The swamp stonecrop, nicknamed "creeping death" by botanists, is so virulent and difficult to kill that it is spreading rapidly and strangling native plants. Since the weed, crassula helmsii, was first discovered growing in the wild in the south-west of England 20 years ago, it has spread to more than 300 sites all over the country.
English Nature, the Government's statutory adviser on wildlife conservation, is to recommend that it be added to the schedule of plants which it is illegal to release into the countryside, joining Japanese knotweed and giant hogweed.
Swamp stonecrop, which is yellow and green with distinctive white petals, was first used as an ornamental plant in gardens during the early part of the century. But the weed, which thrives in ponds and marshes, moved into the wild where it grows vigorously in tight clusters 6ft high smothering other plants and robbing them of oxygen.
For nature conservationists a particularly worrying characteristic is that it is so strong that it has no "die-back" period during the winter which might help check its spread.
Another difficulty is that it has proved resistant to herbicide, though even when it has apparently been killed off it can quickly re-establish itself.
"It's very tenacious," said a spokesman for English Nature. "When treated with sprays it will appear to have been killed. Then when you revisit the site you find it was only lying 'dogo' and lo and behold the little bugger will spring up again."
English Nature estimates that the weed now affects about 320 parks and nature reserves - including Sites of Special Scientific Interest - all over England, Scotland and Wales, with another 200 suspect sites hit.Reuse content