Action: Crassula helmsii: Use salt water to kill plants
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- Two replicated, controlled container trials and two before-and-after field trials in the UK found that seawater eradicated Crassula helmsii.
The tolerance of freshwater plant species, such as C. helmsii, to salt water is variable, but at high salinities it is likely to have adverse effects. However, these effects are likely to extend to native co-occurring plants.
Supporting evidence from individual studies
A replicated, controlled container trial and a small, replicated, before-and-after field trial in 2006–2009 at a grazing marsh in Essex, UK (Charlton et al. 2010) reported that flooding with seawater eradicated C. helmsii, although no statistical tests were carried out. In the container trial, C. helmsii turfs grown in seawater were described as ‘appeared to be dead’ after five months, but those in freshwater and brackish water were still growing. In the field trial no C. helmsii was observed at the end of the seawater flooding period nor 9-11 months after seawater was drained. In the container trials, 15 x 15 cm C. helmsii turfs were placed in 15 l containers and checked every 6-7 days. Four were covered with brackish water (2,000 μS electrical conductance), four with seawater (30,000 μS) and four with freshwater. Field trials were carried out at two sites reported as having ‘abundant and widespread’ C. helmsii. An 8 ha field and a 120 ha field were flooded with seawater to 5 cm above the usual winter water level from April 2006-January 2007 and January-December 2009 respectively. C. helmsii was surveyed between August 2007 and August 2010.
A replicated, controlled study in containers in 2011 in Dorset, UK (Dean et al. 2013) found that increased water salinity led to decreased growth rate in C. helmsii. C. helmsii growth rates were highest in the freshwater control (0.05 g/day), and declined as the salinity of the water increased (0.015 g/day in 2 parts per thousand, 0.005 g/day in 4 ppt and a loss of 0.010 g/day in 8 ppt). After 31 days C. helmsii in the 8 ppt salinity treatment had died. Nutrient concentration did not affect growth rate. Ten gram samples of C. helmsii were grown outdoors in 5 l plastic containers in September-October and sampled after 31 days. Three salinities and a freshwater control were tested at three different nutrient concentrations, and each treatment combination was replicated four times.
A before-and-after trial in a coastal grazing marsh in Hampshire, UK (EPPO 2014) reported that inundation with salt water nearly eradicated C. helmsii, although no statistical tests were carried out. One year after the seawater treatment, 99% of C. helmsii had been killed. The plant only remained in places not reached by the seawater. Two years after the treatment the salt level had reduced to 2 ppt, but the original flora had not returned. The site was flooded with seawater in July 2008 and then naturally filled with rainwater in autumn 2008. C. helmsii cover was assessed in 2009. No details about the depth of flooding or size of site were provided.
- Charlton P.E., Gurney M. & Lyons G. (2010) Large-scale eradication of New Zealand pygmy weed Crassula helmsii from grazing marsh by inundation with seawater, Old Hall Marshes RSPB reserve, Essex, England. Conservation Evidence, 7, 130-133
- Dean C., Day J., Gozlan R.E., Green I., Yates B. & Diaz A. (2013) Estimating the minimum salinity level for the control of New Zealand Pygmyweed Crassula helmsii in brackish water habitats. Conservation Evidence, 10, 89-92
- EPPO . (2014) PM 9/19 (1) Invasive alien aquatic plants. Bulletin OEPP, 44, 457-471