by Andrew Robertson, Trainee Solicitor

A recent report from the Science and Technology Committee has concluded that the UK’s current approach to Japanese knotweed (Fallopia japonica) is ‘overly cautious’ with the need for a more ‘measured and evidence-based approach’ to its effects.


Effects of Japanese knotweed 

Japanese knotweed is estimated to affect around 1.25% of residential properties in the UK at a cost of roughly £200 million per year including the cost of treatment and property devaluation.

While there may be no legal obligation on property owners to treat the plant where it is not a nuisance, its existence should be declared when selling a property. The report noted that in a 2017 survey, 20% of individuals affected by the plant saw the value of their property drop, and 15% saw a potential property deal fall through.

A decade ago, many lenders were unwilling to lend on a property where evidence of Japanese knotweed was found, ‘on the basis that it was proving too difficult to quantify the risk to the lender’.

This problem was eased when the Royal Institution of Chartered Surveyors introduced its assessment framework (2012) which ranked the risk of the plant into four categories. This framework gave many lenders the confidence to begin lending again but they would only do so when treatment plans and insurance-backed guarantees were in place. Despite this, where the knotweed is within seven metres of the property, a lender is still likely to deem it too high a risk.


Are we ‘overly cautious’?

Research by Dr Mark Fennell, Professor Max Wade and Dr Karen Bacon in 2018 would perhaps suggest that we are: ‘While F. japonica is clearly a problematic invasive non-native species with respect to environmental impacts and land management this study provides evidence that F. japonica should not be considered any more of a risk…than a range of other species of plant, and less so than many.’

The report highlighted that those involved in the business of treating and removing the plant agreed that the ‘typical effects…in terms of damage to buildings have been overstated previously, particularly in press coverage of the issue.’

While it is clear that perhaps we should not be as worried about the effect of the plant as is commonly thought, Professor Wade noted that upon the discovery of Japanese knotweed, a homeowner ‘should be seriously concerned. They have a problem, which they need to deal with.’



Given that ‘mortgage lenders in other countries do not treat the plant with the same degree of caution’, one of the committee’s recommendations is a government-commissioned study of international approaches to Japanese knotweed in the context of property sales, with a report on findings due by the end of the year.

Only time and further research will tell if the fear surrounding Japanese knotweed is justified or unfounded.


Further advice

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