Battling the seven-headed hydra: Crassula control in Europe

8 11 2016
Hydra. Seba Albertus (1734-1765). Image from Wellsome Trust

Hydra. Seba Albertus (1734-1765). Image from Wellsome Trust

A contribution by Claire Wordley of Conservation Evidence.

The Australian swamp stonecrop (Crassula helmsii) is a small, unassuming looking plant with incredible resilience. It can survive both baking heat and freezing cold; it can live underwater, on the water’s surface and on land; it can survive being dried out, bleached and sprayed with hot foam; and it can regenerate from tiny fragments. Unfortunately, in the UK and elsewhere in Europe it is an invasive species, choking the oxygen from ponds and shading out other plants with knock-on effects for entire freshwater ecosystems.

Swamp stonecrop, also known as New Zealand pigmyweed, was first introduced to the UK from Tasmania in 1911 and sold in garden centres from 1927 as an ornamental pond oxygenator. Shockingly, despite being documented as an invasive plant in New Forest ponds as early as 1976, its sale in the UK was only banned in 2014. Crassula appears to be spread mostly by people, whether deliberately or accidentally; it appears to be concentrated around car parks, residential areas and areas where equipment such as fishing gear is likely to have come from an infected site. Nearly 20% of 700 UK waterbodies surveyed contained the weed. Since every 10% increase in Crassula corresponds to a 5% decrease in native vegetation, and negative effects of Crassula invasion have been documented for zooplankton, macro-invertebrates and fish, with possible negative impacts on amphibians as well, control and ideally eradication is clearly needed. But what works to destroy this superweed?

Killing the hydra

Crassula helmsii (photo by Benjamin Blondel)

Crassula helmsii (photo by Benjamin Blondel)

Like the seven-headed hydra of legend, Crassula helmsii seems able to regenerate after even harsh treatment and being shattered into tiny pieces. Documenting clearly what works to control this beast – and what does not – is critical. This work has recently been completed by Conservation Evidence at the University of Cambridge, as part of an ongoing series on controlling freshwater invasives. The team has worked to collect all the evidence on different ways of killing Crassula, and experts have scored these for their effectiveness (or otherwise).

One of the most effective ways to knock back Crassula appears to be applying herbicides, particularly glyphosate and diquat or diquat alginate. While each of these performed well to reduce Crassula in many trials – and the use of glyphosate and diquat together led to a 98% reduction in one trial – there are concerns that while the medicine could cure the disease, it could kill the patient. One study in the New Forest noted that native plant cover fell in the treatment sites at a greater rate than in the control sites, and glyphosate appears to be toxic to amphibians. There might also be adverse effects on some bird species, although this could be due more to habitat-level changes than direct toxicity, because other birds appeared to benefit from wetlands being sprayed with glyphosate. Read the rest of this entry »