Friday 19 July 2019

This invasive plant can grow through concrete walls and wipe out the value of your house in just months

Garden Special: Japanese Knotweed is an invasive species that can wipe out the value of your house in a few short months. Tanya Sweeney reports on a plant that is taking hold in Ireland.

Japanese knotweed (right) can wipe out the value of your home in just a few short months.
Japanese knotweed (right) can wipe out the value of your home in just a few short months.

Tanya Sweeney

Imagine you finally bought your dream home. For argument's sake, let's say it's close to transport links and has an expansive back garden.

In it, there's a pretty plant; lush green, flat leaves, bamboo-like structure, that blossoms white in summer and red in winter. Better still, you've heard that the plant has shoots that taste delicious, like rhubarb, and, according to one foodie, are great in jam.

Well, imagine if that very same plant grew up to 20cm a day and three metres deep, its roots getting in under the foundations of your house and invading the structure, Day of the Triffids-style, until it grows under the foundations of your house, choking drains, piercing walls and floors. Welcome to the horrible reality of Japanese knotweed; a plant that experts say Irish homeowners are rarely far away from.

Japanese knotweed is the scourge of both suburbs and countryside, and with very good reason. Left unaddressed, knotweed can effectively wipe out the value of a house. And the Irish property industry, not to mention homeowners, are only just waking up to the full extent of the nightmare now.

Left unaddressed, knotweed can effectively wipe out the value of a house.
Left unaddressed, knotweed can effectively wipe out the value of a house.

Brian Dempsey, partner with DNG, admits that it's the "one thing that would frighten me most" if it surfaced in a house survey.

"In some instances it costs more to get rid of than the house is worth, giving the property a negative value," he explains. "When the roots go in under the concrete, that's where the difficulty lies. You could have a house worth €350,000 and it might cost €400,000 to sort it. From our perspective, that makes it beyond a write-off."

It's a pricey problem to have: in Ireland, it is estimated that Japanese knotweed causes €266m of damage annually, ruining buildings, light rail, road foundations and bridges. In the UK, the cost of removing Japanese knotweed from the London Olympic site in 2012 was around £70m (€83m). A very pricey problem by anyone's yardstick.

One property insider, who wishes to remain anonymous, has admitted that he has seen knotweed writ large in some leafy Dublin suburbs.

Invasive plant control of Japanese Knotweed, Fallopia japonica, near St Keverne, Cornwall, England, UK.
Invasive plant control of Japanese Knotweed, Fallopia japonica, near St Keverne, Cornwall, England, UK.

"On the west coast, we were asked to assess an infestation in the garden of an old stone house undergoing renovation," he says. "The infestation had extended beneath the floorboards; had broken apart a stone balcony and was almost undoubtedly growing through the base of the stone walls. The cost of eradication to make the house safe was estimated at more than its purchase price."

Riverside towns, too, are particularly vulnerable, as river banks erode and knotweed rhizomes (stems) are exposed, fragmented, and carried downstream.

"The fact is that [geographically], Japanese knotweed doesn't discriminate," says Dempsey. "If it's within a mile of your house, there's a very strong possibility that you'll end up with it."

Frances Giaquinto, pictured below, is a botanist and non-native invasive plant species specialist based in Co Clare. "We have been called in to assess Japanese knotweed infestations in a number of back gardens in Dublin," she says. "In some cases it had been planted as an ornamental in the past, and has spread from there, and in other cases it has spread into gardens. Frequently we have seen a knotweed infestation spread laterally across the gardens of terraced houses.

A 'forest' of invasive Japanese Knotweed
A 'forest' of invasive Japanese Knotweed

"It's a highly evolved and successful plant that has readily adapted to Ireland's mild and damp climate," says Giaquinto. "Ireland's economic downturn meant that thousands of hectares originally zoned for development were laid to waste, providing ideal conditions for opportunistic plants, such as Japanese knotweed, to become established."

The knotweed will show up in a typical structural survey, but mortgage lenders, insurance companies and property agents are only now coming to terms with it. As such, there is no official protocol or policy yet in place.

"The UK banks won't lend on a house that has it, but I'm not sure that criteria is here yet," says Dempsey. "It's going to be a situation where more people will be aware of this.

"Surveyors are upskilling all the time and now they can identify it," he says. "I came across a case where a man planted it in his garden, thinking it would make a lovely hedge. The problems arise when people go and dig it up themselves and spread it themselves."

Billy Johnston, of DNG's Donegal office, has seen house sales fall through because of knotweed showing up in a structural survey.

"People try to get rid of it, but they're wasting their time," he says. "The Donegal County Council is very proactive though, and have put up notices on country roads, telling people not to cut the knotweed and bring it home to their own gardens."

Michael Dowling, director of Dowling Financial and chairman of the Irish Brokers Association Mortgage Committee, acknowledges that the lenders don't yet have an approach in place.

"If a valuer doesn't know what Japanese knotweed is, they won't make reference to it in the valuation report required when you take out a mortgage," he explains. "If an issue is raised, it's up to the buyer to renegotiate the price. It's an individual buyer's decision. It would be no different if a property had damp or dry rot."

Similarly, many insurance brokers have yet to come across the problem.

"I'm aware of it, but not aware of any insurance implications and I have yet to experience such a claim," says Tony Hickey of McCarthy Insurance Group.

"In the UK there is a stand-alone policy available in the market, but in Ireland some policies have accidental damage included, and some may or may not include this. If you had an issue with a plant that damaged your neighbour's house, your liability insurance, as part of your home insurance, would kick in."

While it's all too tempting to have a go at kicking the knotweed into touch, it's no overstatement to say that this really is a job for the professionals. Companies like Japanese Knotweed Ireland ( are currently working on projects in Dublin, Kerry, Tipperary, Leitrim and Clare. Depending on the severity, a professional service can cost from €1,000-€10,000.

"Many people have tried to eradicate Japanese knotweed and it hasn't worked," reveals Giaquinto. "This is because eradication is complex and it takes time. No one method is 100pc successful, nor appropriate for all sites. Infestations can be eradicated but it can only work if a three-phase procedure is adopted: site-specific assessment and planning; treatment and post-treatment monitoring."

There is a slight chance that it may return, yet a qualified landscape gardener can help keep it in check: "The risk of re-colonisation must be assessed for each site," notes Giaquinto. "For example, old and well-established infestations may look worse but may be much easier to eradicate than young infestations growing from infested soil that has been disturbed. To ensure eradication, post-treatment monitoring is essential. In the UK, eradication is defined as no re-growth for two years. This means that monitoring may have to continue for three to five years before eradication can be confirmed."

What to do if you suspect knotweed

Get the professionals in: Check that they know the Knotweed's growth habits and understand the risks and pitfalls of each eradication approach.

Don't ignore it: "It's not an offense to have JKW growing at a property," says Fran Giaquinto, "but a neighbour might be breaking the law if they allow a JKW infestation to spread from their land onto a neighbour's property."

Don't pretend it doesn't exist during a house sale: It will be revealed in the structural survey in any case.

Sunday Independent

Editors Choice

Also in Life