Japanese knotweed has been labelled ‘the most dangerous plant in Britain’ and can cross beneath a six-lane motorway. Mike Beevers discusses how the Asian invader remains one of the industry’s most pressing problems.

THE figures speak for themselves. Defra estimates that invasive, non-native species of plants cost the British economy a staggering £2bn a year.

And it estimated that it would cost some £1.56bn to remove Japanese Knotweed alone, even though eradication would prove too costly and too impractical.

Defra has now concluded a consultation, carried out with the active support of the devolved administrations in Scotland and Wales, aimed to help tackle invasive, non-native species. The consultation closed in May and its findings are eagerly anticipated.

Devon County Council is among the many local authorities that takes the knotweed problem extremely seriously.
It points out that the Fallopia japonica was first introduced to Britain by the Victorians as an ornamental plant and was actually awarded a gold medal at a prestigious flower show. It is referred to under its previous name of Polygonum cuspidatum in The English Flower Garden by John Murray. In the 1907 edition it is cited as “easier to plant than to get rid of in the garden”.

In its native countries of Japan, North China, Korea and Taiwan, the weed presents nowhere near the problem it now poses across Europe, America and New Zealand.

With its natural habitat being on the slopes of volcanoes, it is no surprise that the less harsh and more fertile environment of Britain has allowed this plant to flourish to extreme proportions. Furthermore, outside of Asia, the plant has no natural biological enemies to check its spread. In Japan, for example, it is eaten by at least 30 species of insect and six species of fungi.

Japanese Knotweed is perennial and extremely invasive. It thrives on disturbance.

The tiniest piece can re-grow and has been spread by both natural means and by human activity. It soon overruns riverbanks, railway embankments, road verges, gardens and hedgerows, threatening the survival of other native plant species and in turn insects and other animal species.

In areas between land and flowing water, fragments of the plant are dispersed downstream where new colonies form. In the past, fly-tipping and transportation of soil containing rhizome fragments have been a major cause of spread, particularly in the urban environment. Green waste recycling schemes are also sites of potential contamination which is a cause for concern. Local Authorities, including those in Devon, are desperate to find ways of eradicating this serious pest.

The UK-based Japanese Knotweed Alliance was established in November 1999 to highlight the problems posed by this invasive weed and to promote its natural control with natural predators.

The alliance was founded by CABI (formerly the International Institute of Biological Control) the Welsh Development Agency, the Environment Agency, National Botanic Garden of Wales and Cornell University. It has since grown to include British Waterways, Defra, South West Regional Development Agency, Network Rail and Cornwall County Council as the programme has developed

“Doing nothing is not an option,” a Knotweed Alliance spokesman says. “Even in places where there is a concerted effor to control the weed it is still spreading. The balance of nature has been upset by mankind short-circuiting the normal progression of things. By introducing this plant to a new area without any of the checks and balances that it had to deal with in its native range, knotweed will continue to benefit from an unfair advantage.”

The alliance also says that the only current alternative control method requires the widespread and long-term use of herbicides and since knotweed is still in a rapid expansion phase this could only mean increased chemical use rather than the most stated government goal of reduced chemical use.

“Unfortunately, experience shows that chemical treatment can just force knotweed into dormancy,” the spokesman adds.

“Many knotweed areas are near watercourses, so there is a risk of pesticide contamination to the stream or river but often attempts to deal with knotweed by physical means actually exacerbate the problem. A successful biological control approach will greatly reduce the need for these sprays.”