When the German adventurer Philipp Franz von Siebold pilfered a bamboo-like plant from the side of a Japanese volcano and presented a cutting to Kew Gardens in 1850, it was greeted with delight.
Today Japanese knotweed, Fallopia japonica, is one of Britain’s most destructive plants, forcing its way through walls, foundations and road surfaces and costing home-owners and the state about £166 million each year to remove.
The mere report that knotweed exists can make a property impossible to sell or insure — even if you live in a block of flats and the plant emerges in your communal garden (see case study).
Experts say it is on the rise because too many exterminators do not ensure it is dead before they finish treatment programmes. If the roots are not dealt with properly, knotweed will return. “Rhizome [root] segments can remain dormant for up to 20 years, so when the plant is superficially cleared it is possible for it to remain, only to cause problems in later years,” says Rosie McCormick-Paice, a partner at Pemberton Greenish, a law firm.
How can I spot knotweed?
Knotweed usually appears in the form of red or purple shoots poking through your grass, flower beds or patios. These quickly turn into stems and can grow as high as 7ft tall in the space of two months, while the leaves can be as big as 14cm. It flowers in the summer and the leaves fall in autumn. Knotweed can be confused with a number of other harmless plants, including houttuynia cordata and the Himalayan honeysuckle. Wales and southeast England are worst affected.
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How can I get rid of it?
There are two methods of treatment — spray or dig. The most popular is spraying it with herbicides and monitoring the affected area for several years. It is the cheaper option, and one which means your garden will be left intact. However, according to Nic Seal, the managing director of the knotweed removal specialist Environet, this is often not always effective long term. He says that sometimes the only viable solution is to dig up the roots — temporarily destroying a garden. Environet says that the biggest jobs can cost upwards of £100,000. However, if you catch knotweed early, you might get away with costs nearer £5,000. Mr Seal has in the past needed to dig as deep as two metres.
What happens if I do nothing?
If you leave knotweed, it can destroy the foundations of your property. Roots can cause structural damage to buildings, burrowing through floors and even walls. It is not illegal to have knotweed in your garden. However, mortgage lenders will refuse a loan on properties that have knotweed damage — and, sometimes, even on properties where damage was once seen, even anecdotally, and not treated. “Some lenders have reportedly refused to lend on properties where valuers have seen knotweed in the garden or nearby land,” says Ms McCormick-Paice. It is difficult to prove knotweed has been eradicated, so it may be hard to sell your house for several years after treatment, unless you have a guarantee from a specialist. If you live in a flat, keep an eye on your communal garden. The foundations of the block could potentially be at risk from a nearby infestation, and that could make your flat — and those of your neighbours — impossible to sell.
In 2012 a report by the Royal Institution of Chartered Surveyors stated that knotweed was most likely to affect outbuildings, conservatories or boundary walls. If you spot knotweed on neighbouring land, urge them to take action.
What other garden menaces should I watch out for?
Giant hogweed, originally from Russia and Georgia, is a tall, cow parsley-like plant with a thick stem that is sometimes purple-blotched, and topped with clusters of white flowers. Chemicals in the sap can cause blistering and lasting scars.
Himalayan balsam, introduced to Britain in 1839, is often found on riverbanks. The pink-flowered plant reaches over head height and can spread quickly and smother surrounding vegetation.
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Parasitic nematodes are little round worms, some species of which are known to infest grass and damage roots. It has been estimated that there a million species of nematodes on the planet — some are used by gardeners to kill slugs and snails. You can tell if you have a plant-parasitic nematode infestation when grass turns yellow or starts to wilt. There are no chemical pesticides approved for nematodes, but a natural garlic-based product is in common circulation, and was used earlier this month by Crystal Palace’s head groundsman on its turf.
‘The tiniest little bit left there can lie dormant for years’
If Japanese knotweed is discovered in the communal garden of a block of flats, mortgage lenders are likely to refuse loans to everyone living there and no one will be able to sell their property.
That was the worst-case scenario facing Brenda Dunn and her neighbours, who live in a decade-old development of flats and houses in Hindhead, Surrey. This summer they discovered the plant growing rapidly in a hedge at the back of their gardens, along with another section of their land, which has been used in the past for dumping garden waste.
The residents decided on a herbicide spray treatment with Environet, which comes with a five-year guarantee, and a copy of the guarantee was handed to all the neighbours, giving solicitors the proof they needed to agree to deal with any residents who want to sell.
“The tiniest little bit left there can lie dormant for years and then romp back with speed, and we would run the risk of it damaging our foundations,” she says. “Thankfully, everyone living on the estate has a copy of this guarantee, which they can provide to their solicitors if they decide that they are going to sell their house in future.”
