1Introduction
What is Japanese knotweed?
1.Japanese knotweed (Fallopia japonica1) is a fast-growing invasive plant with bamboo-like stems. It has distinctive rhizomes2 (underground structures that resemble roots) that can be more extensive than the above-ground portion of the plant. It was introduced to the UK in the mid-nineteenth century as an ornamental plant in parks and gardens, but has since become a significant nuisance throughout the country as an invasive weed.3 It has been described by the Property Care Association, the trade association for specialists in problems affecting buildings, as “one of the most problematic plant species in the UK and Ireland”.4 It has been estimated that over 2% of development sites and 1.25% of residential properties in Great Britain are affected by the plant, amounting to tens of thousands of sites.5 A 2010 report from CABI, a science-based not-for-profit organisation specialising in agricultural and environmental research, estimated that the total annual costs of Japanese knotweed in Great Britain were £166m per year (equivalent to over £200m in 2018 prices6), including the cost of treating the plant in the rail and road networks and property devaluation.7
2.Japanese knotweed is frequently discussed in the media in the context of property values, on the basis that the presence of this plant can cause difficulties in completing a sale (see Chapter 3). This arises from a popular perception that Japanese knotweed can cause significant damage to buildings, and some mortgage lenders have adopted strict no-knotweed policies which have resulted in prospective buyers withdrawing from a purchase (see Chapter 4). The invasive species consultancy Advanced Invasives described Japanese knotweed as “a menace to homeowners”, as a result of the difficulties that it can cause for prospective buyers to secure a mortgage on the property.8
Figure 1: Japanese knotweed
Image copyright CABI
Our inquiry
3.Our Report focuses on the effects of Japanese knotweed in the ‘built environment’ of buildings, paving, drainage channels and outbuildings. Given the anxiety that the plant can cause for homeowners, and the publication of new evidence relating to the physical effects of Japanese knotweed (see Chapter 2), we issued a call for written evidence on the following issues:
- What scientific evidence exists on the effects of Japanese knotweed on the built environment;
- How the presence of Japanese knotweed in the UK affects mortgage lending decisions and property valuations;
- Whether mortgage lending decisions relating to the presence of Japanese knotweed are currently based on sound scientific evidence of its effects on the built environment; and
- What guidance for the sector currently exists, the impact of existing legislation, and how else evidence-based responses to the presence of Japanese knotweed can be encouraged.
4.We received over 30 written submissions during our inquiry. We also benefitted from a private briefing from Dr Dick Shaw, Country Director for CABI UK, to help shape our initial call for evidence. We invited members of the public to tell us about their personal experience of Japanese knotweed, and received 14 responses. We held a roundtable discussion on Monday 21 January with a small number of individuals affected by Japanese knotweed and a solicitor who had represented clients in such cases, in order to inform our questioning of witnesses. We took oral evidence on Tuesday 22 January from knotweed researchers, remediation experts, a representative of the mortgage lending industry, and the Royal Institution of Chartered Surveyors. We are grateful to all those who contributed to our work.
5.While the focus of our Report is the effects of Japanese knotweed in the built environment, we note that the plant also has wider ecological effects. Where it becomes established, the tall dense summer growth and the mulch-like effect of dead leaves and canes in winter inhibits the growth of almost all other native plant species. It can also lead to problems with water drainage, since “if you have a large, dense stand [cluster of stems] of Japanese knotweed down the side of the river and high rainfall, the water rises in the river and the knotweed will hold it back, which will exacerbate flooding”.9 These effects, and the difficulties in controlling the plant, have led to a range of legislation relating to the spread and disposal of Japanese knotweed (see Box 1 for examples).
Box 1: A selection of relevant legislation
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The wider ecological effects of Japanese knotweed are such that the disposal of the plant is subject to legislation, some of which is relevant to discussion of the effects in the built environment. In particular:
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Case law relating to Japanese knotweed and the law of private nuisance is also developing in this area.
This Report
6.Our Report explores the latest evidence on the physical impacts of Japanese knotweed (Chapter 2). In Chapter 3 we consider the non-physical impacts of this plant in terms of treatment costs and the stigma associated with infestation. Chapter 4 discusses the information and guidance that mortgage lenders use to make decisions relating to Japanese knotweed. Finally, in Chapter 5 we consider the effect on homeowners and the difficulties encountered when Japanese knotweed is present on a neighbour’s land.
1 While we use the term ‘Japanese knotweed’ throughout this Report, we note the submission from Advanced Invasives which clarified that “Japanese knotweed is a term widely used to refer to both the specific species Fallopia japonica var. japonica (commonly; Japanese knotweed) and, quite confusingly, the four key invasive knotweed species in the UK collectively (Japanese knotweed, Dwarf knotweed, Giant knotweed and Bohemian knotweed—referred to as Japanese knotweed senso lato taxa)”. See Advanced Invasives Limited (JKW0032) para 13.
2 More precisely, a rhizome is an underground plant stem which can produce both roots and shoots, and act as an energy store for the plant.
3 CABI, The economic costs of invasive non-native species in Great Britain, November 2010, p33
4 Property Care Association, Code of Practice: Management of Japanese Knotweed, April 2018
5 CABI, The economic costs of invasive non-native species in Great Britain, November 2010, p34–35
6 Using the Bank of England inflation calculator
7 CABI, The economic costs of invasive non-native species in Great Britain, November 2010
10 Wildlife and Countryside Act 1981, Schedule 9
Published: 16 May 2019