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Floating menace in Grand Union Canal

Tim Entwisle

Tim Entwisle

November 7, 2012

No not these! It’s a plant – rare in its natural habitat and a pest everywhere else. Not an unusual scenario in a world where we destroy vast areas of natural habitat at the same time as spreading seed and other bits of plants as widely as we can so they can test out their prowess in a smorgasbord of new and inviting locations.

The Floating Pennyroyal (Hydrocotyle ranunculoides) is a good example. Lynda and I saw a few ominous patches of this nasturtium-like plant in the Grand Union Canal at Brentford yesterday. We walked over from Kew Gardens in the gentle mizzle of a London autumn to the start of what is the UK’s longest canal system at more than 450 kilometres.


In North America, Floating Pennyroyal is becoming rather rare and has been listed as Endangered in some States. Even there, though, it has been spread into new areas by those pesky humans.

Brought to Britain in the 1980s for aquaria and ponds, it soon escaped (i.e. was dumped) into canals and rivers. By 1991 it was well established in the United Kingdom and Ireland. Once in this watery network you can see how it could spread quite easily.

It arrived in Australia about the same time – according to Geoff Sainty and Surrey Jacobs in Waterplants in Australia the first record was in 1983 near Perth – but to my knowledge has not spread in any big way to the eastern States.


In Western Australia and in the United Kingdom, Floating Pennyroyal displaces native plants, gets in the way of animals and boats, and looks unsightly. That is, it does what aquatic weeds do.


The flowers and fruits of the Floating Pennyroyal are small and hidden under the leaves but they are unmistakeably of the carrot family, the Apiaceae. As we walk further along the canal I see, high up on the bank on the other side, what looks like another sinister plant – the Giant Hogweed, a giant plant with a giant name (Heracleum mantegazzianum).

Giant Hogweed is an invasive member of the same plant family, but this one can give you severe skin blisters if you as much as touch it in sunlight. Cue theme music for Jaws.

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Julie Thomson
11 years ago

Interesting post, Tim. Had no idea that pennyroyal was such a nuisance, indeed pest. How was it brought to Britain? From where? That hogweed sounds like something out of Harry Potter. Valdemort’s revenge?

Is this the same pennyroyal plant I buy in a little pot at the herb stall, that claims to be good for arthritis?

It certainly has sinister connotations for waterways with its choking, smothering tendencies …. we have several water-born plant pests here, including some algae that discolours and clogs our streams and dams.
Ours gets a reddish growth sometimes in summer, that a good flush with rain usually disperses…..

Tim Entwisle
11 years ago
Reply to  Julie Thomson

Thanks for the feedback Julie. It seems that this floating pennyroyal (different to the smaller one you would be growing) was brought in for aquaria (fish tanks) and then spread when plant material was dumped in a canal. The same thing has happened a lot in Australia when both fish and plants have been tipped into rivers or ponds from aquaria.

And yes plenty of nasty aquatic weeds out there. Some algae are quite nice of course (I have a special interest in them) but like all living things, if they are in the wrong place under the right conditions they can do bad things!