Japanese Knotweed

What are ‘Invasive Plant Species’

Japanese knotweed, Giant hogweed, Himalayan balsam, Rhododendrons, and New Zealand pygmyweed are classified as 'invasive plant species' that can cause issues for both land and structures. Additionally, 'injurious weeds' like some ragworts, thistles, and docks are under strict control. Japanese knotweed, in particular, has been problematic due to its lack of natural predators in the UK. Recently, homebuyers have expressed concerns about the impact of knotweed infestations on properties they are considering purchasing.

What is Japanese Knotweed (‘Fallopia japonica’)

Japanese knotweed is an invasive, non-native plant primarily found in urban areas, posing significant challenges for property development. This weed can regrow from rhizomes, emerging through gaps in conservatory flooring and patios. It is also commonly seen on the peripheries of farmland, railway properties, and areas where garden waste or topsoil has been discarded. Capable of growing up to 10 cm daily from April to October, even a tiny fragment of root or stem left in the soil can lead to reinfestation. The removal and treatment costs for Japanese knotweed are considerable, with eradication from construction sites potentially exceeding £1,000 per square meter, and the disposal of 'contaminated' topsoil can be costly due to its classification as controlled waste.

A change in approach to knotweed infestation

Recent research suggests that knotweed poses a lower risk of property damage compared to many trees or woody shrubs, and a typical spread distance of 3 meters is more accurate than the previously believed 7 meters. The DEFRA report from 2020 noted that public and media perceptions are often exaggerated relative to the actual physical risk to property. Furthermore, an RICS draft guidance note from August 2021 states that knotweed infrequently causes structural damage to buildings and cannot inflict damage in the same manner as trees or woody plants like buddleia.

Treatments and solutions

The RICS guidance notes state that while removing trees that cause damage is relatively straightforward, treating knotweed is costly, disruptive, and may require several years of management. Addressing knotweed on adjacent properties is challenging due to the lack of rights to enforce treatment on neighboring land. Furthermore, resolving knotweed issues in blocks of flats or conversions can be complex, as the entire block may suffer if the responsible party fails to implement an effective strategy.

There is seldom a quick solution; most remedies involve a mix of removal, potential barrier creation, and a long-term herbicide treatment plan. If the soil isn't treated over an extended period, the plant may resurface, as its roots and rhizomes are highly resistant to treatments.

Management, not eradication, is now the aim

Completely eradicating all traces of knotweed is often not feasible, and a long-term management plan may be a more practical approach. Frequently, the issue is the negative impact caused by the presence of knotweed, with any physical damage being of secondary concern. The RICS report notes that the public's exaggerated perception of the problem greatly exceeds the actual costs of remediation. Regrettably, even after effective remediation of knotweed, properties may continue to suffer from a 'stigma' for several years.

Caveat Emptor and specialist surveys

A seller is not obligated to disclose to a buyer if the property is infested with knotweed. Standard residential conveyancing inquiries do include a question about Japanese knotweed, but it's not compulsory for the seller to respond. However, sellers are liable for misrepresentation if they deny the presence of knotweed knowing it exists. Therefore, sellers may choose not to answer this question if in doubt, leaving buyers to conduct their own inspections and surveys.

If there's any uncertainty about the presence of an invasive species on the property, buyers should arrange for a survey by a qualified residential surveyor or a specialist invasive species consultant.

Criminal offences?

Having the plant on your land is not a criminal offence. However, the Wildlife & Countryside Act 1981 makes it an offence to plant or cause certain plants, including Japanese Knotweed, to grow 'in the wild'. Penalties can reach up to £5,000 and two years in prison. A valid defence is demonstrating that all reasonable steps were taken to prevent this. DEFRA expects landowners to take reasonable measures to contain the plants within their property to prevent them from spreading into the wider environment beyond the owner or occupier’s control. Negligent or reckless actions, such as the improper disposal of garden waste leading to the establishment of an invasive species in the wild, would be considered an offence.

Enforcement Powers

The Infrastructure Act 2015 enables environmental authorities to establish Species Control Agreements (SCAs) with landowners, who in turn agree to manage invasive species. Should the landowner fail to comply, a Species Control Order (SCO) may be issued, mandating the control of the species. Violating the SCO can result in up to 51 weeks of imprisonment and/or an unlimited fine. Additionally, local authorities have the authority under s.215 of the Town & Country Planning Act 1990 and the Anti-Social Behaviour, Crime and Policing Act 2014 to demand the eradication or management of invasive species like knotweed.

Liability to others

The landowner is liable to their neighbors under common law if knotweed spreads onto neighboring property. Typically, building insurance does not cover damage caused by knotweed.

Lenders’ attitudes

You may not be able to sell a property affected by an invasive species if a buyer cannot secure a mortgage due to contamination. UK Finance (previously the Council of Mortgage Lenders or CML) has indicated that the presence of knotweed should be reported on a residential valuation report. Lenders have their own policies regarding this issue. The presence of knotweed is often one of several factors considered by the lender, with the severity level being a key consideration. If mortgage lenders decide to lend on a property with knotweed, they typically require a documented eradication and treatment management plan as a lending condition, along with an insurance-backed guarantee.

Further information

The Invasive Non-Native Specialists Association (INNSA) website offers comprehensive information on invasive plant species. Additionally, the RICS, Environment Agency, DEFRA, The Japanese Knotweed Alliance, and GOV.UK websites provide valuable guidance on these issues. If you suspect that a plant in your garden may be knotweed, the GB Non-Native Species Secretariat website has useful photographs and information that can help with identification.

As always, if you have any questions about this, please give us a call.