Japanese knotweed and the built environment Contents

2Scientific evidence of the physical effects of Japanese knotweed

The latest peer-reviewed research

7.The Great Britain Non-Native Species Secretariat, which coordinates the approach to invasive plants and animals on behalf of Defra, the Scottish Government and the Welsh Government, stated that “Japanese knotweed is infamous for its ability to grow through hard surfaces, such as tarmac car parks and building foundations” (emphasis added).13 The risk of damage of this sort is one of several reasons why Japanese knotweed can cause difficulties when buying or selling property. However, we were prompted to undertake our inquiry by the publication of new research which suggested that the physical effects of knotweed on buildings might not be as significant as previously believed.14

8.Research by Dr Mark Fennell, Professor Max Wade and Dr Karen Bacon published in July 2018 examined the physical effects of Japanese knotweed in comparison with other common plants. Their work (referred to below as Fennell et al) comprised three elements:

9.Professor Max Wade, one of the authors of the study, described the aims of the research as follows:

The question was not, ‘Does it cause damage to properties?’ We recognise that it does. To put it into perspective, we were trying to answer the question, ‘In relation to other plants, what damage does it cause?’.16

The research revealed that while Japanese knotweed could grow through tarmac, disrupt paving and exploit cracks in buildings, the same was also true of other plants that were not subject to the same concerns in the context of selling property. The research acknowledged the structural damage that Japanese knotweed (F. japonica) could cause but noted that this should not be considered any more of a risk than a range of other plants:

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, with respect to capacity to cause structural damage in urban environments, than a range of other species of plant, and less so than many.17

The Property Care Association told us that this paper had “unquestionably opened a debate into the structural implications of Japanese knotweed where it grows close to buildings”.18

10.However, our attention was also drawn to some limitations of this study. Advanced Invasives, a consultancy firm, noted that:

The case study investigating property damage […] is restricted to three streets of derelict housing stock (built prior to 1900) located in northern England. Consequently, it is difficult to generalise to new build housing developments and older stock that is well maintained.

Conservation Land Services Ltd, a knotweed remediation firm, explained further that the use of Victorian properties in the case studies meant that “the houses were built before Japanese knotweed had established in the area and so would not be subject to the effects of Japanese knotweed growing underneath the structures or during their build”.19

11.Advanced Invasives was also concerned that:

[While] rhizome extension is reported as rarely extending more than four metres from above-ground plant growth, [the study] relies upon a small sample size of the contractor questionnaire and only considers relatively smaller knotweed stands [patches of Japanese knotweed] of limited area.20

Nevertheless, the company agreed that the typical effects of Japanese knotweed in terms of damage to buildings have been overstated previously, particularly in press coverage of the issue.21

12.Professor Wade described how, in the absence of high-quality research in this area, Japanese knotweed had developed a reputation for causing significant damage for buildings because of its ability to exploit existing cracks in walls:

We have a tendency—you can see a number of photographs on the internet—to see Japanese knotweed as damaging a building because it is growing up through cracks. You think, “Goodness me, that looks terrible.” However, on the site where we were working, you could see a number of buildings [of the same age] that had cracks and were clearly damaged, but that had no Japanese knotweed.22

He concluded that “undoubtedly, trees are much more damaging and costly than Japanese knotweed. In terms of the built environment, buddleia23 is much more damaging and costly, in my view”.24

13.Nevertheless, we also heard concerns from the remediation firm Environet UK Ltd that the latest research could send the message that Japanese knotweed did not need to be treated, and that without a motivating force this invasive weed might be allowed to spread further.25 We asked Professor Wade to clarify how worried a homeowner should be if they discovered Japanese knotweed growing in their garden. He summarised that “they should be seriously concerned. They have a problem, which they need to deal with”.26 Professor Wade elaborated that this was a concern due to the belief that knotweed could cause damage to the building and the consequence of that belief on the value of one’s house.

Capacity for regrowth and difficulties in management

14.While Professor Wade compared the physical effects of Japanese knotweed to the damage caused by other plants, Dr Dan Jones, representing Advanced Invasives, felt that drawing comparisons between Japanese knotweed and trees was not appropriate since:

controlling and managing Japanese knotweed will inevitably be quite a long-term process. Even following successful control, it is not necessarily dead. Once you have killed a tree for example, it is dead and the problem is removed. Japanese knotweed is quite different in that respect.27

He explained that this was because “the rhizome forms an extensive, resilient and persistent long-term energy store”.28 Indeed, we were told that Japanese knotweed could regenerate from a small fragment of rhizome,29 and that there had been cases identified where “herbicide control appears to have killed the treated Japanese knotweed but in fact has put the rhizome into a dormant condition”.30

15.Ben Lindley, representing Japanese Knotweed Ltd, gave us a vivid description of the problems associated with the difficulty of killing Japanese knotweed:

After three or four years you could find no regrowth, but if you disturb the ground there could be elements of that underground rhizome that retain viability and, therefore it could regrow […] even after successful herbicide treatment.31

Dr Jones summarised that “in terms of chemical control methods, we are talking about long-term sustainable control, not eradication”.32

16.Mr Lindley explained that another distinguishing feature was the way in which Japanese knotweed spreads:

Buddleia and a tree will spread by seed, which has to have light and water to grow, whereas for knotweed the pathway is the rhizome.33 Depending on the size and state of the rhizome, it can produce quite strong growth straightaway.34

Other sources of information

17.We were surprised to find that the Fennell et al study appeared to be the only published research on the question of the damage that Japanese knotweed can cause to buildings. Conservation Land Services Ltd explained that while there was a large volume of academic material on the biology and physiology of Japanese knotweed, there was “very little academic research published on the specific effects of Japanese knotweed on property and structure”.35 More broadly, Professor Wade suggested that there was a need to assess whether the current approach to Japanese knotweed was appropriate or having a desirable effect:

Over the last few decades, UK business and industry, residents, houseowners and so on have spent millions and millions of pounds on Japanese knotweed. We have not stopped and said, “How are we getting on with this? What progress are we making? What can we learn about doing it better? Are we making progress? Are we going backwards?” Surely it is time that we thought about that.36

18.Two other relevant sources of information alongside the Fennell et al study are (i) Japanese knotweed remediation companies and (ii) expert testimony, which we explore in turn below.

Knotweed remediation firms

19.A significant industry has grown around the demand for treating Japanese knotweed, driven by mortgage lender requirements (see Chapter 4). Naturally, these remediation companies have a financial interest in this area, but we heard repeatedly that a wealth of relevant data is collected by them from building surveys, undertaken as part of their work, and that “this means there potentially exists a considerable amount of site-specific data which could be a valuable resource for any future study”.37 Indeed, data of this sort formed one part of the Fennell et al work (see paragraph 8).

20.Japanese Knotweed Ltd, one of several remediation firms who wrote to us, presented an analysis of their own building survey data. They told us that if Japanese knotweed was within one metre of a structure there was a 29% chance that it would cause some damage, which was similar to buddleia.38 We heard that this was similar to the overall findings of the Fennell et al study.39

21.Mr Lindley suggested that remediation companies could make their information available for others to analyse,40 and Advanced Invasives proposed that a national dataset could be assembled by pooling information from a range of remediation companies:

Such a dataset should include a UK-wide impact assessment of knotweed growth on built structures (including old and new build housing) and empirical investigation of knotweed growth rate and form under differing environmental conditions such as regional climate and soil types.41

22.Dr Mark Diamond, Head of Ecology at the Environment Agency, offered to approach the Department for the Environment, Food and Rural Affairs (Defra) to lead on ensuring that gaps in research were filled:

There is a role for developing a cross-department partnership with academia and the sector. The Departments that I am thinking of are the [Ministry for Housing, Communities and Local Government], Defra and the Department for Transport, to look at a prioritised research programme to support rapid advance in the knowledge behind this. […] I can talk to Defra about starting that off, and the GB programme board on invasive non-native species.42

Expert testimony

23.Swansea is one of several parts of the UK that is particularly affected by Japanese knotweed. Sean Hathaway, Swansea Council’s Environment Officer, told us that in two decades of experience with the plant he had only seen it penetrate the inside of a property on three occasions, which reinforced the message from the Fennell et al work that the physical effects may be overstated. Mr Hathaway recalled that:43

One was just inside the wall, by the window, one was up through a cavity wall, and the other was just by a stairway, inside. That was about 10 years ago. It was dealt with very simply, by people using a standard herbicide from a qualified contractor. There were no problems.44

24.Ben Lindley explained that the plant was rarely seen penetrating the inside of buildings because “the natural path of knotweed finds the path of least resistance. Therefore it will not grow into properties if it can grow into normal soil”.45 More serious problems occur, however, “when on construction sites, they ignore Japanese knotweed and build straight over it”, which can result in the plant “growing into properties through suspended or block-and-beam floors or through air vents”.46

25.Mr Hathaway’s view, in common with the Fennell et al study, was that “other species such as bamboo and buddleia cause more problems as do tree roots […] most of the established knotweed control companies agree that damage is over-exaggerated”.47 He added that “the only knotweed stories in the press are scare stories, mostly of houses not being marketable due to knotweed as opposed to being physically damaged”.48

26.Finally, we note that the National House-Building Council (NHBC)’s latest technical note relating to Japanese knotweed stated that “although Japanese knotweed has a vigorous growth and can be difficult to eradicate, it has not been found to cause structural damage. It can grow through, and cause damage to, paved and tarmacked surfaces”.49 This reflects the evidence that we heard during our inquiry.


27.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. This conclusion is supported by the experience of some experts in this area and data from Japanese knotweed contractors. Reactions to the presence of the plant should be in proportion with the actual risk of damage.

28.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.

29.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. Remediation companies collect a considerable amount of data relating to Japanese knotweed as part of their work, and several have indicated their willingness to share this information with others. 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. To support this, 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. This would provide a useful resource for further research and an evidence base to inform guidance in this area. Meanwhile, Defra should consider adding the physical effects of Japanese knotweed to its “areas of research interest” document.

13 GB Non-Native Species Secretariat website, “Our economy”, accessed 26 February 2019.

18 Property Care Association (JKW0010)

19 Conservation Land Services Ltd (JKW0022)

20 Advanced Invasives Limited (JKW0011)

21 Advanced Invasives Limited (JKW0011)

23 Buddleja davidii is a common non-native invasive species of plant, but is not subject to the Wildlife and Countryside Act and its presence is not declared when selling property.

25 Environet UK Ltd (JKW0004)

26 Q2

27 Q5

28 Advanced Invasives Limited (JKW0032) para 6

29 Japanese Knotweed Solutions Limited (JKW0005)

30 Chartered Institute of Ecology and Environmental Management (JKW0024) para 5

33 Japanese knotweed does not produce viable seed in this country, and is spread by the transport of rhizome material—but this can include spread by water during flooding or via the sea as well as human transport of contaminated soil. See Qq40–42 for discussion.

34 Q8

35 Conservation Land Services Ltd (JKW0022)

37 Conservation Land Services Ltd (JKW0022) para 2.1. See also The Knotweed Company Ltd (JKW0014) para 1.3.

41 Advanced Invasives Limited (JKW0011) para 24

47 Swansea Council (JKW0003)

48 Swansea Council (JKW0003)

Published: 16 May 2019