Japanese knotweed and the built environment Contents

3Non-physical effects of Japanese knotweed

30.Alongside the physical impacts of Japanese knotweed, we also sought to understand the wider effects, arising from:

These effects will be particularly relevant to homeowners and developers, and could affect the desirability of a property, and therefore its value.

31.An overall impression of some of these effects was offered by the Crop Protection Association, who pointed to a 2017 survey of 100 individuals affected by knotweed. The survey found that:

Fear and reputation

32.Sean Hathaway, representing Swansea Council, told us that “the problem caused by knotweed in relation to mortgages is not a physical problem caused by the plant but a perceived problem by the lenders caused by mis-information and fear”.51 That fear is reflected in—or, arguably, fuelled by—coverage of the issue in the press. Recent headlines include “Despairing family can’t sell home—because of monster next door that grows eight inches a day, damages foundations and grows through brickwork”,52 and “Fearsome Japanese knotweed that grows eight inches a DAY leaves despairing couple’s garden ‘totally unusable’”.53

33.While research indicates that the physical effects of Japanese knotweed may have been exaggerated in the past, the effects on property values appear to be real and significant. Advanced Invasives pointed to claims against neighbours made in legal cases as a source of information on this, with one noting that:

Continued presence of Japanese knotweed, untreated, [the Claimant’s] land close to the boundary with the claimants’ land had the effect of reducing the current value of the claimants’ land by 10%, from £800,000 to £720,000.54

They noted, however, that in many cases the effect will not be quantified since “buyers will simply reject those properties where knotweed is present”.55

34.We asked witnesses why the presence of a plant with no greater ability to affect the built environment than others should have such an effect on property prices. John Baguley, representing the Royal Institution of Chartered Surveyors, explained that “public perception of the issue of Japanese knotweed is a key factor in the process of valuing property”, and added that:

You have to think about the extent of damage, the cost of repair, the cost of remediation and the ongoing nature of that work. […] trees are very much a one-off kind of repair work, whereas Japanese knotweed needs an ongoing plan, so does that have an impact on the buyer’s decision to buy a property? The answer from chartered surveyors is that it does, so that translates into a potential impact on value.56

Treatment costs

35.Japanese knotweed is usually treated using herbicide or complete excavation of the plant and its rhizomes.57 However, research is also currently being undertaken on biological control using natural predators of knotweed in Japan, including the plant louse (psyllid) Aphalara itadori.58 While this approach could form part of a national long-term strategy for reducing the impact of Japanese knotweed, we were cautioned by Advanced Invasives that it was not currently suitable in a domestic setting for eradication.59

36.We noted in Chapter 1 that control using herbicides only puts the plant into a period of dormancy, from which it could potentially regrow in the future if disturbed. Nevertheless, this is a common form of management since, as Dr Jones noted, completely removing the plant by excavation is “an order of magnitude greater in cost” than treatment with herbicide.60 According to RICS, the costs of treatment by herbicide (in December 2011 prices) were “between £2,000 and £5,000 in total for a typical three-bedroom semi-detached house”.61 When adjusted for inflation the equivalent figures in 2018 would be £2,400 to £6,000.62 In the case of excavation, RICS noted that:

disposal costs range from £25 to £50 per tonne [£30 to £60 in 2018 prices] (not including landfill tax), with the result that excavation of even a relatively small Japanese knotweed infestation can cost several thousand pounds in waste charges alone.63

Treatment of Japanese knotweed has also cost the taxpayer considerable sums of money. The plant was discovered at the site of the Olympic Park in East London during building work for the 2012 Olympic Games and was widely reported to have cost £70 million to clear.64

37.Perhaps unsurprisingly, a significant knotweed remediation industry now exists, which in the past had become “a bit of gravy train”, according to Professor Wade:

I think the story goes back further to 1981 when the Wildlife and Countryside Act came in […] We were then told that, if you had rhizomes in any soil, because there was a risk you might spread it […] the [remediation] business grew and it became a bit of a gravy train. You only have to look at some of the adverts on the internet showing plants growing through concrete and so on. […] we have a mythology rapidly building up about the plant.65

On the other hand, Stephen Hodgson from the Property Care Association argued that remediation firms were simply responding to a market need, and that “If things are required to be done, does it not follow that [it] should be done well? The role of a trade association is to ensure that its members and everybody else can do that work well”.66

Loss of amenity

38.Conservation Land Services Ltd explained that the presence of Japanese knotweed could result in a loss of amenity, since:

where Japanese knotweed becomes present this places restrictions on what may or may not be done within the affected area. For example within a garden this may result in not being able to disturb the ground and so preventing areas of the garden being developed for paths, paved areas or any buildings such as garages or sheds, unless potentially expensive excavation control measures are put in place.67

Stigma and the requirement to declare when selling property

39.While there is no legal requirement for a landowner to treat Japanese knotweed if it is not causing a nuisance, the presence of the plant is expected to be declared when selling a property, seemingly with no upper threshold for how long ago this might have been.68 The Law Society provides standardised property information forms for the seller to complete, which include questions on a range of issues. Property Information Form TA6 (for Freehold properties) includes the following question:

Japanese knotweed is an invasive plant that can cause damage to property. It can take several years to eradicate. Is the Property affected by Japanese knotweed? If Yes, please state whether there is a Japanese knotweed management plan in place and supply a copy.69

40.Advanced Invasives told us that “the wording of the TA6 form is somewhat ambiguous, leaving scope for different interpretation of appropriate answers”.70 For instance, whether or not a property is “affected” by the plant could perhaps be disputed if all material had been removed by excavation, and the use of the word ‘eradicate’ is unclear given that treatment by herbicide leads to dormancy.

Liability for spread to other properties

41.Ben Lindley noted that allowing knotweed to spread to another property could create a liability under the law of nuisance, which can be “a major concern for property owners”.71 This is discussed further in Chapter 5.

Comparisons with approaches taken in other countries

42.While Japanese knotweed is present in many other countries and treated as an invasive weed, we were told that the presence of this plant does not seem to cause such difficulties when buying and selling properties abroad. Professor Wade compared the reaction in the UK with continental Europe, telling us that “they do not even have any laws that govern Japanese knotweed. They certainly have not over-reacted in the same way as we have with regard to properties and insurance”.72 He later clarified that the plant was not ignored in other countries, as it was known to have environmental impacts, but the presence in gardens did not prompt the same level of response.73

43.In contrast, Advanced Invasives argued that this was because attitudes to home ownership also differed in other countries:

Homeownership in the UK is relatively high compared to many of the countries in continental Europe. Like-for-like, per unit housing costs are typically proportionately greater in the UK, and as a result, property market pressure is more acute. Therefore, while Japanese knotweed is present in many other European countries, historically it has less of an effect on the property market compared to in the UK.

Advanced Invasives have found evidence of Japanese knotweed increasingly impacting upon property transactions and associated processes along the eastern seaboard of North America, including in Brooklyn (New York State, US) and the City of Westmount (Montreal, Canada).74


44.The presence of Japanese knotweed can affect the desirability of a property and therefore its valuation, even if the specific physical effects on buildings are not significantly different to other plants. If nothing else, land affected by Japanese knotweed is contaminated with material that has restrictions on disposal methods, makes development (e.g. extensions, garages) on the land more challenging, and comes with a risk of liability if the plant spreads to neighbouring properties. All of these factors will be unattractive to buyers to some extent. This alone might be sufficient to justify the inclusion of a question on Japanese knotweed in the Seller’s Property Information Form, but not the significance attached to it in lending decisions.

45.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. This gives us reason to believe that the UK has taken an overly cautious approach to this plant, and that a more measured and evidence-based approach is needed to ensure that the impact is proportionate to the physical effects of the plant in the built environment. 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.

46.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.

50 Crop Protection Association (JKW0008)

51 Swansea Council (JKW0003)

52 Daily Mail, Invasion of the alien knotweed!, 27 August 2017

53 Daily Mail, Dismay of the Triffids!, 2 June 2017

54 Advanced Invasives Limited (JKW0011) para 12 (quoting Smith v Line case).

55 Advanced Invasives Limited (JKW0011) para 12

57 For further information on treatment methods see CABI Japanese Knotweed Alliance, “Current methods of control in the UK”, accessed 6 March 2019.

58 For further information on the use of psyllids and fungi to manage Japanese knotweed see CABI Japanese Knotweed Alliance, “The potential agents”, accessed 6 March 2019.

59 Advanced Invasives Limited (JKW0032)

61 RICS, Japanese knotweed and residential property, 1st Edition, 2012, para 4.5.1

62 Using the Bank of England inflation calculator and rounding to the nearest £10.

63 RICS, Japanese knotweed and residential property, 1st Edition, 2012, para 4.2.2

64 See, for example Groundsure, Japanese knotweed, 2016, and The Telegraph, Knotweed: the unstoppable scourge of British gardens, 17 September 2016.

67 Conservation Land Services Ltd (JKW0022)

68 See further discussion in Chapter 5.

70 Advanced Invasives Limited (JKW0032)

71 Q6

74 Advanced Invasives Limited (JKW0032)

Published: 16 May 2019