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Troublesome triffids: How to get rid of Japanese knotweed

A Londoner has just been awarded costs and damages of tens of thousands of pounds against a chartered surveyor who didn’t spot Japanese knotweed growing in their garden.

In a case likely to have surveyors checking their botanical guide books, the owner sued not only for the cost of the removal of the invasive plant from their property but also of “making good” the garden and distress and inconvenience suffered.

The judge also took into account the diminution of value of the property. The owner commissioned a full structural survey when purchasing a ground-floor flat in 2014. The survey made no mention of knotweed so the owner went ahead with the purchase.

But the following year, their gardener found signs of the plant. Environet, a company specialising in knotweed removal, confirmed the plant was in three sites in the garden and had been there for at least three years. The owner paid the company to remove it, at a cost of more than £10,000.

Japanese knotweed has long been feared by property owners, and London is a hotspot. Just about now, new shoots of the bamboo-like plant are emerging and will quickly reach a height of two metres, while the strong roots can rampage under fences, damage paths and patios and work their way inside the cavity walls of houses, even emerging two storeys up out of the chimney stack. Removing knotweed from the London Olympics site cost £70m.

If you allow knotweed to escape from your garden into a neighbour’s you could be prosecuted, or given an Asbo for causing a nuisance. It gets even more problematic if you are moving house.

If you have knotweed growing in your garden you must declare it if you sell and this is likely to reduce the value of your property. Mortgage lenders will want an insurance-backed guarantee that the knotweed has been eradicated before agreeing funds to a potential buyer.

Nic Seal from Environet says: “Even when treated there’s still a risk valuers will say there is a residual diminution of the property of two to five per cent.” The costs for knotweed removal in an average London garden, he says, are around £2,500 plus VAT to treat with herbicide and £5,000 to £10,000 to dig it out.

So what should you do if you find knotweed in your garden? If you have a major infestation or you plan to sell your home in the next five years, contact a specialist. Most mortgage companies want a firm associated with the trade body the Property Care Association, so check its website to find a member near you.

However, if you have only a small clump and don’t plan on moving any time soon, you could aim to get rid of it yourself, says RHS chief horticultural adviser Guy Barter. “It’s evil stuff. It’s a lot of work and not feasible if there’s a vast infestation, but you can dig it out with a spade.

“Because it’s classified as ‘controlled waste’, you can’t let any plant material leave the garden. So stack it up to dry on plastic or concrete and then burn it. Or put it in rubble bags and leave it to die for a few years to be sure.”

Barter suggests combining digging up the plant with a herbicide such as Roundup. “But if you’re organic, just dig it up.”

If knotweed is coming in from a neighbouring garden, he says to dig a deep trench on the boundary and line it with a strong root barrier. Try ecomerchant.co.uk for root block impermeable membrane, from £6.25 per linear metre. Knotweed is the most notorious but by no means the only plant likely to have Londoners tearing their hair out. Bamboo makes a great urban privacy screen, rapidly growing to 6ft.

But the roots of “running” varieties can travel 20ft, easily ducking under a fence or wall and popping up next door in the form of thick, spiky shoots. If you plant bamboo, make sure you choose one described as “clump-forming” so it doesn’t get out of control.

If your neighbour’s bamboo is coming into your garden, talk to them about it first, suggests Barter. If they won’t remove it, you could offer to pay for a contractor to do it.

And if this doesn’t work, dig a trench at least 20in deep and line it with a strong root barrier. In this case, a nice clump of bamboo leaves between you and them might be a welcome thing.