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This technical article provides additional guidance on Japanese knotweed.

Japanese knotweed (Fallopia japonica) was first introduced to Britain by the Victorians as an ornamental plant. The 2006 code of practice produced by the Environment Agency on managing Japanese knotweed was amended in 2013. Japanese knotweed is considered to be one of the most invasive plants in Britain.

Since the publication of the superseded 2006 code, various soil screening and sieving methods have become popular methods for rhizome removal. Where conditions are appropriate for these methods, screening can provide an effective means of rhizome removal. Screened soil must still be regarded as potentially containing viable knotweed rhizome and must not be reused off-site or sold for re-use. If soil is taken off-site, it should be disposed of at an approved landfill as Japanese knotweed is classed as a controlled waste.

For warranty purposes, there is no cover provided under the New Homes Policy as Japanese knotweed is not a notifiable contaminant.

Managing Japanese knotweed is the responsibility of the owner/occupier of a site. The owner/occupier of the site has a legal obligation not to allow it to spread. Japanese knotweed spreads easily via rhizomes (root systems) and cut stems or crowns. It has a vigorous growth and is difficult to eradicate.

Japanese knotweed commonly grows up to 3m tall at a rate of 75mm per day. Its rhizomes can extend up to 3m deep and 7m out from the parent plant. Japanese knotweed thrives on disturbance.

There are various control methods available from companies specialising in Japanese knotweed management on development sites and some offer insurance-backed guarantees for its eradication. Therefore, careful consideration should be given to products and methods that claim to quickly eradicate the plant and the guidance for treatment of knotweed recommended in the Environment Agency’s Code of Practice should be followed.

The Code of Practice describes both chemical and non-chemical methods of control along with methods of disposal both on site and to licenced tips. Japanese knotweed can stay dormant for up to twenty years.

If left untreated or is incorrectly treated then the potential for regrowth can be considered to be extremely high. Japanese knotweed can grow through and cause damage to paved and tarmac surfaces and also cause damage to drainage. It is not known for growing through concrete, however it can grow through cracks and gaps in and around concrete finishes/floors both internally and externally of a property. In addition, Japanese knotweed has also been found to grow within external wall cavities and within sub-floor voids.

Summary

If Japanese knotweed is found on the site, the developer should ensure:

• The area is cordoned off where the knotweed is situated in order to prevent machinery/foot traffic from accidentally spreading the material across the site
• The builder/developer seeks specialist guidance, referring the builder/developer to the Environment Agency’s code of practice for dealing with Japanese knotweed on development sites
• That a copy of the Japanese Knotweed Management Plan should be obtained and followed
• If herbicide treatment is to be carried out it is essential the contractor is a competent and qualified person and must have appropriate National Proficiency Test Council certification
• That a copy of any insurance-backed treatment offered is obtained

Note: The 2006 Code of Practice produced by the Environment Agency on managing Japanese knotweed had been amended in 2013 but has now been withdrawn. Developers should follow the guidance on Gov.UK website.

Reference

The Environment Agency’s Guide to managing Japanese knotweed on development sites (Amended July 2013).

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Every care was taken to ensure the information in this article was correct at the time of publication. Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.