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  • Writer's pictureCliff L'Aimable _Chartered Surveyor & Building Engineer

Japanese Knotweed - a misunderstood invasion?

Another Blog Post from University Graduate, RICS Chartered Surveyor and Registered Building Engineer

Mr Cliff L'Aimable (Building Regulations Specialist), who has been writing construction related blog posts since 1999 and was an ex-article writer with technical publications appearing in Trinity Mirror magazines and newspapers in his column called the "Property Doctor". For building regulation advice and a fee quote for building regulations approvals click

In this blog I overview Japanese Knot Weed and the implications on property values and development.

Invasive by nature or just a mis-understood weed….?

Well let me break it down for you. Japanese Knotweed (Reynoutria japonica syn. Fallopia japonica) is a vigorous and invasive perennial weed that forms dense clumps during its growth cycle and establishment within the soils it occupies . Discovered by 18th century Dutch scientist Martinus Houttuyn

(1720–2 May 1798) , he gave it the name, Reynoutria Japonica.

The UK Environment Agency considers Japanese knotweed to be the most prevalent of four invasive knotweed plant species to be found in the United Kingdom.

• Japanese knotweed

• Dwarf knotweed

• Giant knotweed

• Bohemian knotweed (a hybrid)

The zigzag stem structure of the weed, consists of shovel-shaped leaves. White summer flowers form part of its defining characteristics. The stems can reach a height of 2.1m. In the winter, only the dark brownish-red stems are visible above the soil. It propagates via subterranean roots ( called “Rhizomes”).

In the United Kingdom, only the "female variant" of the plant exists. The roots or "creeping rootstalk" are the horizontal underground plant stems that create new shoots and root systems. This plant is considered an invasive species.

The way I see it – this plant is an eager survivor – and not quite a “Triffid -from “Day of the Triffids” !!!

Important facts

· Japanese knotweed is edible and tastes similar to rhubarb. A number of restaurants in London have it on their dessert menus.

· It thrives in sub-zero conditions, as well as in high temperature environments, including therefore on the sides of soils or stratas on the sloping sides of volcanoes.

In the United Kingdom, Japanese Knotweed has no natural antagonists to inhibit its proliferation.

• A finger nail sized rhizome fragment can produce a new plant.

• Elimination of an established growth on a property is practicably impossible. Despite attempts to destroy it with fire or acid, it is essentially indestructible.

• The Japanese make use of the herb as a popular analgesic in traditional medicine. The resulting medication is known as Itadori, which translates to "the tiger - pain reliever."

Presence in the United Kingdom

Every Japanese knotweed plant in England is descended from a single plant that was introduced by Philipp Franz Balthasar von Siebold who was a German physician, botanist, and traveller who lived from 17 February 1796 to 18 October 1866. His studies of Japanese flora and fauna and the introduction of western medicine to Japan brought him to prominence.

This plant was long established in Japan, hence the name. Von Siebold was unaware of the future consequences and damage this plant would inevitably cause on the built environment.

Japanese knotweed was first introduced to the United Kingdom as a fashionable ornamental plant for commercial sale and botanical cultivation in the late nineteenth century, and was then transported to North America.

Interestingly Lilac is a popular garden plant which is frequently mistaken for Japanese knotweed due to its spade-shaped leaves and lush green foliage. If Lilac is cut back, it will send up several invasive suckers, but that is the extent of its invasiveness.

After Von Siebold sent samples back to his native Norway, where the plant spread prolifically once it was introduced into the environment. Samples of the plant were later transported back to Kew Gardens in London.

Kew were quick to note the plant's rapid development and began selling it for use in household gardens. When it was originally identified as a non-native invasive species, it had already spread throughout the nation. Japanese knotweed has spread to nearly every region of the United Kingdom, but is currently most prevalent in Wales.

Japanese knotweed - Problems!

1. Treatment regime

The treatment and control of Japanese knotweed annually costs the economy millions of pounds. A DEFRA report suggests that a staggering £1.5 billion represents the national price tag expended on its control and eradication where necessary.

2. The Spread pattern

The rhizomes can grow to depths of up to two metres and stretch horizontally up to seven metres from the plant's visible portion.

These roots are largely responsible for the knotweed's tenacity, as they can continue to maintain their life for up to twenty years.

It just takes a small quantity of healthy root to give rise to an entirely new plant, so reviving any historical invasion. If allowed to spread or if it is disturbed, Japanese Knotweed can grow beneath pathways and buildings, causing structural damage when it discovers a weakness in the obstructing construction.

Japanese knotweed can spontaneously reappear and regrow at any time, but especially if the contaminated soil is disturbed and spread from one area into another ground location. This may occur as a result of digging or even regular household gardening, as well as extreme floods that transports fragments further afield, where they can flourish.

3. Environmental impact

Its high growth rate allows it to suffocate other species by blocking summer sunlight with a dense canopy of leaves. It also emits allelopathic compounds into the soil, which inhibit the growth of other plants. Large canes that survive the winter can obstruct water channels, increasing the probability of flooding.

4. Problems for commercial enterprises and property developers

As rhizome disruption results in new growth, this can result in significant issues for commercial or private land development. Ignoring knotweed can lead to its regeneration during or after construction, both in hard and soft landscape areas and within or worst still under the built structure of buildings.

If a developer or landowner does not do the essential due diligence and control of Japanese knotweed, they risk legal action for professional negligence, which has obvious repercussions for land designated for development. If knotweed is discovered on a construction site, all work must be halted until a comprehensive survey, treatment, or eradication is conducted. Future land purchases require the location of knotweed to be documented.

5. Restrictions on usage of amenity

Knotweed can inhibit the usage of public areas such as pathways and sports fields, and if left untreated, it can spread to nearby amenity areas, severely limiting the area's use. Due to Japanese Knotweed, home garden amenity use can be severely restricted.

Legal aspects of purchasing and selling infected land

Since 2013, when selling a home, the seller is obligated to indicate on a TA6 form - a conveyancing form for property information – whether Japanese knotweed is present on the property.

Failure to do so or making a misleading statement is a violation of the law that can result in the seller being sued for "misrepresentation."

If commentary or observations relating to knotweed is missing, or misidentified during a professional survey, this is considered "professional negligence."

Developers of a land infested with Japanese knotweed run the danger of being sued for professional negligence if they fail to do the prerequisites relating to due diligence and management measures.

The legal framework

The Antisocial Behaviour, Crime and Policing Act of 2014 was amended to cover invasive non-native species, such as Japanese knotweed.

According to the Royal Horticultural Society (RHS):

You must prevent the spread of Japanese knotweed from your property. Soil or plant material polluted with non-native and invasive species, such as Japanese knotweed, can cause ecological damage and may be categorised as hazardous waste.

• Possessing Japanese knotweed in your yard is not prohibited, and legally, you are not required to remove Japanese knotweed from your property unless it is causing a nuisance, but you can be penalised-fined for causing it to spread in the wild.

• If Japanese knotweed problems emerge in neighbouring gardens, we recommend that you speak or write directly to your neighbours (who may already be taking action to control this difficult weed). Before contacting local council to discuss enforcing the law, you should follow these preliminary measures.

• Homeowners can consider self-control using chemicals for a small, isolated cluster. However, a specialised licensed contractor should be appointed to control, and assure its eradication, and who will be able to dispose of the plant waste at licenced landfill sites once the plants have been chemically treated.

Encroachment across property boundaries

If Japanese knotweed is permitted to grow beyond the property line, this is considered intrusion. This means that if a property has Japanese knotweed growing on its land, it must make every effort to control the knotweed and prevent the spread of this invasive weed onto neighbouring properties.

Persons both ignorantly or deliberately allowing or causing encroachment, spread, onto adjoining land may be served with private nuisance claims. If the underground rhizome of the plant was determined to have spread from your property to another. Encroachment would have occured.

Private nuisance is an act or omission that interferes with, disturbs, or annoys another individual in the exercise or enjoyment of his land ownership or occupation -due to your lack of control of the invasive weed.

Other regulations

The 1981 Wildlife and Countryside Act

The Act regulates the discharge of non-native species into the wild in the United Kingdom. Section 14(2) of the Act makes it a crime to "plant or otherwise cause to grow in the wild" any of the plants listed in Schedule 9, Part II. Research the 1981, Wildlife and Countryside Act.

Waste Disposal Regulations

The Wildlife and Countryside Act regulates the disposal of Japanese knotweed and other invasive plants specified under section 14(2), schedule 9, part II. Waste containing propagules of these plants is designated as controlled waste under the Environmental Protection Act if it is removed from the site of origin.

This still applies to all knotweed material following treatment or excavation, and the trash must only be transported by vehicles registered with the Environment Agency and disposed of at specially licenced landfill sites (of which there are limited numbers in the UK).

When it is possible to retain or dispose of these wastes on-site, it must be done in accordance with Regulatory Position Statement (RPS) 178. This regulates the storage and disposal of garbage on-site, including burial, relocation, and re-use requirements.

The 1991 Environmental Protection (Duty of Care) Regulations

This Act exists to ensure that producers of trash (such as Japanese knotweed) assume responsibility for managing their waste and preventing harm to human health or the environment.

The purpose of the Act is to limit or eliminate harmful waste criminality, such as fly-tipping. The Duty of Care requires anybody who produces, imports, transports, stores, processes, or disposes of controlled waste to guarantee that it is only ever transferred to an authorised recipient. The Environmental Protection (Duty of Care) Regulations of 1991 contain more information.

Effect on the built environment

Over 2% of building sites and 1.25 % of residential homes in the United Kingdom are impacted by Japanese knotweed, equating to tens of thousands of sites. Knotweed can have a negative effect on the value of a property.

According to the Property Care Association (PCA), the Science and Technology Committee analysis reveals that the physical damage caused by Japanese Knotweed is comparable to that caused by other invasive plants and trees. Other plants and trees, however, are not subject to the same level of regulation and do not have the same effect on the sale of a property." However, the Environment Agency has requested additional research (and contacted DEFRA) to better understand the impacts of Japanese knotweed on the built and natural environments.

Control procedures

There are two primary ways of control employed in the industry: herbicide applications (often as part of a programme) and excavation. Despite the fact that numerous novel approaches, including as hot foam, steam, and electricity, are being studied by some companies, there is insufficient proof that they are as successful over the long term and may be more expensive than herbicide and excavation.

Depending on the client's objectives and land use regulations, There are a number of ground remediation techniques, including vacuum excavation, which is employed on areas where environmental considerations (trees or endangered species) or utility considerations (pipes and cables) must be taken into account.

With excavation, knotweed waste material can be left on-site if a permitted method is used, such as cell burial, or it can be totally removed and transported to a landfill that is licenced to accept knotweed waste material.

Mis-management of Japanese Knotweed incidents?

If the Knotweed infestation is left unchecked for several years, it can spread and cause a variety of problems, such as:

• Impedes amenity, land use

• Increasing change of land use costs

• Material damage to hard standing structures

• Decreases property value

• Creates legal problems, claims such as health and safety risks

The Knotweed Management Plan is essential.

The Environment Agency stated that once Japanese knotweed is discovered on private property or a commercial development site, a Knotweed Management Plan must be prepared (KMP).

The survey results will be used to create a written KMP for the property. This comprises an evaluation of the amount of the infestation and the severity of the impact on the present or anticipated future use of the property or land, which can include limits on maintenance and amenity usage activities, and in rare instances, structural damage.

The presence of knotweed does not automatically exclude obtaining a mortgage; a case-by-case approach is typically taken.

Essential proof of a valid Knotweed Management Plan.

Although it may come as a surprise to discover that the property you are attempting to sell contains knotweed, or that the property you wish to purchase does as well, there are treatments available to manage the infestation, and the Knotweed Management Plan is the key.

The Surveying Profession Response.

The "RICS Information Paper 2012" created a framework for evaluating the threat that Japanese knotweed poses to residential property. It designated four risk categories (1-4) with a 7-meter distance from structures and boundaries serving as the defining criterion. In addition to requesting an assessment of minor or substantial structural damage.

The RICS informational brief was helpful in giving a justification for lending on residential properties affected by Japanese knotweed.

Since the original information paper was released in 2012, scholarly research on Japanese knotweed has been published, influencing a review of property impact recommendations and the creation of a new Guidance Note.

The University of Leeds indicated in "a research paper by Fennell et al" in 2018 that the typical rhizome spread was 3 metres, allowing for a new defining distance measurement of 7 metres. According to the study, incidences of structural damage were only likely when knotweed was right close to vulnerable structures.

In 2018, "Jones et al. from the University of Swansea" presented findings regarding the most effective way of utilising pesticide treatment for Japanese knotweed. The eradication of an infestation was easily attainable when the correct procedures were followed.

However, it was recognised that rapid results or complete eradication were not readily achievable with herbicide-only treatment methods.

The Science and Technology Committee of the House of Commons produced a report titled "Japanese knotweed and the built environment" in 2019. The inquiry was prompted by the release of the scientific articles and the expansion of the knotweed lawsuit industry.

The Technology Committee report included a proposal for the RICS to evaluate its knotweed guidance. The report characterised the "7m rule" as a blunt tool that did not represent the most recent scientific findings. It demanded a redesigned evaluation procedure that was "considerably more sophisticated and evidence-based... to reflect the most recent thinking on the relevance of Japanese knotweed."

The result of the requested reassessment is the 2022 RICS Guidance Note "Japanese Knotweed and Residential Property."

What are the major alterations?

The RICS Guidance Note offers RICS members with an updated evaluation for conducting surveys and valuations.

The recommendation incorporates previously published studies on Japanese knotweed. Modifying the accompanying property impact analysis, which advises lenders on whether or not to implement mortgage retention.

When providing advice for non-lending purposes, a property surveyor would always recommend that the client see a specialist cleanup contractor regarding Japanese knotweed.

The contractors' trade association (the PCA Invasive Weeds Control Group) has developed a more extensive assessment of impact and remedial recommendations in their accompanying "Guidance Note on Japanese Knotweed for Professional Valuers and Surveyors."

A professional cleanup contractor's report should inform customers of the precise implications that Japanese knotweed may have on a property, including loss of free use, maintenance restrictions, development constraints, waste disposal expenses, and potential lawsuit impacts.

A specialist remediation contractor should also prescribe the appropriate remedial procedure, such as herbicide treatment or excavation, in accordance with a Knotweed Management Plan for any property afflicted by knotweed and provide an insurance-backed guarantee.

As indicated by the RICS Guidance Note management categories, always seek the services and counsel of a specialist remediation contractor, when selling or purchasing a property affected with Japanese knotweed.

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