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Don't panic: Japanese knotweed may be an alien invader but it can be treated

Japanese knotweed can cause panic among homeowners but Eric Curran, a partner in the Glasgow North office of DM Hall Chartered Surveyors, says that, if caught quickly and treated properly, the plant can be no worse than an outbreak of dry rot.

Japanese knotweed
Japanese knotweed

Invading aliens. Houses under attack. Fearsome female clones which can chew through buildings. It's no wonder people start to lose the plot a bit when their thoughts turn to Japanese Knotweed.

The alien superweed has been causing tales of house prices being slashed by its presence and celebrity homeowners such as Tom Conti comparing its infestation to the classic horror movie The Day of the Triffids.

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But once the last bug-eyed extra has run screaming from the set, perhaps it is time for calmer and more rational voices to prevail. And the fact is that Japanese Knotweed, alarming though it may initially appear, is in reality no worse than any other defect in a house - and it can be dealt with accordingly.

Rather than householders throwing up their hands, panicking and thinking that they will never sell their property, the more realistic message is that, if they catch it quickly and get it properly treated, Knotweed need be no worse than, say, a outbreak of dry rot.

Reading reports in the press, it is perhaps understandable that edgy home owners will be on the lookout for any sign of the shovel-shaped leaves, white blossoms and tough cane-like stems of fallopia Japonica.

Like so many of what we now think of as British garden plants, it was brought to Europe by the plant hunters of the Victorian era who scoured the wildernesses of the globe to find exotica for the collections of the burgeoning middle classes.

Knotweed originally grew on the sides of volcanoes, where it evolved by burying its roots deep underground to survive frequent eruptions, hot ash and flows of molten lava. In its native environment, it rarely grows more than about 18 inches tall.

However, in the more benign conditions of the UK, it grows exuberantly, thrusting tough roots, or rhizomes, deep into the earth and forcing into cracks in concrete, brickwork or road surfaces.

It can be spread from small parts of the plant in garden waste or carried on footwear. Remarkably, a Knotweed leaf can lie dormant for up to 20 years before once again springing to life and causing another infestation. Digging it up will simply increase the plant's density.

So what's the good news? Well, again remarkably, all Knotweed in the UK has developed from one original cutting, which was female and it has not yet been recorded as producing viable seeds. It only reproduces vegetatively.

And though it is difficult to find statistics on the number of properties affected, it is hard to believe it would be any greater than, for instance, the number of houses affected by shrinkages in clay soil causing subsidence.

Lenders, too, though they have shied away from the problem in the past, seem to be taking on board that the risk can be graded, depending on where the Knotweed is in relation to the property.

Knotweed material is regarded as controlled waste and not disposing of it properly would be an offence under the Environmental Protection Act, 1990. Allowing the spread of Japanese Knotweed into the wild is also an offence under the Wildlife and Countryside Act 1981.

So what should home and business owners do if they see the tell-tale signs of the dreaded weed? The first thing is to make a plan, in conjunction with a reputable and reliable contractor, following the Environment Agency's code of practice.

It is not advisable for owners to try to carry out the work themselves, since treatment is highly specialised and technical, and should not be taken lightly. Professional advice is important, particularly since any lender providing mortgage funding would normally seek some evidence of a care plan for the Knotweed over a period of time often supported by a guarantee.

Pulling by hand is recommended to avoid the unwitting dispersal of any cut fragments and with small areas it is best to dispose of the material on site. If it is taken off site it must be disposed of in a licensed waste site under strict conditions.

It is important not to spread soil contaminated with the rhizomes, as they will very quickly regrow, and reproduce the problem elsewhere. Similarly, strimmers, hedge cutters or chippers should not be used.

It is not good enough to take knotweed to council recycling centres, as it will contaminate any compost made by them, and it should go without saying that it is beyond the pale to dump contaminated waste in the countryside.

Like any other house-related defect, the key to Japanese Knotweed is to act fast. Hit it hard, hit it quickly and hit it effectively. That's the way to deal with a blooming nuisance.

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