Attempts to control aquatic Crassula helmsii at Huis ter Heide (Tilburg, The Netherlands), with special reference to dye treatment

Several methods were deployed simultaneously to control, if not eradicate, the highly invasive Crassula helmsii (Australian swamp stonecrop, New Zealand pigmy weed) in a newly created shallow pond adjoining the heath and moorland pools of the Dutch nature reserve Huis ter Heide. Measures included mechanical removal of top soil after draining, followed by extensive covering of pond margins with non-transparent foil, regular manual removal of washed-up plants, and addition of non-toxic dyes. The latter aimed to reduce compensation depth sufficiently to prevent submerged growth below the foil-covered area. Treatment with a mixture of soluble red and black dyes (DyoFix®), commercialized for the control of aquatic weeds and phytoplankton, started in January 2013. Five further additions followed in the course of this year to make up for losses and (starting from July) to increase concentration.
Biomass of submerged vegetation was recorded prior to dye treatment (October 2012) and again in October 2013. Total biomass was substantially higher on the second occasion, mainly due to the continued increase of Crassula which became dominant throughout the pond. The abundance of Eleocharis acicularis did not change markedly, whilst Potamogeton pusillus decreased. Vegetation height increased slightly and the number of macrophyte taxa remained similar. Measurement of photosynthetically active radiation at different water depths showed that prolonged light limitation was unlikely to have occurred even in the deepest part of the pond, despite the use of considerably higher doses of dye than recommended. Consequently, the lack of a negative response was no surprise.
Although pond morphology and water-level changes complicated application in this particular case, effective control of Crassula helmsii by ‘shading’ with dyes appears unlikely given the extreme growth plasticity of this species.
The Huis ter Heide demonstration project is part of the EU co-funded Interreg 2Seas project RINSE (Reducing the Impact of Non-Native Species in Europe; www.rinse-europe.eu), which seeks to improve awareness of the threats posed by INNS, and the methods to address them.