Untangling Japanese knotweed

Published 02/10/2019   |   Last Updated 27/05/2021   |   Reading Time minutes

Japanese knotweed (Fallopia japonica) is a well-known invasive plant. It is very difficult to control once established, outcompetes native species and causes damage to property.

Introduced as an ornamental plant in the mid-19th century, it is now found throughout Wales. It is most common on sites disturbed by human activity, such as railway lines, and is particularly problematic in residential areas.

This article tells you all you need to know about knotweed in Wales and what to do if you find it on your land.

Why is knotweed a concern?

Knotweed is an Invasive Non-Native Species (INNS), meaning that it has been introduced from another part of the world and can spread rapidly causing a number of negative effects. It is very difficult to eradicate; even very small fragments of knotweed stem can give rise to new plants.

The risks and impact of knotweed include:

  • rapid growth (up to 4cm per day);
  • native vegetation dies out underneath it due to a lack of light and water;
  • it leaves bare areas along riverbanks when the plant dies back in winter, which are more susceptible to erosion;
  • it damages structures by growing through weaknesses in asphalt and concrete; and
  • it affects property values and the ability to get a mortgage.

According to Natural Resources Wales (NRW), knotweed costs the British economy around £166 million a year.

Information on the known distribution of knotweed is provided on the National Biodiversity Network Atlas Wales INNS portal.

How does knotweed affect property value and mortgages?

The media is awash with stories of knotweed impacting on house value and mortgages. It can crack tarmac, block drains, undermine foundations and invade homes. The BBC has reported that its presence can be enough to cut a property's value by up to 20%.

However, in early 2019, the House of Commons Science and Technology Committee undertook an inquiry into knotweed and the built environment and concluded that:

A significant industry is built around controlling Japanese knotweed, but we were told that mortgage lenders in other countries do not treat the plant with the same degree of caution. This gives us reason to believe that the UK has taken an overly cautious approach to this plant, and that a more measured and evidence-based approach is needed to ensure that the impact is proportionate to the physical effects of the plant in the built environment…

In response to the inquiry, the UK Government committed to commissioning a study on international approaches and report by the end of 2019.

The Royal Institute of Chartered Surveyors (RICS) has set out a framework (PDF 3.5MB) to help lenders judge the severity of a knotweed problem.

What is being done to control knotweed?

The Great Britain Invasive Non-native Species Strategy 2015-2020 is a joint strategy by the UK, Welsh and Scottish Governments. The Welsh Government highlights that the strategy aims to minimise the risk of INNS by encouraging cooperation between governments, land managers and the public.

The Wales Resilient Ecological Network (WaREN) is a new approach for tackling INNS in Wales. The Wales Biodiversity Partnership (WBP) INNS group (which provides advice and support to the Welsh Government) states that WaREN will increase cooperation between public, private and third sector stakeholders. WaREN’s initial aims are to develop a strategy for tackling INNS collaboratively, scope a toolkit to help evidence-based decision making and to develop a mechanism for identifying funding streams.

Local groups tackling knotweed include the Dee Invasive Non-Native Species Project. This project is a catchment-wide partnership initiative which aims to co-ordinate the control and monitoring of INNS within the Dee catchment to ensure a joined-up approach.

What can you do if you have knotweed on your property?

Many local authorities provide online advice on knotweed. For example, Rhondda Cynon Taf has prepared an information guide on controlling knotweed. Some local authorities, such as Neath Port Talbot and Swansea, also offer a treatment service for landowners.

NRW also has a web-page which includes further information on treatment and control. This also highlights ways for the public to record sightings of knotweed, using free apps, to improve the understanding of this species in Wales through citizen science.

The Welsh Government has also produced a range of guidance documents on knotweed and other INNS.

Depending on the situation, you may wish to seek legal advice if another party is involved. It is not an offence for a landowner to have knotweed on their land. There is also no legal requirement to control or eradicate knotweed or to report its presence. However, the following may apply:

Further reading

The Welsh Government has published the following guidance on knotweed:

Article by Emily Williams, Senedd Research, National Assembly for Wales