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Invasion of the Swamp Monster

November 9, 2016

Crassula helmsii – an invasive aquatic plant. Image: eyeweed | Flickr Creative Commons

In August, we heard from Dr Claire Wordley from the Conservation Evidence Group at the University of Cambridge about the publication of What Works in Conservation, an evidence-based manual reporting the effectiveness of different conservation approaches on a range of ecosystems and species.

Since then, the group has published a new set of findings on the control of freshwater invasive species, based on reviews of recent scientific studies.

Today, we hear from Dr Wordley again, as she writes about the ecological impacts of Crassula – an invasive aquatic plant originally from Australia which is popular with gardeners in Europe – and the effectiveness of different control methods to halt its spread.


Crassula: A ‘Superweed’ from the South

The Australian Swamp Stonecrop is a small, unassuming looking plant with incredible superpowers. 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 it is an invasive species, choking the oxygen from ponds and shading out other plants with knock on effects for entire freshwater ecosystems.

Australian Swamp Stonecrop, also known as New Zealand Pigmyweed (or to give it its Latin name, Crassula helmsii), 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 with 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’?


Crassula blanketing a small pond. Image: Benjamin Blondel | Creative Commons

Killing the hydra

Like the seven headed hydra of legend, Crassula helmsii seems able to regenerate after incredibly 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 together 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 the medicine may cure the disease, but 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 may also be adverse effects on some bird species, although this may be due more to habitat level changes than direct toxicity as other birds appeared to benefit from wetlands being sprayed with glyphosate.

Covering the invasive plant with black sheeting or carpet strips, may, where feasible, provide an alternate approach. While the evidence for the effectiveness of keeping Crassula in the dark is not as strong as the evidence for spraying it, five studies showed very promising results that lightproof barriers can eradicate or severely reduce the coverage of the weed. Sadly, on two sites Crassula recolonised after it was eradicated – indicating that controlling the spread of this plant is likely to be an uphill battle for some time to come. Flooding contaminated ponds with salt water also appears effective at killing Crassula, but salt levels need to be high, as it can survive in brackish water. The lethal effects of salt water are likely to be experienced by native flora – and in some cases fauna.

Since Crassula appears to be mostly spread by people, often on equipment such as nets and rods, effective biosecurity measures to stop the spread will be critical to maintaining areas free from this relentless invasive. Crassula survives drying well, but a 15 minute immersion in 45 °C water led to mortality in 90 % of the plants by one hour after treatment. Experimenting with hotter temperatures and longer immersion times may improve this further.


Crassula growing from an urban storm drain in Australia. Image: eyeweed | Flickr Creative Commons

Treatments to forget

Unfortunately, not all the methods that have been trialled to get rid of Crassula have proven effective. Since Crassula, like other aquatic plants, needs light to grow, aquatic dyes that reduce the light available to submerged plants seemed like a good idea. Unfortunately, in a trial in the New Forest, this proved to be a non-starter, with Crassula cover increasing slightly in dyed pools. Hot foam was another inventive idea – foam stays in contact with the plant for longer than hot water, rupturing the cells of the leaves. Sadly, this was totally ineffective in one trial, and pretty ineffective in another, meaning that this treatment won’t be rolled out to a pond near you any time soon.

Bleach was another failed treatment – adding hydrogen peroxide to tanks containing Crassula did not have sufficient controlling effects to merit field testing, where other plants and native wildlife may be damaged by the chemical. Grazing was also rated as likely to be ineffective or harmful – trials showed varying results, but Crassula cover actually increased significantly in grazed plots in one trial, and did not vary significantly between grazed and ungrazed plots in the other trial.


Crassula covering the shoreline of a lake. Image: Benjamin Blondel | Flickr Creative Commons

Go forth and test

As ever, there are treatments out there that have not yet been tested sufficiently (or at all) – some of which may later prove to be effective. It is up to conservation practitioners who use these methods to test them experimentally and publish the results where they can be accessed by others, enabling the whole community to learn from each manager’s experience.

Combining treatments such as spraying and covering plants with light proof barriers is one method that needs more testing. Other suggestions range from using liquid nitrogen or flame throwers to using fungal-based herbicides and educating the public about the need to decontaminate clothes and equipment between ponds.

Whatever methods people are using to get rid of this plant, it is clear that rigorous collection of more data is needed; on what works to kill Crassula, on what methods lead to an increase in native plant cover, and on what the effects of treatment are on freshwater fauna from zooplankton to fish, frogs and birds. Trials don’t need to be huge to help build our knowledge base, so long as they are well designed – in conservation science, truly every little counts.


What next?

Conservation Evidence will continue to add species to the freshwater invasives synopsis, which already contains 139 actions on American bullfrog, Asian clams, brown and black bullheads, floating pennywort, Ponto-Caspian gammarids, Ponto-Caspian gobies, Procambarus crayfish, red-eared terrapin, skunk cabbage, water primrose and now Crassula helmsii.

The synopsis will hopefully stimulate action to fill in the knowledge gaps, making invasive species control more effective; and when the synopsis is updated in a few years’ time, it is hoped that the evidence base will be much stronger. If not, we could see more freshwater ecosystems irreversibly altered.

4 Comments leave one →
  1. Jonathan Newman permalink
    November 9, 2016 13:42

    A few factual inaccuracies here…..The plant originates in Australia and is not called New Zealand Pygmyweed, it is commonly known as Australian Swamp Stonecrop because of its origin in AUSTRALIA!!!!!, but best just to call it Crassula. Hydrogen peroxide is NOT bleach. While glyphosate may have shown some toxicity to amphibians in trials undertaken 30 years ago, the modern formulations such as Roundup Pro Biactive 360 are totally harmless to amphibians and the only harm that could occur is dropping the bottle on a toad, or treading on a frog while applying the herbicide. You raise the spectre of Crassula and then condemn the ONLY way of treating it by quoting spurious outdated research to demonise herbicides. If realistic long term ecological studies were undertaken more often, an understanding that that seed banks of native species restore the natural habitat over a period of years, and rapid removal of the target weed by herbicides has only a temporary effect. Emergent Crassula is best treated by using an aquatic approved glyphosate formulation and treating any regrowth regularly, or covering with carpet or thick pond liner. Submerged Crassula is impossible to manage effectively without the use of diquat. However, the Conservation Evidence site provides a very good overview of the techniques available for a lot of problematic aquatic invasive species, which just needs reporting accurately….

  2. November 21, 2016 15:57

    Dear Jonathan,

    Thank you for your comment, which raises a number of interesting points. My apologies for the late response, I have been working away from the office recently.

    I did not claim this plant originated from New Zealand, and point out that it was introduced from to the UK from Tasmania (South East Australia) in 1911; however actually it does originate from both Australia and New Zealand according to several sources (,,

    Crassula helmsii is referred to by several common names including New Zealand Pigmyweed by groups such as Plantlife and Froglife, and also on Wikipedia. I deliberately mentioned several common names before switching to using ‘Crassula’ so that readers who had heard different common names would be sure this was the same plant (,,

    Hydrogen peroxide is a bleaching agent with several uses ( About 60% of the world’s production of hydrogen peroxide is used for pulp and paper bleaching. The second major industrial application is the manufacture of sodium percarbonate and sodium perborate which are used as mild bleaches in laundry detergents. Hydrogen peroxide is used to bleach teeth, and, when mixed with ammonium hydroxide is used to bleach human hair. There are of course many other bleaching agents in existence, some chlorine based and some peroxide based, but hydrogen peroxide is most certainly a bleach (

    In retrospect, I should have stated not that ‘glyphosate appears to be toxic to amphibians’ but that ‘some glyphosate based herbicides have shown toxicity in amphibians, although more recent formulations such as RoundUp Pro-Biactive have not so far shown harmful effects in amphibians’ and will ask for that to be amended.
    The comment about toxicity to amphibians should have been linked to a Conservation Evidence page where we report the results of this paper, which while a little out of date is not from 30 years ago: Relyea, R. A. (2005). The lethal impact of Roundup on aquatic and terrestrial amphibians. Ecological applications, 15(4), 1118-1124 ( This is an extract from the abstract: “I assembled communities of three species of North American tadpoles in outdoor pond mesocosms that contained different types of soil (which can absorb the pesticide) and applied Roundup as a direct overspray. After three weeks, Roundup killed 96–100% of larval amphibians (regardless of soil presence). I then exposed three species of juvenile (post-metamorphic) anurans to a direct overspray of Roundup in laboratory containers. After one day, Roundup killed 68–86% of juvenile amphibians. These results suggest that Roundup, a compound designed to kill plants, can cause extremely high rates of mortality to amphibians that could lead to population declines.” The formulation used was Roundup “Weed and Grass Killer”; 25.2% glyphosate plus a POEA surfactant to ensure penetration of the plant cuticle. For comparison Roundup Pro Biactive 360 contains 360 g/l glyphosate, present as 441 g/l (35% ww) of the potassium salt of glyphosate, plus ‘a blend of surfactants’ which Monsanto do not specify.

    There are clearly differences in formulation between different glyphosate based treatments, and you are correct that many of the more recent formulations including Roundup Pro Biactive are far less toxic or so far do not seem not toxic at all at recommended doses ( Howe, C. M., Berrill, M., Pauli, B. D., Helbing, C. C., Werry, K. and Veldhoen, N. (2004), Toxicity of glyphosate-based pesticides to four North American frog species. Environmental Toxicology and Chemistry, 23: 1928–1938. doi:10.1897/03-71; Mann, R. & Bidwell, J. Arch. Environ. Contam. Toxicol. (1999) 36: 193. doi:10.1007/s002449900460).

    However, I believe I am justified in advising a little caution in the use of glyphosate based herbicides, as long term exposure, ‘cocktail effects’ and repeated doses may increase the lethal and sublethal effects, as may temperature and the life stage at which herbicides are applied ( Güngördü, Abbas, Miraç Uçkun, and Ertan Yoloğlu. “Integrated assessment of biochemical markers in premetamorphic tadpoles of three amphibian species exposed to glyphosate-and methidathion-based pesticides in single and combination forms.” Chemosphere 144 (2016): 2024-2035; Gandhi, Jaina S., and Kristen K. Cecala. “Interactive effects of temperature and glyphosate on the behavior of blue ridge two‐lined salamanders, Eurycea wilderae.” Environmental Toxicology and Chemistry (2016); Soloneski, S., Ruiz de Arcaute, C. & Larramendy, M.L. Environ Sci Pollut Res (2016) 23: 17811. doi:10.1007/s11356-016-6992-7); Baier, Fabian, et al. “Effects of a glyphosate-based herbicide on the development of Common toads (Bufo bufo L.; Amphibia) at different temperatures.” EGU General Assembly Conference Abstracts. Vol. 18. 2016.) There is a lethal dose for even the safest herbicides, which should not be seen if application is correct but may result from repeated heavy use or from spills; managers should be aware that issues may result, and also that they should research for the safest formulations.

    Furthermore, some of the formulations which have been shown to be toxic are still available for sale, including those containing polyethoxylated tallowamine (POEA) surfactants which appear to cause the most toxicity to amphibians. Roundup Original, Roundup Transorb, Touchdown Herbicide, Roundup Weed and Grass Killer, Roundup Weed and Grass Killer Ready-To-Use Plus, Roundup Weed and Grass Killer Super Concentrate and Roundup UltraMax to name a few appear to be available for purchase, meaning that many managers could be buying these unsafe glyphosate formulas (although I am aware that not all of these are listed as aquatic safe, many still state that they will not harm wildlife) (formulas shown to be toxic in: Howe, C. M., Berrill, M., Pauli, B. D., Helbing, C. C., Werry, K. and Veldhoen, N. (2004), Toxicity of glyphosate-based pesticides to four North American frog species. Environmental Toxicology and Chemistry, 23: 1928–1938. doi:10.1897/03-71; Mann, R. & Bidwell, J. Arch. Environ. Contam. Toxicol. (1999) 36: 193. doi:10.1007/s002449900460; Dinehart, Simon K., et al. “Toxicity of a glufosinate-and several glyphosate-based herbicides to juvenile amphibians from the Southern High Plains, USA.” Science of the Total Environment 407.3 (2009): 1065-1071; Wagner, N., Veith, M., Lötters, S. and Viertel, B. (2016), Population and life-stage–specific effects of two herbicide formulations on the aquatic development of European common frogs (Rana temporaria). Environ Toxicol Chem. doi:10.1002/etc.3525; Rissoli, Rafael Zanelli, et al. “Effects of glyphosate and the glyphosate based herbicides Roundup Original® and Roundup Transorb® on respiratory morphophysiology of bullfrog tadpoles.” Chemosphere 156 (2016): 37-44.) For a review of the impacts of toxicity of a range of chemicals, including glyphosate, in amphibians I recommend

    I have also only addressed claims of glyphosate toxicity on amphibians, as that was the claim I made, but glyphosate and glyphosate based herbicides have been shown to be toxic to other aquatic organisms ( Cuhra, Marek, Terje Traavik, and Thomas Bøhn. “Clone-and age-dependent toxicity of a glyphosate commercial formulation and its active ingredient in Daphnia magna.” Ecotoxicology 22.2 (2013): 251-262.).

    Finally, I did not state that glyphosate based or any other herbicides should not be used, but flagged up a potential risk that managers should be aware of. We also do not know what surfactants are used in more recent Roundup formulas, so assessing their build-up in the environment and effects of chronic exposure is hard. Management decisions often involve weighing up one harm against another and picking the lesser of two evils; to be honest, more research is needed on cocktail effects, environmental build-up and toxicity to a range of taxa to make a truly informed decision about herbicides. However, we point out that covering with lightproof barriers is another effective approach.

    As for the recovery of native flora, I take your point that there may be recovery from the seed bank in later years, however we did not find papers showing recovery of native flora after any treatment (which is not to say it has not happened). Crassula also appears to recover or recolonise quickly in many cases after removal, meaning that in many cases there was no chance for native plants to recover. My point was simply to report the results of some of the papers we collated and explain that there may not be a swift, favourable and total response to any Crassula treatment yet devised.

    I hope this addresses some of your concerns. Obviously this is a complex issue and we are far from solving it as a community. More evidence is, as ever, key.

    Best wishes,

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