A bush of Japanese knotweed with green shovel shaped leaves and red stems.

All ABOUT JAPANESE KNOTWEED

All your questions about this invasive species answered; how it arrived in the UK, what makes it so problematic, and the issues Japanese knotweed causes. Discover how to get rid of knotweed and a few other interesting things you may not know.

What is Japanese knotweed?

the japanese knotweed plant

Japanese Knotweed (Reynoutria japonica syn. Fallopia japonica) is a fast-growing clump-forming invasive perennial weed that can cause damage to structures and property. It was originally discovered by the Dutch naturalist Maarten Houttuyn in the 18th-Century, who gave it the name it still has today; Reynoutria japonica. Do we have Japanese knotweed under control in the UK? It’s an ongoing issue, and we’re here to help.

The Environment Agency deems Japanese knotweed as the most common of 4 invasive knotweed plant species in the UK, which are:

Japanese knotweed
Dwarf knotweed
Giant knotweed
Bohemian (hybrid) knotweed

Key features are the zig-zag stem structure, ‘shovel’ shaped leaves and white flowers that emerge in summer. The stems can grow up to 7ft high. In winter it looks completely different with only dark brownish red stems being visible above ground. Knotweed spreads via underground root system (Rhizomes). More on how to identify knotweed through the seasons here.

In the UK, we only have the ‘female version’ of the plant. The Japanese knotweed root or ‘creeping rootstalk’ form horizontal underground plant stems that produce shoots and root systems that form new plants.

what does knotweed look like?

A few knotweed facts

Japanese knotweed close up showing hollow fleshy centre

Japanese knotweed is edible and used in some restaurants. It apparently tastes similar to rhubarb.
It can withstand extreme temperatures, happily growing on the side of volcanoes, or in sub-zero climates.
Japanese knotweed has no natural enemies in the UK to prevent its growth.
A piece of rhizome as small as a thumbnail can create a new plant
It cannot be ‘killed’. Despite people trying to burn it with fire or acid, it is pretty much indestructible
The Japanese use the plant in traditional medicine, where it’s a popular painkiller. The resulting medicine is called Itadori, which translates as ‘take away pain’.

How did Japanese knotweed get here?

Originating from Japan (hence the name), every Japanese knotweed plant in the UK derives from one single plant that was introduced by Philip von Siebold in 1850. He was unaware of the environmental impact this plant would go on to have. Knotweed was originally brought to the UK as an ornamental plant for commercial sale and botanical cultivation, and from there was taken to North America in the late nineteenth century.

The plant began to spread after Siebold sent samples back to his native home in Norway, and eventually the cultivated plants were sent back to Kew Gardens. Its quick growth was soon noticed by Kew, who then started to sell the plants for residential garden use. The rest as they say – is history! When it was first identified as being a Non-Native Invasive it was already spreading across the country. Japanese knotweed now grows across almost all areas of the UK, and is currently most prolific in Wales.

Why is knotweed such a problem?

Cost

Japanese knotweed treatment and management costs the economy millions of pounds every year. A DEFRA analysis estimated an astonishing country-wide price tag of £1.5 billion for its control.

It’s growth pattern

The rhizomes can grow to depths of two meters and extend up to seven meters horizontally from the visible part of the plant. These roots are what allows knotweed to spread and are part of the reason that it is so resilient – they can remain capable of supporting life for up to 20 years.

It only takes a small amount of healthy root to give rise to an entirely new plant – and cause the infestation to re-emerge. When allowed to spread, or disturbed Japanese knotweed can grow under footpaths, and buildings, exploiting any weakness within the structure by growing through existing cracks or holes. Contrary to many statements you may read about knotweed – it does not cause actual ‘structural’ damage. It’s important that early stage Japanese knotweed damage is identified so that a treatment plan can be put in place, or excavation can be carried out to completely remove it.

Knotweed can re-emerge and re-grow of its own accord any time, but especially if the contaminated ground is disturbed. This could be through digging or even common household gardening, also by extreme flooding which will carry fragments further afield where they will then establish and grow into new stands. A common myth is that planting knotweed in a pot will contain it – this is not the case.

Environmental impact

Knotweed’s rapid growth rate means it can smother other species, blocking sunlight with a dense canopy of leaves in summer. It also releases allelopathic chemicals into the soil that can stop other plants from growing. The large overwintering canes can block water channels increasing the likelihood of flooding.

Problems for commercial businesses and land developers

As disturbance to the rhizome will generate new growth, this is a real problem for commercial or private land developers. Ignoring knotweed can result in regrowth during or after construction, appearing through hard and soft landscape areas and even within the fabric of buildings themselves if there are any areas it can poke through.

If a developer or landowner does not undertake the necessary due diligence and control of Japanese knotweed, they risk legal action for professional negligence, which obviously has an impact on land marked for development. If knotweed is identified on a development site all works have to be stopped until a full survey and treatment or eradication has taken place. The location of knotweed must be recorded for future land sales.

Restrictions on amenity use

Knotweed can impede public spaces such as footpaths and sports fields, and if left untreated can spread into other amenity areas such as residential or public gardens, play areas or parks. This places severe restrictions of the free unimpeded use of the area.

Legal issues with buying and selling

Since 2013, when selling a property the seller is required to state whether Japanese knotweed is present on their property by ticking a box on the TA6 form – a property information form used for conveyancing. Failure to do so, or to make a false statement is deemed an illegal act that has consequences such as the seller being sued for ‘misrepresentation’.

If a professional survey is carried out and knotweed is either missed or mis-identified this is classed as ‘Professional negligence’

When developing a site affected by Japanese knotweed, if the necessary due diligence and control of Japanese knotweed has not been undertaken, the developers risk legal action for Professional Negligence. Ignoring knotweed can result in regrowth during or after construction, appearing on the property long after the site has been handed over to a Managing Agent – thus causing issues for the Agent and homeowners alike.

knotweed and the law

New knotweed legislation

An amendment to the Anti-social Behaviour, Crime and Policing Act 2014 includes invasive non-native plants including Japanese knotweed. RHS Guidelines state:

It is not illegal to have Japanese knotweed in your garden, but on your property you should aim to control this invasive non-native plant to prevent it becoming a problem in your neighbourhood. If it has a “detrimental effect of a persistent or continuing nature on the quality of life of those in the locality”, the legislation could be used to enforce its control and property owners may be prosecuted
Where problems with Japanese knotweed occur in neighbouring gardens, we suggest that you speak or correspond directly with your neighbours (who may already be taking action to control this difficult weed). These informal steps should be taken before contacting your council to talk about action under the legislation
Homeowners can consider control themselves for a small, isolated clump. However, a specialist professional company will be skilled at control, ensure eradication and can dispose of the plant waste at licenced landfill sites

knotweed Encroachment issues

If Japanese knotweed is allowed to spread beyond the property boundary this is deemed as encroachment. This means if a property has knotweed growing on their land they should make every effort to control and prevent this invasive weed from spreading onto a neighbouring property. Encroachment would be deemed as having occurred even if just the underground rhizome of the plant had been found to have spread from one property to another.

Actionable private nuisance claims can be served against persons found responsible for having allowed or caused encroachment to take place. A private nuisance is an act or omission which is an interference with, disturbance of or annoyance to a person in the exercise or enjoyment of his ownership or occupation of land, and can lead to legal action or even a court case.

knotweed control Industry legislation

knotweed Waste Disposal Regulations

The disposal of Japanese knotweed and other invasive weeds listed under section 14(2), schedule 9, part II of the Wildlife and Countryside Act, is legislated. Under the Environmental Protection Act, waste containing the propagules of these plants is classified as controlled waste if removed from site of origin. This still applies to all knotweed material after treatment or excavation, and requires the waste to be transported only on Environment Agency registered waste carrier vehicles and disposed of at specially licensed landfill sites (of which there are limited numbers in the UK).

Where there is an option to retain or dispose of these wastes on-site, it needs to be done in accordance with the Government’s Regulatory Position Statement (RPS) number 178. This controls how the waste can be kept and disposed of on-site, with specifications for burial, relocation and re-use.

Environmental Protection (Duty of Care) Regulations 1991

This Act exists to ensure responsibility is taken by the producers of waste (such as Japanese knotweed) for managing their waste and avoiding harm to human health or the environment.

The Act aims to reduce or eradicate harmful acts of waste crime, such as fly-tipping. The Duty of Care incorporates a responsibility on anyone who produces, imports, carries, keeps, treats or disposes of controlled waste to ensure it is only ever transferred to someone who is authorised to receive it. Read more on The Environmental Protection (Duty of Care) Regulations 1991.

The Wildlife and Countryside Act 1981

The Act provides the primary control for the release of non-native species into the wild in Great Britain. It is an offence under Section 14(2) of the Act to “plant or otherwise cause to grow in the wild” any plants listed in Schedule 9, Part II.. Read more about the Wildlife and Countryside Act 1981.

Impact of knotweed on the built environment

It has been estimated that over 2% of development sites and 1.25% of residential properties in Great Britain are affected by Japanese knotweed, amounting to tens of thousands of sites in total. Evidence of knotweed on property can have a negative impact on its value.

According to the Property Care Association (PCA): “The Science and Technology Committee report advises that the latest research into Japanese knotweed suggests that the physical damage to property is no greater than that of other disruptive plants and trees. However, other plants and trees are not subject to the same levels of control and do not have the same impact on the sale of a property.” The Environment Agency has asked for further investigations to be carried out (and have approached DEFRA) to better understand the affects of knotweed on the built and natural environments.

knotweed Control methods

There are two main methods of control used in the industry: Herbicide treatments (usually via a programme) and Excavation. Although several new methods are being trialed by some companies, such as hot foam, steam or electrical usage, there is not enough evidence that these are as effective long-term and may cost more than herbicide and excavation.

At JKL we use the best and most cost-effective options for each client, based on their needs and the land use requirements, this can mean a bespoke plan using several methods. We have several methods of excavation, including vacuum excavation which is used on sites where either environmental aspects (trees or endangered species) or utilities (pipes and cables) need to be considered.

With excavation the knotweed waste material can be left onsite if using an approved method such as cell burial, or completely removed and taken to an approved landfill site that is licensed to receive knotweed waste material.

japanese knotweed removal questions

Knotweed clump looking up to the sky with overhead services

What happens if knotweed is not controlled?

If the Japanese knotweed infestation is left unchecked for several years, it can spread and cause a range of issues including:

Impeding amenity land use – more on that in this BLOG
Increase change of land use costs – more on Japanese knotweed issues for construction in this BLOG
Damage to hard standing structures – more on Japanese knotweed damage in this BLOG
Lowering property value – more on selling a property with Japanese knotweed in this BLOG
Creating legal issues – more on Japanese knotweed and the law in this BLOG

Or simply find out why KNOTWEED IS NOT TO BE IGNORED!

The importance of a Japanese Knotweed Management Plan

The Environment Agency states that once you find Japanese knotweed on private land or commercial development site, it is essential that you set up some form of Knotweed Management Plan (KMP).

It’s the survey findings that will go on to form a documented KMP for the property. This includes an assessment of the extent of the infestation, and severity of impact to the current or proposed future use of the property or land, which can include restrictions on maintenance and amenity use activities, and in rare cases damage caused to structures.

The presence of knotweed does not automatically prevent a mortgage from being obtained, with a case-by-case basis approach often adopted some mortgage lenders will agree. Evidence of a suitable KMP is paramount. Whilst it might be a shock to find out that the property you are trying to sell has knotweed, or indeed, the property you intend to buy is affected, there are treatments available to manage the infestation which will be clearly outlined in the KMP.

What’s included in a KMP?

The KMP provides you with the survey report, picture evidence of Japanese knotweed (or other invasive weeds) found onsite, a site drawing locating the knotweed and a recommended plan of action for remediation.

The quotation within the plan may include several methods depending on the site. If there are known change of land use proposals for the property, these will also be evaluated to decide the appropriate control method. After the works have been carried out a Completion Report is provided as evidence of works undertaken.

The future

RICS knotweed guidelines

The Royal Institution of Chartered Surveyors (RICS) Guidance Note published in January 2022, on which our Director Ben Lindley is directly named as a contributor set the standard by which Japanese knotweed is controlled. The RICS’ Standard was governed and overseen by the Standards and Regulation Board (SRB), whose aim is to develop the ethical and competence of the profession and set standards for service delivery.

The new Note was later superseded by a new document that was reissued in October 2022 as a Professional Standard, however the principals outlined in the document remained the same.

The RICS information paper 2012 established a framework for assessing the risk to residential property posed by Japanese knotweed. It specified four risk categories (1-4), using a distance of 7m from buildings and boundaries as the defining measurement. Along with asking for an assessment on minor or major structural damage. The information paper was instrumental in providing a rationale that enabled lending on residential properties impacted by knotweed.

Since the original information paper was published in 2012, academic research into Japanese knotweed has been published that has influenced a review of property impact guidance and development of the new Guidance Note.

In 2018 a research paper by Fennell et al from the University of Leeds reported that typical rhizome spread was 3m, opening consideration for a different defining distance measurement to 7m. The paper also reported that cases of material damage to a structure were only likely when knotweed was immediately adjacent to susceptible structures.

A further research project by Jones et al from the University of Swansea published findings in 2018 on the optimum method of controlling Japanese knotweed with herbicides. Control of an infestation was readily achievable when properly undertaken. However, fast results or total eradication were not recognised as readily attainable from herbicide only treatment methodologies.

In 2019, the House of Commons Science and Technology Committee published a report on an enquiry titled ‘Japanese knotweed and the built environment’. The enquiry had been promoted by the publication of the research papers and a growing litigation industry surrounding knotweed.

Japanese Knotweed Ltd provided written evidence to assist with the enquiry, and a company Director, Ben Lindley, was invited to provide oral evidence at the committee meeting, along with seven other industry stakeholders.

The Technology Committee report made recommendations, one of which was for the RICS to review its guidance on knotweed. The report described the ‘7m rule’ as a blunt instrument that did not reflect the latest scientific evidence. It called for a revised assessment process which was ‘much more nuanced and evidence-based … to reflect the latest thinking on the significance of Japanese knotweed’.

The RICS Guidance Note “Japanese Knotweed and Residential Property” 2022, is the result of the requested re-assessment.

What are the main RICS changes?

The RICS Guidance Note provides an updated assessment for RICS members in conducting their surveys and valuations.

The guidance takes onboard previously published research into Japanese knotweed. Modifying the associated property impact assessment, which advises lenders on needing to apply mortgage retention or not.

When advising for non-lending purposes the property surveyor will always advise the client to seek advice from a specialist remediation contractor about the Japanese knotweed.

The contractors’ trade association (the PCA Invasive Weeds Control Group) have produced a more detailed assessment of impact and remediation recommendations in their associated Guidance Note ‘Japanese Knotweed – Guidance for Professional Valuers and Surveyors’.

A specialist remediation contractor, such as Japanese Knotweed Ltd, will advise clients of the specific impact knotweed may have on a property, such as loss of free use, maintenance restrictions, limitations on development, waste disposal costs, and potential litigation impacts.

A specialist remediation contractor will recommend the appropriate remedial action to take, such as herbicide treatment or excavation under a Knotweed Management Plan for any property affected by knotweed and offer the provision of an Insurance Backed Guarantee.

So, ‘Caveat emptor’ buyer beware and as advised by the RICS Guidance Note management categories, always seek advice from a specialist remediation contractor, such as Japanese Knotweed Ltd, when looking to sell or buy property impacted by Japanese knotweed.