trial period. A biodegradable agent composed of plant oils and sugars was applied as a very hot foam (above 97 °C for 2 s) to five ponds twice during autumn 2011 and autumn 2013. No treatment occurred in 2012, and two ponds were only partially treated in 2011 and 2013, because of high rainfall. Aquatic dye treatment was additionally applied to these two ponds. C. helmsii coverage was assessed in five random 0.25 m² quadrats within each treatment area in winter and summer from 2011-2014, and also in seven control ponds.
Crassula helmsii: Use dyes to reduce light levels

A replicated, controlled study in 2011-2014 at waterbodies in the New Forest, UK (Ewald 2014) reported that treatment with aquatic dye, along with other treatments at some sites, did not reduce cover of C. helmsii, although no statistical tests were carried out. Average coverage of C. helmsii was 72% before and 75% at the end of the dye treatment, compared to 63% and 70% respectively at control sites. The study also showed that coverage of native plant species fell from 17% to 11% at treatment sites and from 17% to 14% at control sites over the trial period. Several other treatments (mechanical removal, herbicide, hot foam) were also used at some sites during this trial. A combination of Dyofix blue and black pond dyes were applied to six ponds on 5-6 occasions between August 2011 and December 2013. C. helmsii coverage was assessed in five random 0.25 m² quadrats within each treatment area in winter and summer from 2011-2014, and also in seven control ponds.
Crassula helmsii: Use grazing to control plants

A small, replicated, controlled study in 2009 at four ponds in the New Forest, UK (Ewald 2014) found that excluding grazing did not reduce the cover of C. helmsii. There was no significant difference between average cover of C. helmsii between ungrazed areas (42%) compared to grazed exclosures (26%). There was no difference in cover of plant species of conservation importance in ungrazed areas compared to grazed areas (7% vs 10%). Exclosure fences were erected in March 2009 to create ungrazed areas in four ponds with at least 75% C. helmsii cover. Grazing was mainly by ponies and cattle, but the area was also used by deer, pigs and donkeys. Exclosures included plants under the water and on the bank. Cover of plants in five random quadrats was surveyed in each pond in autumn 2009.