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Japanese Knotweed – Keeping claims under control

December 2020
David McArdle and Jon Quirk

Japanese Knotweed (“Knotweed”) has achieved notoriety over the years as a result of the perceived damage it can cause to vegetation and property. Despite recent research suggesting that it may not, in fact, be as big a problem as previously thought, it continues to be a source of prolific professional negligence claims against surveyors. In this article we address the changing perceptions of the plant, the nature of claims against surveyors and the arguments that a surveyor or his professional indemnity insurers might raise by way of defence.

What is Japanese Knotweed?

Knotweed is a large, highly aggressive and invasive weed, sometimes known as “elephant ears”, “donkey rhubarb” or “monkey weed”. It grows in many conditions and is often seen alongside railway tracks, on river banks and on waste ground. It has large, hollow stems which look similar to bamboo, and heart shape green leaves.

It has a reputation for causing damage to property because its roots (rhizomes) have been thought to grow as far as seven metres horizontally and two metres deep underground, cracking concrete and damaging foundations. As a result, mortgage lenders will often refuse to lend to anyone purchasing a property with Knotweed on the premises. Despite its fearsome reputation, a report commissioned by the government in 2019¹ suggested that Japanese Knotweed may in fact pose no greater threat to property than that of other invasive plants. The existing (and arguably outdated) RICS assessment framework for surveyors observing Japanese Knotweed uses a distance of 7 metres as its threshold for concern; research suggests that a more nuanced and evidence based approach carried out on a case-by-case basis is required. The RICS guidance is currently under review and a new assessment framework is expected in due course.

Claims against surveyors

Over the years surveyors have seen a wealth of Knotweed related professional negligence claims. The common allegation is that the surveyor failed to identify Knotweed on the purchased property, or a neighbouring one, and/or underestimated the scale of the problem and the costs of remedial work. To succeed in any such claim, a claimant must show that the surveyor (1) owed a duty of care to identify Knotweed on the property concerned (2) was in breach of that duty of care and (3) that the breach was causative of the claimant’s loss. The appropriate measure of such loss is a further issue of contention.

Duty of care

Did the duty to observe/discover Japanese knotweed fall within the scope of the surveyor’s retainer at the relevant time? This may well depend on the standard of survey that the surveyor was asked to provide. If, for example, the surveyor was retained to carry out a straightforward mortgage valuation report, then any duty of care will be limited to reporting on those areas of Japanese Knotweed reasonably apparent from a brief visual inspection. Whilst a valuer may owe a duty to report on Japanese Knotweed where this is likely to have a material effect on a property’s valuation, a court is unlikely to find that there has been a breach of that duty in circumstances where Japanese Knotweed was obscured, concealed or where identification would require a more detailed inspection of the garden otherwise unsuited to a valuation report

Arguments regarding scope of retainer are particularly likely to apply where it is alleged that a surveyor should have identified Japanese Knotweed on neighbouring/adjacent property. A surveyor is considerably more likely to be expected to check and identify Japanese Knotweed on a neighbouring property where he has been instructed and paid to carry out a full survey than he would be when carrying out a basic mortgage valuation report. This point was well illustrated in the case of Michele Davis v Marshalls (Plumbing and Building Development) Limited & Connells Surveying and Valuation Limited [2018]. Here, the report commissioned was a basic mortgage valuation report and the Knotweed was in the neighbouring property’s grounds. The court found that inspecting the neighbouring land was not within the surveyor’s duty of care and Ms Davis’ claim failed.

It is always necessary to consider how hard the surveyor should reasonably have looked so as to identify Knotweed on site. No surveyor has any duty to go digging around hunting for Knotweed. As such, the duty to identify the plant would probably be considered greater in summer than in winter, when the plant is dormant and therefore much less visible. It is always very helpful if the surveyor has photographs taken at the time to evidence whether the Knotweed was reasonably visible when surveying the site.

In the well-known Knotweed case of Ryb v Conway Chartered Surveyors [2019] the defendant surveyors were found liable for failing to identify Knotweed in the grounds of the ground floor flat that Mr Ryb was purchasing. Mr Ryb was awarded the diminution in value of the property as well as the costs of Knotweed remedial works. It should be noted, however, that there were some exceptional circumstances in this case: Mr Ryb had commissioned the most thorough level of survey available (not just a basic mortgage valuation report); he had issues with his sight and was therefore dependent on the surveyor to identify any problems for him; the survey was carried out in late summer when the Knotweed would have been in full bloom and easily identifiable; and, finally, the surveyor had no photographs, measurements or notes from the time of the inspection nor any relevant training on Knotweed. It was, as they say, “the perfect storm”.


It is useful to consider in each case whether or not the Japanese Knotweed concerned was actually present at the time the surveyor’s inspection was carried out. If the Knotweed was not present at the time of the survey, then clearly it could not have been spotted by a surveyor, even if the survey was carried out with reasonable care and skill. The burden will be on the claimant to prove causation, which it will often try to do by disclosing a copy of the Knotweed removal survey (“the Knotweed survey”). This will inevitably surmise as to the likely presence of Knotweed at the time of the surveyor’s inspection.

Knotweed can grow quickly, sometimes as much as 70cm per week. It is therefore worth considering the length of time which has elapsed between the date of the surveyor’s inspection and the Knotweed survey. It is quite possible that, even though the Knotweed was there at the time of the Knotweed survey, it was not there at the time that the surveyor completed his or her report. It may also be that some areas of Knotweed were there at the time of the surveyor’s inspection, whereas others were not. This will naturally affect the extent to which the claimant is entitled to recover from the surveyor any losses incurred. Again, the importance of contemporaneous evidence to rebut the claimant’s assertions as to the presence of Knotweed should not be underestimated. Photographic evidence, careful notes and measurements will all be of great assistance and will often be the only means of demonstrating the true state of affairs at the relevant time.


Further arguments arise as regards the correct assessment of loss in cases where the claimant has successfully established a breach of the surveyor’s duty of care. Commonly, a claimant will seek redress for an alleged diminution in value of the property, on the basis that it is blighted by the presence and potential return of Knotweed, and also unmortgageable. Often claimants will seek to recover remedial costs in addition to the diminution in value. Various issues arise with this approach:

  1. Such plans are commonly included with herbicide treatment, costing a few thousand pounds, and it seems can be obtained in isolation for as little as £100 online
  2. Often there is no residual diminution in the value of the property after treatment has taken place. The courts have been reluctant to accept that the resulting loss in value is anything more than a very modest percentage of the property’s value (in the Davies case the court, in a seemingly arbitrary fashion, commented that the diminution in value would be no more than 3%).
  3. As such, the real measure of loss is arguably the cost of the appropriate Knotweed treatment plus whatever small residual diminution in value there may be if any. Finally, as such, the real measure of loss is arguably the cost of the appropriate knotweed treatment, plus whatever small residual diminution in value there may be.

It is also worth considering whether or not the claim can be deflected to the property’s vendors. Question 7 of the Law Society’s Property Information Form asks the vendor whether the property is “affected by” Japanese Knotweed and, if so, whether there is a management plan in place. If a false answer is given, a misrepresentation claim could be made. The difficulty here is the arguably ambiguous nature of the question – whether or not a property is “affected” by Knotweed is arguably very subjective – for example, is the property still “affected” if the plant has been removed by excavation?


Despite suggestions that Japanese Knotweed may not in fact be as big a problem as was once thought, the plethora of Knotweed claims farmers ensures that there remains a continued flow of Knotweed related surveyors’ claims. This may change if, in due course, RICS updates its 2012 Japanese Knotweed assessment framework to reflect the latest evidence on Knotweed’s effects. In the meantime, surveyors are encouraged to keep contemporaneous and accurate records (especially photographs) from the time of inspection. They and their insurers will then be well placed to argue in defence of any allegations which might arise.

¹ Japanese Knotweed (Fallopia japonica): an analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension – Mark Fennell, Max Wade & Karen L. Bacon, 2018

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