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Japanese Knotweed latest findings: Mild threat or wild menace?

By Jan Moys

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Japanese Knotweed has gained a fearsome reputation as an invincible and destructive weed that can threaten the value of properties within its reach. Historically, mortgage providers have been cautious about lending against affected properties. But what is the real threat level posed by this pernicious perennial? New research from a team of ecologists at Leeds University suggests its destructive capacity may have been exaggerated.

Introduced into the UK in the eighteenth century, Japanese Knotweed was prized for its decorative qualities and as a possible solution for concealing and stabilising railway embankments. However, in recent years it is commonly referred to as a “scourge” in the press, and even named a “tarmac-smashing thug” by The Telegraph.

Japanese Knotweed is certainly invasive – and pervasive. The plant spreads extremely quickly and the smallest pieces of cut stem can take root, making the plant very difficult to kill off. Knotweed’s negative impact on the environment, with native plants struggling to survive under its dense foliage, is not in question. However, a certain amount of hysteria appears to have amplified the plant’s ability to cause damage to residential buildings.

New findings

New research published in July 2018 shows that Japanese Knotweed does not cause as much damage to buildings as had previously been thought. The team from Leeds University surveyed 51 contractors and 71 surveyors who provided insights on 122 properties infected with Japanese Knotweed.

They found that structural damage to those properties as a result of the weed was rare, and only occurred where existing structural weaknesses were present. On the other hand, trees, climbers and shrubs were shown to cause notably more damage to residential buildings.

Dr Mark Fennell, Principal Ecologist at AECOM, led the study and commented on the findings:

“We found nothing to suggest that Japanese knotweed causes significant damage to buildings – even when it is growing in close proximity – and certainly no more damage than other species that are not subject to such strict lending policies.”

Knotweed and mortgage providers

We asked Greg Cunnington, Director of Lender Relationships and New Homes at Alexander Hall, whether he had seen any changes in the attitudes of mortgage lenders towards homes affected by Knotweed.

“Lenders, and the surveyors whose opinion a lender will be guided by, have definitely shown a softening approach to Japanese knotweed in the last couple of years,” he said.

“Historically the mere mention of Japanese knotweed being in the vicinity of a property was enough for a lot of lenders to immediately decline an application,” he added. “However, as there has been a more sophisticated understanding of Japanese knotweed and the treatments available, a much more flexible approach has now been taken by the majority.”

He also explained that, although he had not noticed any marked changes since the Leeds University report, surveyors and lenders would be constantly assessing the subject. He said he was “very confident” that the new research would have an impact and “increase comfort behind the scenes” for buyers seeking mortgages on affected properties.

So what’s the real risk with Knotweed?

Although its ability to damage residential buildings seems to have been exaggerated thanks to sensational news reports, its vigorous growth can affect the following areas of a property:

  • Drains and other underground services can be disrupted by Knotweed roots
  • Paved areas such as patios and pathways can be affected by Knotweed shoots growing through joints
  • Garden walls with shallow foundations can be undermined by the weed
  • Outbuildings such as greenhouses, sheds and even garages can be affected
  • Conservatories can be affected in a similar way to outbuildings
  • Gardens can be overrun by Knotweed, reducing the positive effects of a well-planned outdoor space

Even when the plant appears to have been cleared above ground, any sections of the plant can remain dormant underground. Regrowth can occur years after an initial treatment, so what’s the solution for homeowners faced with this pernicious pest?

The solution

Since its inclusion in the Wildlife and Countryside Act 1981, fly-tipping of Japanese Knotweed cuttings has been illegal. Anything other than professional extermination attempts are highly discouraged due to the ability of even the smallest section of the plant to regenerate and take root. These measures have restricted the spread of the plant in rural areas, but urban gardens and brown belt land have become a prime breeding ground.

The Environment Agency has rejected proposals for a national strategy to eradicate the species due to prohibitive costs running into billions of pounds. The agency commissioned this map to track the distribution of the species around the country, and relies on the efforts of private homeowners and landowners to manage the spread of the plant.

Expert help

IN 2012, the Property Care Association (PCA) established the Invasive Weed Control Group to help people identify qualified, regulated specialists to help with infestations. Amateur attempts to remove the plant can easily make the problem worse, and professional solutions require a sustained approach over a number of years to completely remove the plant.

Progress is constantly being made in finding more effective ways to tackle Japanese Knotweed. A team of bioscientists at Swansea University has spent five years testing 19 methods used to control the weed, eventually patenting a method known as 'The 4-Stage Model'.

The team highlights the fact that effective treatment of the plant relies on applying the right herbicide at the correct time of year. They also state that any thorough solution to an infestation will take 3-5 years of careful treatment. Click here for more on their findings and approach, as well as a link to the full research paper.

Responsibilities: buyers and sellers

A number of legal cases in recent years have seen landowners successfully sued for allowing Japanese Knotweed to spread to surrounding properties. However, it’s worth bearing in mind that the RICS' Japanese Knotweed and residential property report emphasises, “most infestations … can be controlled without prohibitive costs.”

So what are the main responsibilities of buyers and sellers when it comes to Japanese Knotweed?


Sellers are legally obliged to check for the presence of the weed on their property. If found to be present, this must be marked on the TA6 property information form. Any attempt to hide the existence of Knotweed on a property could result in being sued.

It is the seller’s responsibility to provide an appropriate management plan for dealing with the infestation using an approved contractor (see Invasive Weed Control Group mentioned above). These firms should be able to offer a warranty, protected by an independent insurer, for the eradication work they undertake.

This provides both seller and buyer with peace of mind, knowing that a full course of treatment has been committed to. This is especially important given the research findings from the team at Swansea University, who state that full eradication can take between 3-5 years of consistent work.


The buyer’s surveyor will assess the impact level of any Japanese Knotweed infestation on, or around, a property, based on the RICS report mentioned above.

Infestations are categorised from 1-4. Category 1 is where the plant is found more than 7m away on land neighbouring the property, and Category 4 where the Knotweed is found within 7m of the property’s main buildings, conservatories or garages.

Greg Cunnington explains how lenders will respond when Knotweed has been found:

“A lender will typically want to know the details on the location, severity and treatment in place for any Japanese knotweed highlighted,” he explains. “As such, if these details are available upfront an intermediary can approach the lender and surveyors pre-application to ensure they would be comfortable with this.”

For more information and advice:

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