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Last Updated: Thursday, 22 September 2005, 08:40 GMT 09:40 UK
What's the problem with Japanese knotweed?
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Japanese Knotweed
Looks harmless but is a menace
Japanese knotweed has to be removed from the 2012 Olympic site in east London. Why is that such a problem?

With bamboo-like stems and clusters of creamy flowers, Japanese knotweed sounds exotic.

But it holds the title of the UK's most invasive plant and has become the subject of horror stories. Its removal from the 2012 Olympic site in east London could cost hundreds of thousands of pounds.

Flourishing in any soil - however poor - Japanese knotweed spreads relentlessly, overwhelming other plants and damaging ecosystems.

With the ability to grow through walls, tarmac and concrete it can wreck roads and buildings. Experts say a new plant can grow from a piece of root the size of a garden pea.

The Victorians are to blame for introducing it into the UK. They thought it was beautiful and brought it over from Japan, but they soon got fed up when it started overtaking their gardens. They dug it up and threw it out, spreading it to the wild.

Natural enemies

The problem is now so great the government estimates that controlling the weed countrywide would cost 1.56bn. Planting it or dumping it can lead to two years in prison, a large fine, or both, under the Wildlife and Countryside Act 1981.

Its rapid spread in the UK is largely down to its toughness and the fact it has no natural enemies, says Richard Shaw of CABl Bioscience, which is heading a project to find a natural control for the weed.

"They don't have a problem with it in Japan because its natural enemies keep it under control," he says.

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An infestation reduces land value and removing it is expensive and time consuming. With roots that can spread seven metres wide and three metres deep, huge amounts of soil have to be dug up and there are strict legal restrictions for getting rid of it.

Contaminated soil is classed as controlled waste. It has to be buried at least 10 metres deep on site or taken away by a licensed operator to a designated landfill site and buried to a depth of five metres.

Many landfill operators do not accept it and those that do need advance warning of its arrival and charge a premium rate because of the requirement for deep burial.

Imported bugs

Treating the weed with chemicals is cheaper but can take years, according to experts. Removing it has delayed the completion of a 1.5m road near Fareham in Hampshire for three years because the county council has decided to poison it at a cost of 5,000. Removing it would have been quicker but would have cost 250,000.

There is also concern about the toll heavy herbicides have on other plants and animals.

Other systems for containing its growth - such as using root barriers - are used but experts say no method is guaranteed to eradicate the weed. Often it just lies dormant and there is evidence of it starting to flourish again after 22 years if it is disturbed, says Mr Shaw.

The main hope in tackling the menace is to import bugs from Japan to help control its rampant spread. Doing nothing is not an option as it will continue to spread unchecked across the country, says the Japanese Knotweed Alliance.


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