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Publish date

23 March 2023

What is Japanese Knotweed?

This plant is native to Japan, Taiwan and Northern China. It was introduced into the UK as an ornamental plant and has subsequently become naturalised. It is a fast growing plant which is expensive and difficult to eradicate. It can take years to remove and occasionally it can damage buildings. Finding it on a development site can therefore pose serious problems for developers due to the cost and delay involved in dealing with its removal. The House of Commons Science and Technology Select Committee’s 2019 report on Japanese Knotweed suggests that structural damage cause by knotweed to property is no greater than other disruptive plants. However, knotweed is potentially more difficult to eradicate which is why it must be taken seriously.

Guidance on eradication of Japanese Knotweed

The Environment Agency’s 2007 guide for developers on “Managing Japanese Knotweed on development sites” withdrawn by the Environment Agency in 2016 as best practice guidance is no longer provided by them.

Basic advice from the Environment Agency on how to prevent knotweed spreading is available but it does not contain the same level of detail. It would seem therefore that developers will need to consult Japanese Knotweed eradication experts in order to establish what the best approach is for the control or eradication of knotweed on their site.

It should be noted that as more has been learnt about Japanese Knotweed a more measured approach is beginning to be taken to its presence on sites. As part of this process RICS have published a new Professional Standard (for residential properties) to replace their 2012 Information Paper. This sets out a revised assessment methodology to help valuers and surveyors objectively describe the scale of a Japanese Knotweed infestation to ensure that more balanced and measured decisions can be made on its impact.

Legal implications

Liability could arise in several ways:

•    Common law: A landowner who allows Japanese Knotweed to escape onto neighbouring land potentially is open to civil claims including for private nuisance and/or trespass. Once there is an encroachment there is physical interference with the neighbouring land and losses flowing from this (including a claim by an adjoining owner for diminution in value) are potentially recoverable

•    Wildlife and Countryside Act 1981: The plant carries criminal liability for those who plant it or allow it to grow in the wild under the Wildlife and Countryside Act 1981. It is not an offence to have it on your land, but failure to take reasonable measures to control it or being negligent or reckless about that occurring, could amount to the offence of causing it to grow in the wild. The penalty can be up to 2 years’ imprisonment and/or an unlimited fine

•    Invasive Alien Species (IAS) Regulation: Following Brexit this regulation has been retained in the UK to prevent (and manage) the introduction and spread of invasive alien species which have an impact on biodiversity and the environment. It introduces criminal offences particularly in relation to repeat offenders whose actions would result in environmental damage. These offences are in addition to those in the WCA 1981. The maximum penalty is a two years’ imprisonment

•    Species control agreements and species control orders: In 2015 new provisions were added to the Wildlife and Countryside Act 1981 to enable certain environmental authorities (including the Environment Agency) to seek a Species Control Agreement (SCA) with a landowner of affected land, requiring the landowner to deal with an invasive non-native species such as Japanese Knotweed. If the landowner breaches the terms of the SCA or there is a need for urgent action a Species Control Order (SCO) can be made to compel the owner to control the plant or allow the environmental authority to control it. Failure to comply with an SCO can also lead to imprisonment and/or an unlimited fine

•    Community Protection notice under Anti-social Behaviour, Crime and Policing Act 2014: Local Authorities (or the Police) have powers to serve community protection notices on individuals or organisations if their conduct has a detrimental effect of a continuing nature on the quality of life of those in the locality and/or is unreasonable. These powers were not specifically designed in relation Japanese Knotweed. However, guidance surrounding the powers makes it clear they are to be used flexibly

•    Local Authority powers: Local Authorities may also have the right under s.215 Town and Country Planning Act 1990 to serve a notice on a landowner to remedy the condition of the land within a specific period of time. Failure to comply can lead to the Local Authority stepping in and carrying out the works at the landowner’s expense, prosecution and a fine of up to £1,000

•    Waste offences: The plant constitutes controlled waste under the Environmental Protection Act 1990 and must be disposed of safely. If offsite this will mean at a licensed landfill site. This means that once the plant is cut back or removed from the ground, there can be liability for unlawful disposal

•    Misrepresentation: If you are selling a site that has knotweed or you do not know for certain that it does not have knotweed you must be very careful not to misrepresent the position to your purchaser. The standard enquires for both residential and commercial property ask questions about Japanese Knotweed and recent case law has confirmed that unless you know for certain (as you have carried out all the appropriate investigations) you may be liable for misrepresenting the position in relation to Japanese Knotweed even if your answer is given in good faith. Specific advice on how to answer this question, based on the facts of your sale, should be obtained from your conveyancing solicitor.

Due diligence

If a buyer is intending to develop a site they should consider commissioning a specific knotweed survey and assessment, either from a surveyor with the relevant expertise, an environmental consultant or a specialist knotweed consultant.

How is land remediated?

New biological methods of control are being trialled using aphids. At this time, it is understood these trials have not been as successful as it had been hoped.

More traditionally, the best options are a combination of digging and burning the roots and chemical spraying. Control with just herbicides can take up to three years depending on the size of the problem.
Digging it out, and/or using binding and membranes is much quicker but more expensive.

The preference is usually for treated soil, free from Japanese Knotweed, to be re- used on-site. Soil removed from an affected site can only be taken to a licensed landfill.

Some local authorities require a Japanese Knotweed management plan prior to any disposal works being undertaken.

A number of companies specialise in Japanese Knotweed control. The Japanese Knotweed Alliance, Phloram and the Property Care Association provide practical information on their websites and details of specialists. It is best to speak to a specialist as to what type of treatment is right for your site as each infestation is unique and should be considered individually.

Cost implications

The developer should cost the options for disposal and management on the site. The time it takes for removal means that a development may be delayed which will have cost implications. In addition, the cost of removal needs to be factored in, even the digging out, removing and disposing of an area 1m² could cost many thousands of pounds. There is no substitute for good environmental surveys and early advice on options for its management and disposal.

Land remediation relief scheme

Tax relief may be available for the cost of some of the remediation works that may be required to deal with Japanese Knotweed under the Land Remediation Relief Scheme. This was extended in 2009 to encourage the remediation of derelict land blighted by contamination including Japanese Knotweed. It allows companies to claim a deduction in corporation tax for revenue and capital expenditure incurred in remediating certain contaminated or derelict sites. For more information click here.

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