Japanese knotweed: 7 plant points to note

A report by Environet UK at the end of 2021 discovered that Japanese knotweed – a non-native and aggressive weed – had knocked an estimated £11.8 billion off the value of UK property. While it’s lush leaves and dainty white flowers may look attractive, its potential to scupper a transaction is something buyers and sellers need to be aware of.

Despite its bad reputation, identification and successful treatment of Japanese knotweed almost always result in favourable outcomes for sellers. Here are the 7 most important points you need to know when it comes to the plant:-

1. Even though Japanese knotweed is a well-known horticultural pest, only around 4% of UK properties actually have a problem with the plant. It is an invasive species that is classed as a weed but it’s not illegal to let it grow in your garden.

2. When you sell a property affected by Japanese knotweed, this must – by law – be disclosed on the Property Information Form (TA6). Disclosure also pertains to Japanese knotweed that has spread from a neighbouring property or from surrounding land – any plant within 7 metres of the property’s boundary must be mentioned.

3. Some lenders won’t approve a mortgage on a property where there is Japanese knotweed as it represents a threat to the structure of the building. Its roots and rhizomes can damage drainage systems, foundations and walls – harming a home’s current and future value, and potentially making it unsellable in the future.

4. If you suspect Japanese knotweed, you will need to instruct a Property Care Association (PCA) approved surveyor, who will assess the plant and its posed risk. They will grade the problem using a categorisation system set out by the Royal Institution of Chartered Surveyors – a system widely used by mortgage lenders to assess risk.

5. Eradicating Japanese knotweed needs the involvement of a specialist company. The plant’s rhizomes are buried very deep in the ground and simply pulling out what’s above ground is very rarely enough – the stems merely snap and the plant regrows quickly, sprouting up to a metre a week in spring. There are also strict laws governing the disposal of Japanese knotweed, set out by The Environment Agency.

6. Japanese knotweed doesn’t always have a detrimental effect on a property’s value. Reports suggest the plant can reduce a home’s value by around 5% but in cases where there has been a successful insurance-backed treatment (see below), the full market sales value is often achieved.

7. It is essential that any eradication work is carried out by a specialist Japanese knotweed contractor that offers an insurance-backed treatment plan accepted by mortgage lenders (also known as knotweed IBG, Japanese knotweed indemnity or knotweed insurance-backed warranty). With this in place, mortgages on properties that have Japanese knotweed are usually granted, although the lender will usually want the treatment finished before completion takes place, and it may also require a larger deposit.

If you have any questions about Japanese knotweed – as a buyer or a seller – please call us for guidance.