RICS Draft Guidance Note: Japanese knotweed and residential property, 1st edition

2 The Japanese knotweed problem

2.1 Understanding the problem

Japanese knotweed is certainly a challenging issue, but the reality does not mirror common public perception. Hence the need for this guidance note to provide clarity and confidence in the market when Japanese knotweed is encountered, through consistency in the initial assessment and provision of advice to clients by RICS members.

It may also be helpful to consider Japanese knotweed in the context of other plants. Brambles can take over gardens if left uncontrolled and gardeners well understand the difficulties of dealing with persistent weeds such as bindweed. The concept of zero tolerance of weeds does not align with weed management generally where the requirement is for regular management rather than seeking permanent eradication.

Experience and research in recent years mean that the real problems posed by Japanese knotweed in a domestic residential setting are now better understood. For example:

  • Japanese knotweed rarely causes structural damage to substantial buildings such as dwellings. The 2018 paper by Fennell et al explains why the biology of Japanese knotweed means it is incapable of causing damage in the same way as trees or woody plants such as buddleia. Large stands or growths of Japanese knotweed, if left uncontrolled, can damage lightweight structures, retaining walls, paths, hard standings, drains and other ancillary features.
  • The treatment of Japanese knotweed is expensive, disruptive and can affect the quiet enjoyment of a property for a number of years. By contrast, killing or removing buddleia or trees is comparatively straightforward.
  • Ground affected by Japanese knotweed cannot be developed without taking special precautions, typically including the removal and disposal of contaminated soil by appropriately licensed operators. This will increase development costs.
  • Remediating Japanese knotweed growth in a neighbouring property or on adjacent public land is more problematic than growth solely on the subject property because the property owner lacks effective control over the treatment regime.
  • Japanese knotweed that crosses property boundaries can sometimes result in expensive and frustrating legal action that is often fuelled by 'no win, no fee' organisations.
  • The resolution of Japanese knotweed problems can become complicated where it affects flats in blocks or conversions. In some circumstances it is possible for the saleability of all flats in a building to be affected if the responsible person or body (whether freeholder, management company or individual owner) does not adopt a suitable approach. Flats may be remote from the Japanese knotweed but in rare circumstances leaseholders may ultimately be left with few options other than legal action.

It is acknowledged that the presence of Japanese knotweed can be a significant impediment to the sale and purchase of a property and it is known to affect both value and saleability. This effect is currently likely to be similar whether the Japanese knotweed is close to the dwelling, at the bottom of a long garden or actually damaging ancillary features. It is the presence of the Japanese knotweed that is the problem, while any damaging effects are secondary.

Unfortunately, with Japanese knotweed the original problems in the market became to a large extent self-perpetuating, with the Defra report describing 'linguistic alarmism' in the media as a significant influence on public opinion. The exaggerated public perception of the problems caused by Japanese knotweed has meant that the impact in the marketplace is often out of all proportion to the cost of remediation. Even where Japanese knotweed has been effectively remediated, experience has shown that properties may retain a 'stigma' for some years afterwards, with a lingering, if diminishing, negative perception and a corresponding adverse impact on saleability.

This places valuers in a difficult position. They understand the reality of the physical problems that Japanese knotweed can cause but they must 'follow the market and not lead the market'. Their role is to take a holistic approach and reach a professional judgement by striking a balance that accounts appropriately for market sentiment and the facts relevant to the property being valued. This paradox is acknowledged in the Defra report which states that 'Valuers recognise all of this but must reflect public perception and the resulting impact on values'. Section 7 of this guidance note provides guidance to valuers on the factors that may need to be taken into account when valuing a residential property affected by Japanese knotweed.

2.2 Typical scenarios

A typical Japanese knotweed infestation in a residential property may result in a loss of amenity, some disruption to landscaped areas, driveways, paths, etc. and possibly damage to footings or foundations of lightweight structures but it is very rare for there to be structural damage to the foundations of dwellings. Typical effects and affected areas that may be encountered are:

  • Gardens: In most cases there is a loss of amenity. This may range from a minor inconvenience to a major loss.
  • Patios, paths and driveways: Often there may be no major damage to patios, paths and driveways relating to the presence of Japanese knotweed. However, like many other plants, Japanese knotweed can sprout up between patio slabs, joints in concrete driveways and cracks in brick paving. If the plant is allowed to grow unconstrained slabs may be lifted. Tarmac surfaces are also susceptible to damage, particularly around edges, and certainly if laid on top of Japanese knotweed.
  • Boundary, garden and retaining walls: In the main, well-built boundary, garden or retaining walls should resist damage from Japanese knotweed. However, like many mature shrubs and trees, very mature stands of Japanese knotweed (with massive root 'crowns') can undermine or 'push-over' garden walls over a period of years.
  • Outbuildings: As with many other plants, mature stands of Japanese knotweed can worsen existing damage to lightweight, insubstantial and poorly founded outbuildings such as garden sheds, greenhouses and, in very rare cases, poorly built garages.
  • Conservatories: Although the effects may be like those described for outbuildings, owners understandably attribute greater importance to these structures. Where serious difficulties are encountered it is usually due to a conservatory having been constructed on top of mature, untreated Japanese knotweed, due to inadequate site clearance, rather than Japanese knotweed 'invading' the conservatory from a nearby location.
  • Drains and other buried services: Like other trees and shrubs, Japanese knotweed roots/rhizomes can exploit existing cracks and gaps in e.g. drainage pipes in the search for water, potentially causing further damage and, in some cases, blocking drains. Large, densely packed mature stands of Japanese knotweed can disrupt drain runs where allowed to grow unconstrained for many years.

2.3 Implications of the Environmental Protection Act 1990

The Environmental Protection Act 1990 contains legal provisions that designate Japanese knotweed-contaminated soil as 'controlled' waste. Only properly licensed organisations may remove this waste from a property and they must take it to appropriately licensed waste facilities. This can have serious implications for owners who want to develop their property. In properties affected by Japanese knotweed, large amounts of contaminated soil can result from activities such as:

  • adding an extension to the main building
  • redesigning the garden and
  • maintaining and repairing the property following a Japanese knotweed infestation (for example, re-laying paths and drains).

The need for licensed removal of this contaminated soil and any dead plant material will obviously add to the cost of the work. The cost of removing Japanese knotweed-contaminated soil and possibly protecting works with a root barrier can be significant. Guidance from the Environment Agency is that waste should be minimised and various proportionate solutions to reduce the amount of waste are often available but the financial impact of Japanese knotweed on this type of work must not be underestimated.

2.4 Wider environmental implications

While re-directing the focus of Japanese knotweed on its wider impact, rather than solely as a risk to buildings, it is important also to be mindful of wider environmental implications. Ignoring Japanese knotweed is not an acceptable strategy, either from a local or national perspective but remediating Japanese knotweed may involve choices between the use of herbicides over a number of years or the removal of substantial volumes of contaminated waste soil to landfill sites. Individually these pose significant environmental issues in themselves but together they have national implications. In each individual case the objective should be to implement the minimum acceptable intervention to achieve the desired level of management or control at the lowest environmental cost.

As the Defra report recommends, there should be 'an increasing awareness that eradication of Japanese knotweed is not a helpful objective, and that the focus should be on management and control.'

Section 6 of this guidance note briefly describes remediation options but more detail is provided in the PCA document Japanese knotweed - Guidance for Professional Valuers and Surveyors.