What is Japanese Knotweed and how has the advice around dealing with it changed?

Japanese Knotweed is native to Japan, Taiwan and Northern China.  It was introduced into the UK as an ornamental plant and has subsequently become naturalised.  It is a fast growing plant which is expensive and difficult to eradicate.  It can take years to remove and cannot simply be covered over as it forces its way through concrete and tarmac and occasionally it can damage buildings.  Finding it on a development site can therefore pose serious problems for developers due to the cost and delay involved in dealing with its removal and also for home owners as it has been widely assumed that Japanese Knotweed poses a structural risk to building foundations.

Japanese Knotweed was included in the original list of invasive alien species in the Wildlife and Countryside Act 1981. This made it an offence to ‘plant or otherwise cause Japanese Knotweed to grow in the wild’ and classified soil or plant material likely to contain Japanese Knotweed as ‘controlled waste’ under the Environmental Protection Act 1990.

The RICS information paper published in 2012 provided surveyors with an assessment methodology for assessing the risk posed by Japanese Knotweed to a property, with the emphasis very much placed on the need for eradication. However, this guidance has recently been reviewed and updated by RICS.

Why the change?

A research paper by Fennell’s in 2016 re-assessed the risk of damage to property by Japanese Knotweed in comparison to other trees and shrubs and showed no higher risk. It appears from the introduction to the new RICS guidance note that the decision to issue their revised note was brought about by the House of Commons Science and Technology Committee publishing a report on an enquiry titled “Japanese Knotweed and the built environment in 2019”  which criticised the approach used in the 2012 RICS guidance note and in 2020, Risk & Policy Analysts (RPA), published a report on behalf of the Department of Environment, Food and Rural Affairs (Defra) into Japanese Knotweed. This passed comment on the risks posed by Japanese Knotweed and was critical of the overly cautious approach by RICS towards Japanese Knotweed given that it does not necessarily cause more damage to buildings than other plants.

The New Guidance

RICS have now provided some further draft guidance notes on Japanese Knotweed, which show a shift in the way that Knotweed is considered. The new draft guidance note replaces the previous guidance note from 2012.

The new guidance shifts the way that Japanese Knotweed is looked at, so that it is controlled and managed rather than looking for it to be eradicated and this can be seen in the change of language used within the guidance note.

The emphasis is placed on seeking to assess individual properties and then provide for the management of Japanese Knotweed to foster reassurance amongst lenders and buyers. In particular, the guidance now distinguishes between properties where Japanese Knotweed has actually caused structural damage and those properties where is simply in the vicinity of buildings.