RICS Draft Guidance Note: Japanese knotweed and residential property, 1st edition

6 The management of Japanese knotweed

6.1 The Japanese Knotweed Management Plan

When Japanese knotweed has been identified at a property further investigation should be carried out by an accredited member of a recognised trade body (currently the Property Care Association (PCA) and the Invasive Non-Native Specialists Association (INNSA)). A report should be provided and, depending on the findings, a Japanese Knotweed Management Plan (JKMP) should be prepared. The JKMP can provide the necessary reassurance to all interested parties that a Japanese knotweed problem is being properly managed.

The options for remediating an infestation of Japanese knotweed will depend on the commercial choices and preference of the contractor but the JKMP should reflect current legislation and the latest guidelines and practices as set out in the most up-to-date Codes of Practice (e.g., PCA, INNSA or Great Britain Non-Native Species Secretariat (GBNNSS)).

The Japanese Knotweed Management Plan should include the following features:

  • a description of the property with an accurate record of the Japanese knotweed infestation
  • a scaled plan with dimensions, supporting photographs are also useful
  • full details of the contracting organisation and a description of the methods to be used to eradicate, manage, or control the Japanese knotweed
  • a treatment schedule that is updated as treatments are carried out
  • on completion there should be a certificate that confirms the treatment is complete and that the Japanese knotweed at the property has been remediated. A recommendation for long term monitoring may be made, especially with respect to herbicide treatment.

Valuers and surveyors should take account of this range of information when deciding whether the evidence of previous treatment programmes is adequate. Other features of the JKMP providing additional reassurance to lenders:

  • the current owner may have to pay all costs associated with the management plan upfront, so that the treatment programme can be completed without relying on financial support from subsequent owners
  • the management plan should be transferable to any subsequent owners
  • the management plan should cover the whole of the property and not just those affected parts identified by the original valuer or surveyor
  • an appropriate warranty or guarantee from the contractor for a five- or ten-year period and third-party insurance cover that will ensure that the warranty remains valid in the event of insolvency of the original contractor.

Important note: Although it is unlikely that Japanese knotweed will return following the successful delivery of a professional treatment plan, the biology and nature of the plant, and site variations, means that in some circumstances regrowth can occur. The JKMP guarantee should, therefore, include details of how to proceed if regrowth does become apparent.

6.2 Remediation options

Once it is established, eradicating Japanese knotweed can be challenging and usually a more economical objective is to achieve effective control. Valuers and surveyors should be aware of the range of approaches that can be taken by professional contractors but it is beyond the scope of this guidance note to do more than briefly outline alternative remediation options. For detailed information, including typical costs and the pros and cons of the alternatives, the complementary PCA publication Guidance for Professional Valuers and Surveyors should be consulted.

For residential properties there are usually only one or two remediation options realistically available, either chemical control using herbicides or physical removal by excavation, but the implications of the necessary treatment will vary depending on the circumstances on-site.

Chemical control is the application of herbicide to Japanese knotweed plants over a period of several growing seasons. This is often the most economical treatment option but will usually require a minimum of four years of treatment and monitoring before control can be assured.

Excavation involves the removal of contaminated soils to a licenced waste-management facility, which can involve significant volumes of soil. Even when volumes are minimised by careful site monitoring, excavation of even a relatively small Japanese knotweed infestation can cost several thousands of pounds in waste charges alone for excavation, transport, and landfill taxes.

It should be appreciated that the treatment area will not only be the visible Japanese knotweed area but will typically include an area of 2m to 3m from the nearest visible knotweed. For example, a 2m x 2m stand of knotweed may require an area of 6m x 6m to be treated. Excavation could involve removal of that area of soil, typically down to a depth of 1m or 2m.

Some alternative remediation methods, such as partial excavation, on-site burial, geotextile installation or encapsulation with membranes, may be appropriate for large sites or developments and can significantly reduce waste disposal costs. It is unlikely, however, that such methods, including biological control, would be suitable for standard residential properties.