Not welcome at Chelsea: the Japanese 'Triffid' ravaging Britain's gardens

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An alien invader is taking over Swansea, cracking concrete, pushing into homes and knocking over gravestones. Since it is virtually impervious to chemicals and pollution, fire is the only sure way of driving it back.

An alien invader is taking over Swansea, cracking concrete, pushing into homes and knocking over gravestones. Since it is virtually impervious to chemicals and pollution, fire is the only sure way of driving it back.

The Welsh city is merely the worst sufferer from Japanese knotweed, which has spread inexorably across the British countryside from Land's End to the northern tip of the Isle of Lewis. The weed is such a menace - a fragment the size of a fingernail is enough for it to get a grip - that you are breaking the law if you plant it.

"Japanese knotweed is an absolute nightmare," said TV gardener Charlie Dimmock. "It takes over embankments and scrublands and is incredibly difficult to get rid of. The most important thing people should understand if they find it in their gardens or elsewhere is that they must not throw it on compost heaps or rubbish dumps, because that is the way it spreads. The only safe way to get rid of it is by burning it."

Swansea has appointed a full-time knotweed officer to deal with the plant, but the city estimates it would cost nearly £10m to eradicate it. Spending on herbicides has reached £10,000 a year without result. Japanese knotweed, which has a flecked bamboo stem, long fluffy white flowers and orangey yellow roots up to 20ft long, is so persistent it grows back within weeks.

Because all the plants in the country have been found to be female, the whole invasion is believed to originate from just one plant. The footnotes of newsletters from local naturalist societies chart its spread. Introduced in the early 19th century by a Dutch horticulturalist, the plant was first noticed in the wild in 1900, and it had reached a rubbish-tip in Langley, Middlesex, two years later. By 1908, a botanist had spotted it near Exeter. It was in Suffolk by 1924, West Yorkshire in the early 1940s and Northumberland in the 1950s. A decade later it was everywhere.

"It really is like the Day of the Triffids," said Simon Ford, regional nature conservation adviser for the National Trust in Cornwall, which has embarked on a £50,000 three-year programme to try and conquer the weed. "Nothing seems to stop it.

"Not only is it destroying wildlife, it is also forcing itself into many of our archaeological sites. Near Land's End, for example, we have several Cornish mine engine houses and the weed has come right up into the middle of them. The worst areas are the river valleys, because it is so easily transported by water. In Rocky Valley near Tintagel, for example, knotweed covers 100 per cent of the land - the coastal grass and primroses have simply disappeared."

Just outside Swansea, in the village of Treboeth, the 19th century Caersalem Newydd Baptist Church is just one of the ancient buildings being damaged by the weed. "It is simply beyond control," said the Church secretary, Denise Rees. "The whole graveyard is covered with it and it's grown so high you can barely get in at all. It's even pushed over some of the older headstones. We've had quotes for as much as £8,000 to try and eradicate it, but we simply cannot afford to do it."

As a last resort, scientists are now being sent to the home of the Japanese knotweed in search of a solution. "Our plan is to try and find the natural enemies that are keeping it in control in Japan, where it is not a pest at all," said Dick Shaw, a weed biocontrol scientist who will lead the team from the inter-governmental organisation CABI Bioscience. "We know that there is at least one promising beetle that damages it very heavily. But we have to make sure it is specific to this plant.

"Japanese knotweed, for example, is in the same family as rhubarb and shore dock, which is an endangered species. We would therefore have to make sure that any beetle or fungus attacked only the knotweed and not these other species.

"If we find what we are looking for, screening would be carried out in a quarantine facility and only then would we apply to the Department of the Environment to release it everywhere."

Such caution is wise: there are several examples of attempted "natural solutions" which have turned out to be worse than the problem. The cane toad, for example, was introduced from India to the sugar cane fields of Queensland to deal with beetles which were devastating the crop. Not only did the beetles remain unscathed - they live too high in the cane for the toads to reach them - but three Australian states are now plagued by the ugly, noisy amphibians.

Cane toads breed fast, crowding out native species, eat almost anything that moves and are hard to kill. They are poisonous, repelling predators and often killing cats or dogs who pick them up in their mouths. Australian websites are full of wry humour about the relative merits of golf clubs and cricket bats for dealing with the creatures; one tip is to weaken them by shoving them in a freezer before attempting to kill them.

Even the common house sparrow, suddenly in short supply here, is regarded as a pest in North America. It was introduced by a New Yorker who hoped they would exterminate the caterpillars infesting trees in Madison Square, where he lived.

Should Britain consider importing a beetle to control another introduced species, the Japanese knotweed, it might be salutary to remember the old lady who swallowed the fly. As the song records, she swallows a spider to catch the fly, a bird to catch the spider, a cat to catch the bird and so on until she ends up swallowing a horse. The song concludes: "She's dead, of course."