Floating menace in the Grand Union Canal

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.