Seeds of Change - is Japanese knotweed undergoing a radical image overhaul?

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Seeds of Change - is Japanese knotweed undergoing a radical image overhaul?

Published 24 May 2019

With news that RICS is in the process of updating its guidance on how surveyors should deal with Japanese knotweed, the House of Commons Science and Technology Committee (“Committee”) has just published a report on “Japanese knotweed and the built environment” calling on further research to be done by the Environment Agency and DEFRA in order to “fill [the] knowledge gaps” and, in addition, to report on the international approaches to Japanese knotweed which appear to be far more relaxed than in the UK.

The Committee noted that there was surprisingly little academic research on the physical effects of knotweed on the built environment, notwithstanding the effect that its presence can have on property value.

Public anxiety, encouraged by the media, has been stoked by images of knotweed growing through concrete and stories of it causing “serious structural damage” to buildings. This has not been helped by the statement of the Great Britain Non-Native Species Secretariat that “Japanese knotweed is infamous for its ability to grow through hard surfaces, such as tarmac car parks and building foundations”.

However, the Committee’s report, produced following a one-off evidence session on the effect of Japanese knotweed on the built environment, resolutely concludes that “the latest research suggests that the physical damage to property from Japanese knotweed is no greater than that of other disruptive plants and trees”.

The real issue, it seems, is not that Japanese knotweed is particularly dangerous or destructive, but that, compared with other invasive plants, it is simply more difficult to eradicate.

In reaching its conclusions, the Committee analysed the following issues:

  • Scientific evidence on the effect of Japanese knotweed on the built environment;
  • How the presence of Japanese knotweed affects mortgage lending decisions and property valuations in the UK;
  • Whether mortgage lending decisions are based on sound scientific evidence;
  • The current guidance and legislation and how to encourage an evidence-based approach.

In respect of the available evidence of the effects of knotweed, there is very little out there. The Committee, therefore, relied heavily on the research paper published by the University of Leeds which concluded that, while Japanese knotweed could grow through tarmac, disrupt paving and exploit cracks in buildings (although not, as the public has been lead to believe, grow through concrete), the same was also true of other invasive species which are not, however, subject to the same scrutiny by mortgage lenders. Indeed, it was found that trees are much more damaging and costly to property owners than Japanese knotweed. The problem, therefore, remains that, while trees and other invasive plants can be killed off relatively easily, Japanese knotweed cannot. Accordingly, it requires long-term sustainable control which, while relatively inexpensive, is time consuming and may interfere with the use of one’s property.

The Committee also highlights that, while a significant industry has been built around controlling Japanese knotweed in the UK, mortgage lenders in other countries do not treat the plant with the same degree of caution, suggesting that the UK’s approach is overly cautious and not evidence-based. The Committee has, therefore, invited DEFRA to commission a study of international approaches to allow consideration of alternative angles.

To date, while various limited guidance has been published by the Government and the PCA on the management, treatment and disposal of Japanese knotweed (some since withdrawn[1]), the main source of guidance for surveyors and lenders has been the RICS Information Paper which introduced the categorisation Japanese knotweed growth based on its distance from any buildings. This guidance was originally greatly welcomed, with a number of mainstream lenders swiftly changing their lending policies and confirming that properties would be mortgageable as long as a treatment plan was in place.

However the Committee felt that this categorisation, (known as the ‘seven metre rule’), has since been applied too stringently, too often being used as a blunt instrument in mortgage lending decisions, to the extent that the presence of knotweed within seven metres of a property boundary is stigmatising that property, regardless of any risk of damage.

This has caused unnecessary problems for home owners and an evidence-based risk framework is much overdue. RICS states on its website that its Information Paper is “no longer current”. The Committee proposes that any new framework take into account issues such as the size of the infestation, the distance from any building, the age of the building and the potential risk of damage including soil conditions and the paths that the plant could exploit. RICS is expected to consider this as part of its review of its guidance which will involve meetings to examine the current guidance and approach and to consider the development of appropriate tools for the evidence-based assessment and reporting of Japanese knotweed.

As part of its investigations, the Committee contacted a number of lenders, including those who had more stringent lending criteria in respect of Japanese knotweed. The feedback was that lending policies may well be revised following any update from RICS to its 2012 paper. Accordingly, a fresh set of guidelines from RICS may be what the market requires in order to clarify, inform and ensure a proportionate and consistent approach.

In addition, it is understood that the Environment Agency and DEFRA are in the process of commissioning further research into Japanese knotweed and the Committee has suggested that the major national Japanese knotweed remediation firms (who are in possession of substantial amounts of data) should also be engaged with a view to establishing a national database.

The Committee has also recommend that the Law Society review the wording of the Property Information Form in conjunction with RICS, in respect of whether the requirement to declare the presence of Japanese knotweed should be removed where it has been treated by excavation and there has been no re-growth within a certain period.

In summary, we are certainly seeing a period of change which will hopefully provide some clarification and standardisation of the approach to Japanese knotweed by the public, lenders and surveyors alike which should result in a more reasoned attitude to the weed.

[1] The Environment Agency’s code of practice dated 2006 entitled “Managing Japanese knotweed on Development Sites” was withdrawn in 2016 as it no longer provides best practice guidance.


Polly McBride

Polly McBride


+44 (0)117 918 2723

Duncan Greenwood

Duncan Greenwood


+44 (0)113 251 4760

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