Deep in the jungle undergrowth, the pretty lime-coloured leaves of the Japanese knotweed offer welcome shade on a blisteringly hot sunny day in rural Buckinghamshire.
It is no wonder that wealthy Victorian landowners fell in love with this exotic plant, adding the fashionable ornamental species to their botanical gardens.
Yet these days, if you spot its oriental spade-shaped leaves and its speckled pink stem in your garden, alarm bells should start ringing. You could be facing a home disaster. Forget the plant horror of John Wyndham’s The Day Of The Triffids – Japanese knotweed is the real thing.
Menace: Our reporter Toby Walne sprays the dreaded Japanese knotweed
It grows up to a foot in a week and not only strangles other plant life, but it can destroy the foundations of your home and cause thousands of pounds worth of damage. It can also knock ten per cent off the value of your house.
And if that were not bad enough, once it is discovered in the garden, your home may become unsellable unless you spend thousands of pounds getting experts to dig it up – or chemically destroy it.
Knotweed is a greater threat than ever before. A wet spring, followed by sunshine, has provided it with the perfect conditions to spread like wildfire. And with global warming not going away despite the hot air at this month’s G7 Summit, knotweed is not going anywhere either.
I am in Buckinghamshire with weed control company Environet – and it doesn’t take long to stumble across the menace on a quiet country path ten miles south of Aylesbury.
The knotweed does not lash out like a Triffid, and its delicate foliage gives no clue to the plant’s power to wreak havoc.
I am drawn in by its three-inch-wide leaves that look similar to heart-shaped bindweed that wraps around it. The nearby stinging nettles seem far more dangerous.
Yet while studying its dappled bamboo-like green stems that are speckled purple and red, Environet director Luke Walton points out how prolific this plant’s growth is.
He says: ‘Look up and you can see that at some points the knotweed is almost ten feet high.
‘It stretches along this footpath for 20 feet, but what isn’t obvious is that it also goes back into the undergrowth for a further 30 feet. You could spend £20,000 hiring a professional to dig up this jungle while chemical treatment might cost at least £5,000 and require at least five years of spraying to kill it off.’
Now we have identified this Japanese menace, it is time to fight back. Wearing white boiler overalls with gloves, boots and a 16-litre knapsack sprayer weighing 20 kilograms, I start liberally spraying the plant using a three-foot lance that emits a toxic mix that you cannot buy across the counter.
It starts off easy work, but ten minutes into a two-hour task I begin to wilt under the intense heat.
Walton points out that I would have to visit this spot twice a year and spray for at least the next five years to beat the knotweed beast.
He says: ‘It might look bad above ground but that is just the tip of the iceberg. The real damage is done underneath with a network of root stalks known as rhizomes that can spread undetected for years.’
Under the Wildlife and Countryside Act 1981 it is an offence to let knotweed grow in the wild. But it is not illegal to grow knotweed in your garden unless it causes a ‘detrimental effect of a persistent or continuous nature on the quality of life of those in the locality’.
In this case the local council can order you to spend thousands of pounds to destroy it.
Since 2013, house sellers also have a legal requirement to state whether Japanese knotweed is on their property when they fill out an obligatory ‘TA6’ form – paperwork that is used for property conveyancing.
SNIFFER DOGS CAN ROOT IT OUT
Weapon: Mick can sniff out the knotweed
A new weapon in the war against Japanese knotweed is the sniffer dog.
Working in partnership with Environet, dog handling firm RFA Security is training up canines to detect Japanese knotweed rhizomes – smelling these root stems up to four feet underground.
The dogs are trained in the same way as those that find explosives.
As puppies, they play with tennis balls placed next to Japanese knotweed.
A sniffer dog can detect knotweed even if you cannot see it – including when it lies dormant in winter or it has been cut back to avoid detection by a home seller.
Two-year-old Mick is one of two fox-red labradors trained by RFA Security to detect knotweed.
Let off the leash, Mick licks nearby leaves to cleanse his pallet and sticks his nose high in the air in search of the specific scent.
Ignoring a ten-foot high forest of knotweed above ground, he only focuses on what is beneath the soil – scraping frantically with his paws once he has found some knotweed rhizome.
The secret is in Mick’s nose – which has 300million receptors linked to an area of the brain dedicated to detecting scents that is proportionally 40 times larger than that of a human.
The sniffer dog service does not come cheap at £900 but Mick is currently one of the best Japanese knotweed detectors on the planet.
If you are buying a home and knotweed is discovered in the garden, then a mortgage provider may insist on it being eradicated before agreeing to lend you money. Failure of a seller to be honest on the TA6 form can result in them being sued.
If the dreaded plant is caught early enough, it can be conquered. The Royal Horticultural Society says the most effective and simplest method to tackle a limited outbreak of knotweed is with a glyphosate-based treatment such as Roundup Tree Stump Weedkiller.
The plant can be cut back in the summer with just an eight-inch hollow stem left overground. You then drip feed the weed killer into the hollow so it sucks up the poison – and hopefully kills underground growth.
But it usually requires at least three to four seasons of treatment. A professional knotweed eradicator costs more, but they will have access to a more powerful weed killer that might tackle the outbreak in a shorter period. They should also provide a certificate to show that the work has been done and the problem stamped out. The company should also include a guarantee that if the problem comes back within five or ten years, they will treat it for free as part of the initial contract.
Colin Peters, of weed control specialist Perfect Ground Solutions in Wareside, Hertfordshire, says: ‘You do not need to have sleepless nights over Japanese knotweed – there are sensationalised stories about the damage it can do when in reality it is usually not a major problem.
‘You can pay £100 or so for a site visit from an expert to diagnose the situation and look at how it might be treated. If you are lucky, a couple of repeat treatments at a couple of hundred pounds might be enough. The final bill may come in well under £1,000 and it will be money well spent.’
But Peters warns that doing nothing is not a smart option – given knotweed’s propensity to spread.
Trying to dig it up yourself might do more harm than good by accidentally disturbing rhizomes in the soil. Taking away contaminated soil and treating a persistent clump of knotweed can cost at least £5,000.
