Action: Crassula helmsii: Use lightproof barriers to control plants
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- Five before-and-after studies in the UK found that covering Crassula helmsii with black sheeting or carpet strips eradicated or severely reduced the cover of the plant. However, C. helmsii was reported to have progressively recolonized two of the sites where it had been had initially been reported as eradicated.
Covering vegetation with lightproof barriers, such as matting or black sheeting, can control growth and eventually kill plants by preventing photosynthesis.
Supporting evidence from individual studies
A before-and-after field trial in 2003 at waterbodies in a nature reserve in South Yorkshire, UK (Bridge 2005) reported that covering C. helmsii with black plastic and soil killed all plants, although no statistical tests were carried out. C. helmsii was covered with black plastic and topped with 1 m of soil in March 2003. No details about the area covered, duration of treatment or subsequent monitoring was provided.
A before-and-after field trial in 2003-2004 at a single pond in Bedfordshire, UK (Wilton-Jones 2005) reported that covering plants with black polythene eradicated C. helmsii, but it recolonized the site within a year and no statistical tests were carried out. Before the trial C. helmsii was estimated to cover 5% of the pond, and was eradicated after the treatment. However, one year after the treatment finished C. helmsii had recolonized the pond. The authors suggest this was due to plants which survived in surrounding areas not covered by the polythene. The 12 m2 pond was covered with opaque black polythene weighed down with stones for six months between autumn 2003 and spring 2004.
A before-and-after study at a single lake in Dorset, UK (Dawson & Warman 1987) found that covering C. helmsii with dark material killed the plant, although it slowly recolonized, and no statistical tests were carried out. Two months after the dark sheeting was applied, the underlying C. helmsii was killed. However, after this the plant progressively recolonized the site. Typar geotextile sheeting was used to cover 50 m2 of C. helmsii. No control or comparison, and few details of the experiment (e.g. timing, water depth), were provided.
A before-and-after study in 2000 at a single lake in a nature reserve in Hampshire, UK (Stone 2002) reported that covering with black sheeting killed C. helmsii, although no statistical tests were carried out. Black sheets (20 x 10 m) were secured tightly over submerged and exposed areas of C. helmsii for six months including summer. Few details of the site or methods were provided.
A before-and-after study in 2002-2004 at a single pond in Surrey, UK (Anonymous 2004) reported that covering C. helmsii with carpet strips followed by the application of glyphosate reduced the area of the plant, although no statistical tests were carried out. One year after glyphosate treatment approximately 80% of C. helmsii had been killed, although it is not clear whether this was a direct result of the use of carpet or was due to herbicide application. In autumn 2002, strips of carpet were placed over C. helmsii and weighted down 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.
- 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
- Wilton-Jones G. (2005) Control of New Zealand pygmyweed Crassula helmsii by covering with black polythene at The Lodge RSPB Reserve, Bedfordshire, England. Conservation Evidence, 2, 63-63
- Dawson F.H. & Warman E.A. (1987) Crassula helmsii (T. Kirk) cockayne: Is it an aggressive alien aquatic plant in Britain? Biological Conservation, 42, 247-272
- Stone I. (2002) War against Crassula – one year on. Enact, 9-10
- (2004) Chemical control of Australian swamp stonecrop (New Zealand Pygmy Weed) Crassula helmsii. The National Trust Conservation Newsletter, 8, 2-3