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 It’ll be withdrawn from sale by next year.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.