Action: Crassula helmsii: Use dyes to reduce light levels
- One replicated, controlled study in the UK found that applying aquatic dye, along with other treatments, did not reduce coverage of Crassula helmsii.
Dye treatments work by reducing the amount of light that penetrates through water, and therefore reducing photosynthesis in plants under the water. This could potentially reduce the growth rate of C. helmsii. However, a study in the Netherlands suggested using soluble red and black dyes did not substantially reduce the amount of light penetrating the water. The authors concluded effective control was unlikely given the extreme adaptability of C. helmsii (EPPO 2014).
For studies covering the use of lightproof barriers to inhibit growth see ‘Use lightproof barriers’. Increasing turbidity or using shading by vegetation to reduce growth are covered under the intervention ‘Alter environmental conditions to control plants’.
EPPO (2014) PM 9/19 (1) Invasive alien aquatic plants. EPPO Bulletin, 44, 457-471.
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 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.