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Property transactions and Japanese Knotweed

Japanese Knotweed is an invasive species of plant which can cause damage to properties if left untreated, particularly with drains and other buried services, paths and driveways, boundary/retaining walls, outbuildings, conservatories and gardens.

Your legal responsibilities for invasive plants and injurious weeds

If you have invasive plants or injurious weeds on your premises, you have a responsibility to prevent them spreading into the wild or causing a nuisance. You must not plant or otherwise cause to grow in the wild any plant prescribed under the Wildlife and Countryside Act 1981.

Identifying Japanese Knotweed

Japanese knotweed begins to grow in early spring and can grow in any type of soil, no matter how poor. It can grow as much as 20 centimetres per day, and can reach a height of 1.5 metres by May and 3 metres by June. It does not produce viable seeds in the UK, but instead spreads through rhizome (underground root-like stem) fragments and cut stems. How to detect Japanese knotweed:

  • produces fleshy red tinged shoots when it first breaks through the ground;
  • has large, heart of spade-shaped green leaves;
  • has leaves arranged in a zig-zag pattern along the stem;
  • has a hollow stem, like bamboo;
  • can form dense clumps that be several metres deep;
  • produces clusters of cream flowers towards the end of July; and
  • dies back between September and November, leaving brown stems.

If Japanese knotweed encroaches on my land, what is my neighbour’s responsibility?

In the unfortunate instance that a neighbouring property is responsible for Japanese knotweed encroaching onto your property, then a Species Control Agreement (SCA) can be entered into between all persons involved. Failure of a responsible party to recognise a SCA can result in a Species Control Order (SCO) being issued under the Non-Native Specis Code of Practice.
Due to the destructive nature of the invasive weed, Japanese knotweed is covered by additional legislation under the Anti-Social Behaviour Act. This legislation focuses on residential areas and anyone ignoring orders to control their knotweed problem can be charged and receive:

  • on the spot penalties of up to £100;
  • criminal prosecution and fines of up to £2,500; and
  • Companies and businesses can also be fined up to £20,000.

Effective treatment of Japanese Knotweed

Once it is established, getting rid of Japanese Knotweed can be challenging. According to the Royal Horticultural Society, ‘eradication requires steely determination’. Below we have set out some treatment options and evaluate their suitability for use in the commercial and residential setting.

1. Excavation of the plant and it’s roots

Knotweed-infested soils can be excavated and removed to an off-site, appropriately licensed, waste-management facility. The volume of excavated soils can extend to 3m vertically and 7m horizontally from the above-ground growth, resulting in large volumes of waste soil. Disposal costs range from £25 to £50 per tonne (not including landfill tax), with the result that excavation of even a relatively small Knotweed infestation can cost several thousand pounds in waste charges alone. Typically, off-site disposal can result in total treatment costs in excess of £10,000.

However, it is possible to reduce the volume of Knotweed-infested waste soils by segregating rhizomes and crowns. There are a variety of picking, sorting and screening techniques that can achieve this.

2. On-site burial and/or encapsulation with membranes

Knotweed can be excavated and then buried on-site, but unless it can be covered with 5m or more of overburden, a specialist root barrier membrane should be installed to fully or partially encapsulate the Knotweed-bound soil, to prevent any regrowth. A root barrier membrane can also be used to encapsulate Knotweed where space does not allow burial at all.

One on-site burial technique is the use of vertical barriers to prevent Knotweed crossing boundaries. This is a popular approach if adjoining landowners are not co-operating with a cross-boundary programme to treat Knotweed infestation. However, vigorous Knotweed growths can often breach vertical root barrier membranes unless they are several metres deep and this should not therefore be relied upon as an effective treatment on its own.

3. Biological control

Biological control involves the introduction of a ‘pest’ species that will attack and control the target ‘host’ species (in this case, the Knotweed). It is effectively a grazing system, whereby the growth of the Knotweed is controlled to a level that keeps it in check. Biological control does not result in the death of the Knotweed, as this would be counterproductive to the pest species that feeds on it.

A trial is currently underway using a Japanese sap-sucking insect, which could prove an effective biological control agent for Japanese Knotweed. However, it is unlikely that this will change current legislation on the issue.

4. Chemical control

Chemical control is the application of specialised herbicides to Knotweed plants over a period of several growing seasons. This is often the cheapest treatment option costing between £3,000 and £5,000 in total for a small commercial building or a typical three-bedroom semi-detached house. However, this method can take more than three years to be effective.

Where space is limited and property boundaries are closely located, planned and managed chemical control is the only realistic option for most properties in terms of practicality, cost and the need to satisfy lender requirements.

Impact on value

By quantifying the likely cost of treatment and any necessary repairs in this way, the impact of Japanese Knotweed can be taken into account in the valuation process and reflected in the same way as any other defect or item of disrepair. If a professional survey/report shows that Japanese Knotweed is unlikely to cause serious structural damage to buildings and that it can be controlled in a domestic context without incurring prohibitive costs, it can start to be considered as just one of the many factors to be taken into account when preparing a valuation.

If treatment is deemed necessary in cases where Knotweed is present in the grounds but no damage has been caused to the property itself, the sole expense may simply be the cost of the treatment. In some circumstances, this may have no adverse affect on value.

Lawyer’s responsibility?

Property lawyers must ensure that they carry out adequate due diligence regarding knotweed. Its presence can adversely affect the value, marketability and insurability of land and buildings. There is no blanket policy from lenders which prevents them from lending on properties that have knotweed, although the difficulty in treating it has led to some historical reluctance to lend.

Buildings insurance policies generally do not cover damage and problems caused by knotweed. Where work is done to eradicate knotweed, it is worth considering whether to use a specialist company that provides a guarantee of ongoing treatment cover if the knotweed regrows or the original work was faulty. This guarantee may be backed by insurance, and so can offer protection if the original company has ceased to trade. Many of these products have been designed to meet the demands of insurance companies, banks and building societies.

The normal rule of caveat emptor (buyer beware) means that, where property is sold or let, the seller or landlord is not under a duty to disclose any information about the physical condition of the property. The buyer or tenant will usually raise pre-contract enquiries to establish more information using the standard commercial pre-contract enquiries (CPSE.1 (version 3.4) General pre-contract enquiries for all commercial property transactions).

  • Enquiry 8.1(d) asks for details of rising damp, rot, any fungal or other infection or any infestation.
  • Enquiry 15.4(b) asks for details of hazardous substances or contaminative (or potentially contaminative) material.
  • Enquiry 15.7 asks for details of any environmental problems relating to the property.

Arguably, enquiries 8.1(d) and 15.7 apply to knotweed not least because the Environment Agencies’ knotweed code of practice refers to knotweed infestations and so approaches it as an environmental problem. It is unclear whether knotweed is a contaminative, or potentially contaminative, material.

If the buyer is particularly keen to enquire about knotweed, it should raise a supplementary enquiry: “Please confirm that there is no Japanese knotweed on the property”. The buyer could extend the enquiry to request confirmation of the absence of any invasive non-native plants (“INNS”) (including knotweed). The buyer may also want to ask whether the seller is aware of the presence of knotweed (or other INNS) on adjoining land.

It is unlikely that the seller will give a definitive answer. If the seller does not occupy and has not inspected the property it is likely to reply “The buyer should rely on its own inspection and survey”. If the seller replies “Not so far as the seller is aware” without making reasonable efforts to check the position, it may be held liable for misrepresentation if it subsequently transpires that knotweed was present on the property at the date of that answer. This will not always be the case. The seller could potentially reduce this risk by a statement such as “not so far as the seller is aware but it has not inspected the property since [ ] and gives no warranty as to the absence of knotweed. The buyer should make its own enquires and inspect”.

Alternatively, it may be that the seller has obtained a knotweed guarantee from a specialist company, the benefit of which can be assigned to the purchaser on the sale of the property, usually for a small administration fee, in which case the seller should consider revealing the existence of the guarantee.

If you would like any more information in relation to this article then please feel free to contact me via email:

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