Action: Crassula helmsii: Chemical control using herbicides
- Seven studies (including one replicated and controlled study) in the UK, found that applying glyphosate reduced Crassula helmsii. In one before-and-after study at a single site glyphosate applied in combination with diquat reduced C. helmsii by 98%. Another before-and-after study at a single site found that covering C. helmsii with carpet before treating with glyphosate resulted in an 80% reduction in the plant.
- Three out of four studies (including one controlled study) in the UK found that applying diquat or diquat alginate reduced cover or eradicated submerged C. helmsii. One before-and-after study at a single site found that applying both diquat and glyphosate reduced C. helmsii by 98%. One small, before-and-after trial found no effect of diquat or diquat alginate on cover of C. helmsii.
- One out of two studies (including one replicated, controlled study) in the UK, found that treating submerged C. helmsii with dichlobenil in container trials led to partial reduction in its biomass. One small before-and-after field study found no effect of dichlobenil on C. helmsii.
- One replicated, controlled container trial in the UK found that treatment with terbutryne partially reduced biomass of submerged C. helmsii. The same study found reductions in emergent C. helmsii following treatment with asulam, 2,4-D amine and dalapon.
Application of chemical herbicides may offer a tool for localized management of C. helmsii, provided regulatory approvals are in place. Many herbicides apart from glyphosate are no longer approved for aquatic use in Europe. Vegetation above the water may require a different approach to that below the water, meaning that more than one herbicide may be required to eradicate all C. helmsii from a site. The most commonly-used herbicide on emergent vegetation is glyphosate and on submerged vegetation is diquat. Additives (adjuvants) can be used to improve the efficacy of application of chemical treatments. Retreatment after initial application of herbicide is often required.
The intervention ‘Surround with wire mesh’ discusses the use of mesh to surround treatment sites and minimize spread of disturbed plant fragments during herbicide application.
Supporting evidence from individual studies
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.
A controlled container experiment in 1988-1994 in southern England, UK (Dawson 1996) reported that control of C. helmsii varied between herbicide types, although no statistical tests were carried out. Treatment of low-biomass, underwater C. helmsii with diquat reduced biomass by 100%. Submerged C. helmsii biomass was also reduced by diquat-alginate (97% reduction), dichlobenil (66%) and terbutryne (48%). Glyphosate caused the greatest biomass reduction in trials on plants above the water (82%). Emergent C. helmsii biomass was also reduced by asulam (66%), 2,4-D amine (55%) and dalapon (51%). Trials of higher herbicide concentrations on higher biomass of C. helmsii showed that diquat and diquat alginate reduced biomass by at least 85% at all concentrations. However even at the highest concentration, C. helmsii was not eliminated. The biomass reduction caused by glyphosate was lower and more variable (between 5% and 80%). Fourteen 0.25 x 0.25 m turfs were grown in either deep or shallow water in 300 l tanks. Low-biomass trials used 13-16 kg/m2 fresh weight C. helmsii and took place in October 1988 and summer 1989 and 1990. High-biomass trials took place in spring 1993 and 1994 using up to 50 kg/m2 C. helmsii. Glyphosate and diquat were applied between 1x and 50x usual concentration. Trials lasted 5-12 weeks.
A small, controlled, before-and-after trial in 1992 in two lakes in West Sussex, UK (Child & Spencer-Jones 1995) found that spraying C. helmsii with glyphosate reduced plant height and cover, but dichlobenil and diquat had no effect (although no statistical tests were carried out). C. helmsii cover decreased from 100% to 60% 11 days after spraying emergent plants with glyphosate, but did not change in the control plot. Thirty five days after spraying, treated plants were shorter (4 cm) than control plants (15 cm). At the same lake, a small plot of submerged C. helmsii was treated with dichlobenil but this had no effect (no data provided). At the second lake, diquat alginate did not affect cover of submerged C. helmsii (before: 100%; 35 days after: 95%). In a second trial at the same site, diquat did not affect cover of submerged (before: 95%; 16 days after: 95%), floating (before: 70%; after: 70%) or emergent plants (before: 15-20%; after: 15%). Glyphosate with an active ingredient concentration of 2.5 kg/ha was sprayed onto leaves at three 1 x 6 m plots in August. One control plot was left untreated. Dichlobenil granules were applied at 150 kg/ha at a 1 x 4 m plot in July. Diquat alginate, containing 100 g/l active ingredient, was applied at 10 l/ha to the water surface in August. Diquat, containing 200 g/l active ingredient, was applied at 25 l/ha to shallow areas and 50 l/ha to deep areas in September. Sites were monitored twice before (June-July) and three times after (August-October) treatments.
A before-and-after trial at a single waterbody in a nature reserve in 2000-2002 in Hampshire, UK (Stone 2002) reported that treatment with diquat and glyphosate herbicide nearly eradicated C. helmsii, although no statistical tests were carried out. The treatment cleared 98% of C. helmsii from the lake, and ‘no major regrowth’ was reported two years after treatment. The diquat-based herbicide Reglone was applied at 50 l/ha in March 2000, and again at 35 l/ha one month later. The glyphosate-based herbicide Roundup was sprayed onto vegetation around the edge of the lake in November.
A before-and-after study in 2001-2004 at waterbodies in a nature reserve in Kent, UK (Gomes 2005) reported that application of either diquat-based herbicide or glyphosate killed most C. helmsii plants, although no statistical tests were carried out. Spraying once, using diquat on field ditches and glyphosate on the margin of a gravel pit, killed 70% of C. helmsii. A second application of diquat the following year was recorded as being ‘partially successful’ (no data given). However the authors reported that re-growth of C. helmsii was ‘noticed annually’ at sites treated with both chemicals. The diquat-based chemical Reglone was sprayed onto 50 m2 ditch at a rate of 10 l for 300 m in August-September 2001 and 2002. Glyphosate was applied at an unknown concentration in August-September 2004.
A before-and-after field trial in 2004 at a single waterbody in a nature reserve in South Yorkshire, UK (Bridge 2005) reported that treating C. helmsii with glyphosate-based herbicide partially destroyed the plants, although no statistical tests were carried out. Spraying with glyphosate killed approximately 50% of C. helmsii, but did not eradicate it. Glyphos biactive was sprayed on exposed plants in a shallow pond at 5 l/ha in July-August 2004. No details of the size of area treated or monitoring were provided.
A before-and-after study in 2002-2003 at a single pond in Surrey, UK (Anonymous 2004) reported that covering plants with carpet strips and then applying glyphosate reduced the area of C. helmsii, although no statistical tests were carried out. Approximately 80% of C. helmsii was killed, although it is not clear whether this was a result of herbicide application or was due to the use of carpet to prevent light reaching the plant. In autumn 2002, strips of carpet were placed over C. helmsii wherever possible on the pond edge and in shallow water. In July-August 2003 the carpet was removed and the glyphosate-based herbicide Roundup was applied twice (concentration not given). C. helmsii cover was assessed in June 2004.
- 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 report.
- Dawson F.H. (1996) Crassula helmsii: Attempts at elimination using herbicides. Hydrobiologia, 340, 241-245
- Child L.E. & Spencer-Jones D. (1995) Treatment of Crassula helmsii - A case study. Plant Invasions: General Aspects and Special Problems, Kostelec nad cernymi lesy, Czech Republic, 195-202.
- Stone I. (2002) War against Crassula – one year on. Enact, 9-10
- Gomes B. (2005) Controlling New Zealand pygmyweed Crassula helmsii in field ditches and a gravel pit by herbicide spraying at Dungeness RSPB Reserve, Kent, England. Conservation Evidence, 2, 62-62
- Bridge T. (2005) Controlling New Zealand pygmyweed Crassula helmsii using hot foam, herbicide and by burying at Old Moor RSPB Reserve, South Yorkshire, England. Conservation Evidence, 2, 33-34
- (2004) Chemical control of Australian swamp stonecrop (New Zealand Pygmy Weed) Crassula helmsii. The National Trust Conservation Newsletter, 8, 2-3