It's one of developers' biggest headaches, but Japanese knotweed (Fallopia japonica) has become big business for firms set up to control it - so much so that established operators are now complaining of a further invasion of inexperienced contractors making undeliverable promises to clients. This is already leading to court cases, in which clients try to sue contractors when shoots of the plant resurface in sites declared weed-free.
BALI technical director Neil Huck says: "If you see anyone offering a '100 per cent guarantee', that should set alarm bells ringing, since there can be no such thing."
Complete Weed Control (CWC) managing director Ian Graham agrees. "Customers are being misled," he says. "I'm concerned about companies that say they can deal with it in a very short time, without removing the soil. If someone offers you something that sounds too good to be true, it probably is."
Developers have been a particular target, he says. "Often they are dealing with big sites worth millions, which they want to develop quickly and move on. If someone says to them, 'in 12 months' time, the problem will be gone', that's what they want to hear. But contractors can't guarantee that it won't resurface." Customers are starting to wise up though, he adds. "They find it refreshing to be told there is no silver bullet, and you can't just kill it stone dead."
Graham reckons there are hundreds of contractors offering invasive-weed control. "It has become a bigger part of our business, and some of our franchisees specialise in it," he says.
Increasing public awareness of the problem caused by the weed, and the difficulty of eradicating it, has been good for business, he says. "People in local councils are more likely now to see it as something they have to deal with.
"The increase in house prices has also meant it's now worth developing pockets of land that weren't viable before," he adds. These often have to be decontaminated, as they may have lain unused for some time and had soil and other waste fly-tipped on them.
And with growing awareness has come the need for staff to know how not to make the problem worse. "Small problems can be made larger by moving soil inappropriately," he says. "I've seen countless cases where a problem in one corner has been spread right across the site. Contractors paid to do, say, fencing, may not be aware of the effects of digging. There's a need to educate all people coming on-site."
One of the most high-profile cases of a Japanese knotweed problem has been the Olympic Park site in north-east London. Here, Huck has provided training to enable staff to both identify invasive species and to avoid spreading it around and beyond the site. Contaminated areas are effectively quarantined, with vehicles entering and leaving being sprayed with herbicide to contain any spread. But it is by no means unusual. According to one estimate, a grid of square-kilometre blocks over the UK would show more than half contain the weed. And as it is absent from many unpopulated areas, the implication is that most populated areas suffer from it to some extent.
Other invasive species such as giant hogweed and Himalayan balsam also pose problems, but according to Graham: "These are not the same concern because they're not so destructive." Not only must land owners and managers control the problem before a site can be developed, they also face a legal responsibility not to allow it to encroach upon neighbouring land. Even unused sites can be a liability requiring action. But contractors agree that there is no one-size-fits-all form of treatment.
Director of research and development Martin Chapman of contractor TCM says factors affecting the most appropriate means include cost, area, soil type, proximity to water and time. "It takes a whole growing season to treat chemically, but if the client hasn't got that, you're looking at cell burial or removal from the site," he says.
And the rising demand for treatment has spurred the quest for more reliable and less costly forms of treatment. According to Huck: "There is a lot of research going on at the moment, but the techniques involved have yet to be proved." Huck's firm, Ground Control, already uses "all the recommended techniques", he says, including placing contaminated soil on a root-proof membrane, then turning it each year to stimulate growth, leaving rhizomes susceptible to chemical treatment. Other operators will actually feed rhizomes with fertilisers to promote growth before spraying, he adds.
TCM is engaged in research with the University of Bath and Writtle College, looking at the efficacy of various methods, including stem injections. But Chapman admits to being sceptical of the value of injections. "There have already been a lot of trials, but they've been inconclusive," he says. "Some appear to have been successful, but in some older trials, it has come back years later."
The problem with chemical treatments is how to get the chemicals into the plant without shocking it into dormancy, he adds. And while a completely fail-safe solution has yet to be found, contractors' hands are increasingly tied by herbicide regulations.
Graham says: "We've lost some useful chemicals already, but using non-residual herbicides means we have to do repeat applications. We must safeguard those chemicals where nothing else will do."
He is concerned by moves made at European level to further restrict herbicide use in public areas, believing that the UK already has adequate safeguards on their use. "We are streets ahead of the rest of Europe in terms of control," he says.
On the other hand, a benefit to developers and contractors came in December when the Treasury announced that it would extend tax relief on land remediation to on-site and off-site treatment of knotweed-contaminated soil, but not to its disposal in landfill. But none imagine the problem going away any time soon. Graham says: "We have to talk about control rather than eradication."
TREATMENT: THE OPTIONS
On-site burial of contaminated soil under an impermeable membrane topped off with 5m of topsoil.
Spraying foliage with herbicide. Inexpensive but rarely effective on a single application. Soil may be agitated or even fertilised to bring dormant rhizome fragments into leaf.
Injecting herbicide into stems. Said to be more effective than spraying, but long-term effectiveness is disputed. Allows more controlled and economical use of chemicals than spraying but difficult to apply systematically on larger sites.
Off-site disposal of soil in a licensed landfill site. The costliest option, and likely to become more so as the tax and planning systems work to discourage landfill.
TREATING WEEDS ON WATERWAYS
A fleet of 30 purpose-built "weed boats" of varying design leads British Waterways' fight against invasive weeds.
British Waterways ecologist Chris John says: "We treat different invasive species for different reasons. Japanese knotweed has structural implications, giant hogweed (Heracleum mantegazzianum) is a health and safety risk, whereas aquatic weeds cause problems for navigation."
Staff are trained to NPTC or BASIS standard in order to apply those herbicides specifically allowed near water courses. "Loss of chemicals has made weed control more difficult, but we mainly use glyphosate against knotweed and it is licensed for use near water," says John.
In the water, he says, different regions face different problems, though floating pennywort (Hydrocotyle ranunculoides) "is on a march", spreading rapidly from the South East to most other parts of the country. "Mechanical removal is probably best, but it needs careful handling. If it breaks, you get inter-nodal regrowth."
Subsequent application of glyphosate would be made more effective with the use of TopFilm, a soya-based adjuvant from Waterland Management that "drip feeds" herbicidal agents to the plant, he adds. The Pesticides Safety Directorate is currently assessing the product for approval.