Japanese knotweed and the built environment Contents


Japanese knotweed (Fallopia japonica) is a fast-growing invasive plant with bamboo-like stems. It was introduced to the UK in the mid-nineteenth century as an ornamental plant in parks and gardens, but has since become a significant nuisance throughout the country as an invasive weed. It has been estimated that over 2% of development sites and 1.25% of residential properties in Great Britain are affected by the plant, amounting to tens of thousands of sites in total.

Given the anxiety that the plant can cause for homeowners, and the publication of new evidence relating to the physical effects of Japanese knotweed we were prompted to hold a one-off evidence session on the effect of Japanese knotweed in the ‘built environment’ of buildings, paving, drainage channels and outbuildings.

In this Report we find 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 that are not subject to the same controls and do not have such a substantial ‘chilling’ effect on the sale of a property. However, Japanese knotweed has some distinguishing features that are relevant in this context. Japanese knotweed is particularly hard to eradicate compared with other plants, requiring multi-year treatment with herbicide or excavation. This is not the case with trees or plants such as buddleia. There is also an ongoing risk that the plant will regrow, either because it is only made dormant by herbicides or because fragments of the plant remain in the soil.

We conclude that there is surprisingly little academic research on the physical effects of Japanese knotweed in the built environment, despite the impact that the presence of Japanese knotweed can have on a property sale. We welcome the Environment Agency’s offer to approach Defra and others with a view to ensuring that research is commissioned to fill knowledge gaps. We recommend that the Environment Agency should also convene a meeting with the major national Japanese knotweed remediation firms to explore how a national dataset could be assembled from this information and how companies could contribute to this on an ongoing basis to inform academic research which seeks to better understand Japanese knotweed.

A significant industry is built around controlling Japanese knotweed, but we were told that mortgage lenders in other countries do not treat the plant with the same degree of caution. We recommend that Defra commission a study of international approaches to Japanese knotweed in the context of property sales to further inform discussion of this issue, and report by the end of the year.

We recommend that the Law Society review the wording of the question in its Property Information Forms in consultation with the Royal Institution of Chartered Surveyors and others. In particular, it should consult with experts to determine whether the need to declare previous Japanese knotweed problems should expire if the plant has been treated by appropriate excavation and there has been no re-growth within a certain period. It should do this by the end of the year.

The existing RICS risk assessment framework for Japanese knotweed has ensured that in many cases lenders have the confidence to lend against properties affected by Japanese knotweed, so long as there are funded treatment plans and insurance-backed guarantees covering the treatment in place. These can be expensive for homeowners looking to sell, but they often provide a route for the buyer to secure a mortgage.

The ‘seven-metre rule’ that forms part of the 2012 risk assessment framework is being used as a blunt instrument in some mortgage lending decisions. It does not reflect the latest scientific evidence. RICS itself notes that the framework is “no longer current”, but in the meantime it is still forming the basis of mortgage decisions. This framework lacked a clear and comprehensive evidence-base and yet is causing significant problems to some house vendors and purchasers. A much more nuanced and evidence-based risk framework is urgently needed to reflect the latest thinking on the significance of Japanese knotweed, in relation to the size of the infestation, the distance from the property, and the potential risk of any damage. We are pleased to hear that following our evidence session RICS has convened meetings of stakeholders and influencers to update its 2012 assessment framework for Japanese knotweed to ensure that its policies reflect the most up-to-date evidence. We hope that RICS will complete this update as soon as possible and certainly no later than the end of this year.

The challenge of resolving disputes relating to Japanese knotweed is diminished if a more measured and evidence-based approach is taken to Japanese knotweed. Nonetheless, we conclude that, in most if not all circumstances, where disputes between landowners relating to the encroachment of Japanese knotweed persist these are not usually best resolved by means of litigation, which can be both expensive and protracted. We recommend that, in consultation with the Civil Mediation Council, the Government produce additional guidance on dealing with such disputes, recommending that mediation via an accredited mediator be normally used, subject to the agreement of the parties involved, as the initial route to resolution of the dispute if it offers value for money, while explaining that this would not prevent an aggrieved party from having recourse to litigation if efforts to achieve a mediated settlement do not succeed.

Published: 16 May 2019