UK JAPANESE KNOTWEED CASES

29,536 Confirmed Cases Analysed – See How Your Area Is Affected

Japanese Knotweed was originally imported into Britain from Japan back in the nineteenth century, when engineers thought it would stabilise and beautify railway embankments.

Growth in confirmed cases

There are areas of the UK where knotweed is known to be more prevalent due to its introduction on railway lines and through industrial activity.

Analysis of the 29,536 confirmed cases in the UK show the regions with the largest growth in live cases over the last 5 years:

County

Total Confirmed Live Cases

5 Year Change

South Yorkshire

1,111

+77.19%

Hampshire

123

+73.24%

West Sussex

155

+72.22%

Kent

217

+64.39%

Powys

923

+61.93%

North Yorkshire

215

+58.09%

Cardiff

280

+53.01%

West Yorkshire

1,762

+52.82%

Surrey

200

+49.25%

Shropshire

146

+47.47%

National Average

29,536

+27.91%

Based on our analysis of NBN Atlas data, there are a further 19,702 unconfirmed cases of Japanese knotweed, with thousands more in the UK likely to remain unreported.

We have produced a range of maps and charts showing the distribution of confirmed Japanese Knotweed cases across the UK:

Japanese Knotweed survey

On 27/07/2021 we carried out a YouGov survey of 907 people.

Every respondent was a homeowner based in the UK. Specifically, they either owned their own home outright, with a mortgage or through a shared ownership scheme.

Here are the top-level findings from our survey:

  • Four in five would walk away from buying a property affected by knotweed.
  • Of those who would consider purchasing a property with a known knotweed invasion, almost two-thirds of buyers would expect at least a 5 – 10% discount on the expected sale price.
  • One in twelve had never even heard of Japanese Knotweed.
  • Nearly half (47%) didn’t recognise a knotweed plant from our line-up of plant images.

Please find the full data from our survey below.

Identifying Japanese Knotweed

Shoots

Distinctive red and green shoots are likely to emerge from the ground in Spring.

Lobed Leaves

The leaves are smooth and green in colour, with a ‘spade’ or ‘love heart’ shape that is quite recognisable.

White Flowers

During late summer the plant will usually bloom with long clusters of cream-coloured flowers.

Record keeping

There has been a sizeable drop in UK record-taking for Japanese Knotweed over the last 18 months, likely caused by the impacts of Covid-19.

Records provide the opportunity for evidence-based decision making on invasive plant and wildlife species in the UK.

Record taking is vitally important to the eco-system as it helps with conservation as well as the planning and management of invasive species such as Japanese Knotweed.

We would like to take this opportunity to encourage Fallopia Japonica (Japanese Knotweed) record submission. If you know of any cases in your local area, please follow these steps:

  1. Locate your Local Environmental Records Centre (LERC) using the ALERC Finder tool.
  2. Submit your record to your local ecology organisation.
  3. If you are unable to locate your local organisation, submit your record(s) via iRecord or iNaturalistUK.

About this data & our methodology

All records are used with written permission or under creative commons. Licenced records include CC-BY, CC-BY-NC, CC0 and OGL.

We wish to thank and acknowledge NBN Atlas and the 49 data providers without whom, this map would not have been possible. See a full list of data providers below.

We only used ‘Confirmed Cases’ in our analysis. In the interests of accuracy, ‘Unconfirmed Cases’ were not used.

Records with an end date are classed as ‘Resolved Cases’ – where knotweed has been successfully treated. These are included in our interactive map and labelled as such.

Data Providers

In our interactive map, the owners of each record are listed as the ‘Rightsholder’.

In no particular order, we wish to thank each of the following organisations, without whom this map would not have been possible:

NBN Atlas, West Wales Biodiversity Information Centre, South East Wales Biodiversity Records Centre, BSBI, BIS for Powys & Brecon Beacons National Park, Scotland’s Environment Web and Biological Records Centre, Environment Agency, Cofnod – North Wales Environmental Information Service, Central Scotland Green Network Trust, Nottinghamshire Biological and Geological Records Centre, The Wildlife Information Centre, Merseyside BioBank, Thames Valley Environmental Records Centre, Yorkshire Wildlife Trust, Leicestershire and Rutland Wildlife Trust, CEDaR and Biological Records Centre, The National Plant Monitoring Scheme partnership, National Trust for Scotland, Sheffield and Rotherham, Wildlife Trust, North Devon AONB, Fife Nature Records Centre, NatureScot, Derbyshire Biological Records Centre, North East Scotland Biological Records Centre, Suffolk Biodiversity Information Service, Derbyshire Wildlife Trust, Outer Hebrides Biological Recording, Essex Wildlife Trust Biological Records Centre, Caring for God’s Acre, The British Association for Shooting and Conservation, Royal Botanic Garden Edinburgh, Botanical Society of Britain and Ireland and Biological Records Centre, Natural Apptitude and Biological Records Centre, Steve Woodward, Leicestershire and Rutland Environmental Records Centre, City of Edinburgh Council, Natural England, Severn Trent Water, Charnwood Borough Council, Greater Manchester Ecology Unit, Rotherham Biological Records Centre, Cumbria Biodiversity Data Centre, Manx Biological Recording Partnership, Norfolk Biodiversity Information Service, Isle of Wight Local Records Centre, Staffordshire Ecological Record, Dorset Environmental Records Centre, Hertfordshire Natural History Society Flora Group, Environmental Records Information Centre North East, Cambridgeshire & Peterborough Environmental Records Centre, Bristol Regional Environmental Records Centre.

These records remain the property of their respective rightsholders.

Survey data

1. Would you purchase a property affected by Japanese Knotweed?

  1. Yes – 103 (11%)
  2. No – 728 (80%)
  3. I don’t know what Japanese Knotweed is – 76 (8%)

2. Those who answered ‘Yes’ – what reduction in sale price would you expect for the affected property?

  1. 0 – 5% – 33 (32%)
  2. 5 – 10% – 26 (25%)
  3. 10 – 15% – 13 (13%)
  4. 15 – 20% – 19 (18%)
  5. 20% or more – 12 (12%)

3. Please select the plant you believe to be Japanese Knotweed from the images below (5 images were shown)

  1. Himalayan Honeysuckle – 120 (13%)
  2. Japanese Knotweed – 482 (53%)
  3. Houttuynia Cordata – 174 (19%)
  4. Mountain Fleece – 84 (9%)
  5. Impatiens Glandulifera – 47 (5%)

We are happy for external websites and publications to use these results, provided they credit Horticulture.co.uk as the source.