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Matt Montgomery and Andrew Bowyer look at dealing with Japanese knotweed - and the implications of ignoring it.

The Environment Agency (EA) recently put Japanese knotweed at the top of its 'most wanted' list of foreign species. The Department for the Environment, Food and Rural Affairs has estimated the cost of controlling Japanese knotweed at £1.56bn.

Introduced to Britain as an ornamental plant by the Victorians, Japanese knotweed flourishes anywhere and can penetrate minute cracks in mortar, concrete and tarmac, which in turn can lead to structural damage. It spreads at an alarming rate from the smallest fragment of existing plants, making cross-contamination all too easy.

The proliferation of Japanese knotweed has been linked to fly tipping and inappropriate disposal of garden waste. As a result, it is an offence under the Wildlife and Countryside Act 1981 to encourage its spread by disposing of it carelessly, cutting it or moving earth contaminated by it. Soil containing knotweed is classed as 'controlled waste' and its disposal is accompanied by stringent rules, upheld by the Environment Agency.

Despite this legislation and government guidance on dealing with the problem, many planners, site managers, landscape architects and engineers are surprisingly ill informed about the most basic aspects of control or treatment.

The fact is that failure to address an infestation can lead to long term structural problems, lengthy delays and hefty fines as well as posing serious liability questions under professional indemnity cover.

The building industry is acutely affected by Japanese knotweed because sites typically targeted for development can be the biggest sufferers. Disused or unmanaged brown field sites may be infested because of irresponsible dumping of contaminated waste; roads, rail and waterways can fall prey to its spread by transport vehicles and water flow.

The cost of removing the plant from construction to regulated land fill sites is inflated by the government imposed Landll Tax. The charge currently stands at £18/t of controlled waste, but is set to rise by £3/t each year with the objective of reaching a rate of £35/t by 2012.

Methods used to eradicate Japanese knotweed can carry substantial time and cost implications.

Spraying is only effective on areas not subject to soil disturbance, which virtually rules out land planned for imminent redevelopment using heavy machinery.

Spraying with herbicide can take four years to eradicate it, depending on the type and strength of chemical used. While this is the least expensive method of treatment, most building schemes cannot afford to abort work for such long periods while the threat is removed.

Excavation of Japanese knotweed and its removal to a regulated land fill site is the most common method of removal. Alternatively, contaminated waste may be buried on site, beneath a geotextile membrane and 5m of clean fill material.

Under EA guidelines, both methods can involve digging out an area up to 7m in radius around the plant and 3m in depth, making it far more costly and disruptive than spraying.

A 50m 2 infestation removed using digging and dumping or burying costs, on average, £160,000, compared with £4000 using spraying.

Digging methods can easily create tens of thousands of cubic metres of contaminated waste, but can cut treatment times to just a few weeks.

In the bid to tackle the knotweed challenge a number of specialist fims have emerged. Seeing this as a potentially lucrative source of income, some environmental engineers may be tempted to undertake the remedial action itself in addition to specifying the solution. This is dangerous ground as professional indemnity (PI) policies will not cover manual work - rather they cover professional advice given.

In choosing to undertake the remediation, there may be a temptation to sub-contract the manual labour and simply manage the process. But here lies another pitfall. Were the subcontractor to get it wrong and commit an offence under the Environmental Protection Act, the engineering practice would be liable. The practice would then struggle to get its PI policy to meet any nes imposed should a successful prosecution be made, and these nes can be severe enough to have a damaging impact on any business's bottom line.

Given that Japanese knotweed has been found on perhaps the highest profile construction project in the UK - the London 2012 Olympic site - general awareness of the problem caused by the plant has risen among insurers. There is now the very real possibility they could view it in the same light as they do asbestos and toxic mould and impose further limitations on cover under PI policies.

Matt Montgomery is senior surveyor in invasive plant management at Leyden Kirby Associates.

Andrew Bowyer is an account executive at Howden Insurance Brokers.

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