Japanese Knotweed
Risk Data for Every UK Area

Japanese knotweed (Fallopia japonica) is an invasive plant species originally from East Asia. It was introduced to the UK in the mid-19th century as an ornamental plant. It grows rapidly — up to 10cm per day in summer — and its root system (rhizomes) can extend 3 metres deep and 7 metres from the visible plant, causing structural damage to buildings, drains, and hard surfaces.

Yes. Japanese knotweed can reduce a property's value by 5% to 15% or more, depending on the severity of the infestation and its proximity to the building. Some mortgage lenders will refuse to lend on properties with untreated knotweed. However, with a professional treatment plan and insurance-backed guarantee (IBG), many lenders will consider the property.

It is not illegal to have Japanese knotweed on your land. However, under the Wildlife and Countryside Act 1981, it is an offence to plant or cause it to grow in the wild. If knotweed spreads from your property to a neighbour's land, you could face a private nuisance claim. Under the Anti-social Behaviour, Crime and Policing Act 2014, councils can issue Community Protection Notices requiring landowners to control it.

Herbicide treatment typically costs £2,000 to £5,000 and takes 3 to 5 years to complete. Excavation and removal costs £5,000 to £20,000+ depending on the volume of contaminated soil. The right approach depends on the size of the infestation, the urgency (e.g., if you're selling), and whether an insurance-backed guarantee is needed for mortgage purposes.

South Wales, the West Midlands, Greater Manchester, West Yorkshire, and South West England consistently have the highest knotweed densities. Industrial heritage, canal networks, Victorian housing, and river corridors are the primary factors. Our county-by-county pages provide detailed local risk assessments.