March 15, 2019

Knot Alien Invaders!

Lois G. Paine

Invasive non-native species, of which Japanese knotweed is one, are sometimes referred to in the United Kingdom (UK) as “alien species.” They include plants introduced by accident or deliberately into a natural environment where they are not normally found––causing serious negative consequences. Japanese knotweed also has spread to the United States and is causing problems there, but the United States does not yet have legislation addressing Japanese knotweed like that found in the UK, which is briefly summarized here. A recent case has raised even higher the profile of this plant and the costs associated with it.

Japanese knotweed was introduced in the UK in the nineteenth century as an ornamental plant. It has bamboo-like hollow, smooth, purple- to green-colored stems up to 2.5 cm (1 inch) in diameter. It is now the most invasive plant in the UK, and between spring and the end of autumn grows phenomenally fast by its rhizomes (roots) and stems—up to 4 inches a day. It can grow up to 6.5 feet in height, and its rhizomes can extend 22 feet horizontally and 10 feet vertically. It can affect drains, streams, walls, and grow through tarmac. It appears to have no natural enemies in the UK. It increases the risk of riverbank erosion when it dies back in the autumn, exposing bare soil, and can create a flooding hazard if the dead stems are washed into the streams. Assuming 0.5 percent of the total land area of Britain may be affected by Japanese knotweed (and this is likely to be an underestimate), the UK government has estimated the costs of eradicating Japanese knotweed from the UK at £1.6 billion (over $2 billion).

The UK regulatory body, the Environment Agency, published a Knotweed Code of Practice in 2006, updated in 2013 (withdrawn in 2016 but used as guidance in the 2018 Court of Appeal case referred to below). The Code of Practice stated that greenhouse trials have shown that as little as 0.7 grams of rhizome material (10mm in length) can produce a new, regenerated plant within 10 days. Rhizome material may remain dormant for long periods, possibly as long as 20 years. Heat treatment cannot be relied upon to kill the knotweed completely. Current government advice states that spraying with chemicals can be an effective treatment to stop these plants from spreading. Only approved pesticides may be used, and it usually takes three years of annual spraying to be effective. Burying must be at a depth of at least two meters (over six feet) with a membrane material covering the knotweed. Permits and consents may be needed when using or disposing of these chemicals or burying knotweed, as discussed below.

The UK Council of Mortgage Lenders has published a policy at stating that the presence of Japanese knotweed may affect the valuation of a property and might be an issue for customers whose property is affected but who find it difficult to afford treatment costs. Under the policy, valuers who inspect property for mortgage purposes are instructed to report to lenders where knotweed is present and where lenders and customers therefore are likely to need professional help with remedial work. It notes that the pre-contract inquiries that prospective purchasers seek as part of the UK legal process of purchasing a property ask whether Japanese knotweed is present. The English Law Society Property Information Form TA6, which the seller of a residential property is usually required to complete, includes a specific inquiry that asks whether “the property is affected by Japanese knotweed” and, if it is, whether there is a Japanese knotweed management plan in place. The form also asks for a copy of any such plan.

In the UK, several statutes and regulations cover issues associated with Japanese knotweed. Section 14(2) of the Wildlife and Countryside Act 1981 (WCA 1981) states that “. . . if any person plants or otherwise causes to grow in the wild any plant which is included in Part II of Schedule 9, he shall be guilty of an offence.” Japanese knotweed is one of the plants listed in Schedule 9. Attempting to cut knotweed also is an offense, as this causes it to spread. Anyone convicted of an offense under section 14 may face an unlimited fine and between six months and two years’ imprisonment. Releases do not have to be deliberate or intentional to be prohibited, although section 14(3) provides a defense if the defendant can show that all reasonable steps were taken, and all due diligence was exercised to avoid committing an offense. Under the WCA 1981, licences can be granted giving exemptions from section 14. The WCA 1981 also has been amended (new section 14(4)(A) and Schedule 9A) to allow for species control agreements (SCAs) and species control orders (SCOs) relating to species including Japanese knotweed. Environmental authorities have powers to enter a voluntary SCA with an owner of premises for operations to eradicate, control, or prevent the return of a species such as knotweed. An SCO can be imposed on an owner if, for example, the owner does not comply with an SCA or agree to an SCA. A Code of Practice has been issued by the Government on SCAs and SCOs in 2017 at

Section 215 of the Town and Country Planning Act 1990 provides local authorities (local government bodies) with a discretionary power to require landowners to clean up “land adversely affecting the amenity of the neighbourhood,” which could be relevant to control of Japanese knotweed. Local authorities also have the power to undertake clean-up works themselves and to recover costs from the landowner.

On a broader geographic scale, the European Union (EU) Invasive Alien Species Regulation 2014 (1143/2014) seeks to address the problem of invasive non-native species (INNS) in a comprehensive manner across the EU and requires member states (currently including the UK) to eradicate and manage certain key INNS (including knotweed). It required member states to introduce penalties for breach of this regulation. EU regulations are directly applicable in member states without the need for further local legislation. A consultation on this regulation and how to implement it in England and Wales recently took place (, and in July 2018 the governments’ response ( was to use existing regulatory frameworks to enforce the regulation, and, where possible: (1) implement civil sanctions, and (2) create new criminal offenses enforcement to deal with severe or persistent offenses.

The most important international instrument is the Convention on Biological Diversity (CBD), which became effective in 1993. It requires contracting parties (of which there are 196, 168 signatories), as far as possible and as appropriate, to prevent the introduction of, control, or eradicate, those alien (i.e., non-native) species that threaten ecosystems, habitats, or species. The CBD has done much to provide an overarching international framework for action, including the development of a set of guiding principles. The UK and EU are parties to the CBD. The United States has signed but not ratified the CBD.

There has been a recent important UK case on tort liability for Japanese knotweed that will have far-reaching and expensive ramifications. That case is Network Rail Infrastructure Ltd v. Williams & Another, EWCA Civ 1514 (2018). The claimants both owned homes in Wales located in front of a Network Rail (NR) track. (NR owns and operates the railway infrastructure in the UK.) Behind the claimants’ houses, the railway embankment was infested with Japanese knotweed, which was estimated to have been there for 50 years. It had encroached under both properties, though there was no actual physical damage. NR had actual and constructive knowledge of the risk of damage and loss of amenity, and its intermittent treatment plan over the years was found not to be adequate or reasonable. The judge found NR liable due to the disturbance of the quiet enjoyment of both claimants, and found that because there was a risk of future damage and because of mortgage lenders’ attitudes, both claimants had a claim for a loss of amenity. He declared that the presence of knotweed on NR’s land had caused a loss of enjoyment to the claimants’ properties and awarded damages of £10,500 ($13,500) toward diminution of the property value and £5,000 ($6,500) for the cost of removing and disposing of the knotweed.

NR appealed this decision on two grounds. First, NR claimed that the mere presence of knotweed on their railway embankment and the economic loss suffered could not give rise to an actionable nuisance. NR also claimed that the finding that there was a causal link between NR’s breach of duty and the residual diminution in value of the claimants’ properties was wrong.

The Court of Appeal ruled in July 2018 that private nuisance is a violation of property rights. It could be broken down into different categories, which were examples of this violation, including nuisance by encroachment, physical injury, and interference with quiet enjoyment. However, some direct physical interference with (not necessarily damage to) the claimant’s land is always necessary.

On the other hand, the court stated that the proposition that damage is always essential for a nuisance claim must be treated with considerable caution. It stated the concept of damage here was “a highly elastic one.” The judge referred to Japanese knotweed as “a pernicious weed” and said, “Japanese knotweed and its rhizomes can fairly be described, in the sense of the decided cases, as a ‘natural hazard.’ They affect the owner’s ability fully to use and enjoy the land. They are a classic example of an interference with the amenity value of the land.” He also stated that “[t]he purpose of the tort of nuisance is not to protect the value of property as an investment or a financial asset. Its purpose is to protect the owner of land (or a person entitled to exclusive possession) in their use and enjoyment of the land as such as a facet of the right of ownership or right to exclusive possession.”

The importance of this case lies in clarifying the case law on economic loss in the tort of nuisance. The spread of the knotweed rhizomes to the claimants’ land caused sufficient change to the land to trigger a right of action in nuisance, even though the knotweed had not caused any physical damage. It had caused interference with the claimants’ quiet enjoyment of their property. So, both claimants were entitled to damages for the diminution of the value of their homes.


Lois G. Paine

Ms. Paine is a senior environmental and charity lawyer at British Telecommunications plc in London, United Kingdom. She may be reached at