The problem of an escaped water plant

If like me, you’ve kept goldfish (Carassius auratus auratus) in cold-water fishtanks, you’ve probably included aquarium plants. My pet goldfish were a joy and inspiration during my baby-naturalist childhood. So it’s a shock to learn that a plant I grew in my fishtanks is an alien species, so invasive that it’s being banned from sale in Britain.

I knew this plant as ‘curly pondweed’ but its posh name is Australian swamp stonecrop (Crassula helmsii). [See my note at the end of this post.] It’s sometimes mislabelled as Tillaea recurva or Tillaea helmsii. The Centre for Ecology and Hydrology (CEH) tells us that C. helmsii has invaded ponds and waterways across the British Isles. It grows fast, shading other plants and taking oxygen from the water.

Once C. helmsii is in a body of water it’s almost impossible to get rid. The CEH factsheet that I’ve just linked to reminds me of something I knew all those years ago: this plant can regrow from tiny fragments. So mechanical control doesn’t work. You can keep returning to your pond or stream, raking away the stonecrop, but you’ll rake away anything that may be entangled in it, and the stonecrop will soon be back.

You can resort to weedkiller (herbicide). In the European Union, diquat dibromide is banned for aquatic use but people are appealing against that ban, when the weed to be controlled is C. helmsii. Glyphosate (Roundup) works but there’s evidence that it’s a hazard to human health. Another herbicide called dichlobenil is also banned. Anyway, for me, weedkiller would be a last resort because it can disrupt the whole ecosystem.

So what’s left? Biological control is a very appealing idea. It can go wrong as I’ve mentioned under my ‘biological control’ tag, but with enough forethought, perhaps it will work against C. helmsii. My fellow blogger Suzy Wood at the Centre for Agricultural Bioscience International (CABI) tells us about research into biological control for this invasive weed. Here’s a CABI report on that ongoing project.

Despite the problem of its invasion, C. helmsii is still on sale in garden centres and aquatic nurseries. Not for much longer, though. It’ll be withdrawn from sale by next year.

If you’re setting up a fishtank or a pond, you’ll want aquatic plants. Now would be a good time to look at alternative plants. Plants which, if released, won’t add to a problem of invasion in our ponds and waterways.

[Edit] Apparently I may have been mistaken about this plant. Please scroll down to see what my fellow blogger Rachel at ecologyescapades says in her comments about invasive water plants.


About argylesock

I wrote a PhD about veterinary parasitology so that's the starting point for this blog. But I'm now branching out into other areas of biology and into popular science writing. I'll write here about science that happens in landscapes, particularly farmland, and about science involving interspecific interactions. Datasets and statistics get my attention. Exactly where this blog will lead? That's a journey that I'm on and I hope you'll come with me.
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5 Responses to The problem of an escaped water plant

  1. Rachel says:

    I think it is so stupid, and so bad, that for years this plant, and other invasive aquatic plants, have been on Schedule 9 of the Wildlife and Countryside Act 1981, making it illegal to ‘plant or cause to spread in the wild’ known invasive species, and yet nothing has been done, until now, about the sale of these plants. Just because you put it in a pond in a garden, does NOT mean it will not spread beyond the garden fence.

    • argylesock says:

      Yes indeed. Thank you for telling me about the 1981 Act. Does your work involve clearing invasive plants?

      Since writing this post I feel guilty about having introduced C. helmsii into the garden pond that was one of my projects during the early 1980s. But I feel annoyed too, because if I’d known that this was an invasive plant, I’d never have introduced it. I’d have been cautious the same way I was cautious about duckweed (Lemna spp.)

      • Rachel says:

        You don’t have to feel guilty – many people have done the same, especially with curly waterweed, a different aquatic invasive, that is sold as an ‘oxygenating’ plant. It is – only it benefits itself!

        I have come across so many ponds during newt surveys with these two species in, and I am making it my mission to inform owners of what these species are – it might be too late for the pond they are in, but the more people that know about them, and spread the word, the less likely they are to be used in new ponds. Just about every person I have spoken to was sold it in the mistaken belief that it would be good for the pond, as told by garden centres.

        My work doesn’t involve clearing invasive plants, no, this is generally something done by specialists, as for things like Japanese Knotweed there are specific chemicals and timetables, laws to follow about removal and destruction… it is a whole job on its own. However, when writing reports and we have found invasive plants, we note this and make ‘recommendations’.

        • argylesock says:

          Oh so there’s another plant that looks like the one I introduced! This one? Which of the Latin names shown on that page would you use for it?

          Now I’ll never be sure whether I introduced that plant or C. helmsii. I was just a kid. Thank you for saying that I shouldn’t feel guilty. To tell the truth, guilt isn’t part of my emotional repertoire! but in a general sense, I do know that I’m responsible for my actions.

          • Rachel says:

            Lagarosiphon major – yes – it doesn’t look quite the same as the one you posted about, but it is the most common one sold in garden centres as an oxygenating plant. It can grow up to 6m thick in deep water, and you have to hack it out in cubes because it all clings together.

            We are all responsible for our actions, but we can only be as responsible as our ignorance at the time allows us!

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