Over the past few years, Japanese knotweed has gone viral in more ways than one. As Britain's most aggressive, non-native plant, Japanese Knotweed is now affecting 5% of UK homes, according to invasive plant removal specialists Environet.
Analysing the data, generated from the over 57,000 known cases across Britain, Environet has revealed the top 20 places in the UK where the weed is most prevalent.
Click or scroll on to find out if you live near a Japanese knotweed hotspot...
Japanese knotweed is a herbaceous perennial native to Japan, China and parts of Korea. It was introduced to the UK in 1825 by German botanist Philipp Franz Balthasar von Siebold, who was unaware of the devastating impact it would have.
According to the RHS, Japanese Knotweed has "bamboo-like stems which grow to 2m (6.5ft) tall”. Aggressive and fast-growing, the plant can grow up to 10cm a day and is classed as ‘controlled waste’ under the Environmental Protection Act 1990. Japanese knotweed can quickly take over areas of land, causing damage to lightweight structures, drains and even walls.
Now, let’s reveal the 20 places in England and Wales where Japanese knotweed is most pervasive. Is your home region on the list?
Through its interactive heatmap tracker Exposed™: The Japanese Knotweed Heatmap, Environet has collated almost 60,000 cases of Japanese knotweed to help it build a picture of the worst infested areas.
The information has been gathered by both the Environet team and members of the public who can report possible sightings by uploading images which are verified by the experts. The data takes into account the size of each area and calculates infestations per square kilometre (km²).
Users can enter their postcode to see the prevalence of Japanese knotweed in their area or an area they are considering buying a property in, with hotspots highlighted in yellow, orange and – in areas with the highest density of cases – red.
Some of the results may surprise you. Our list of the 20 areas reporting the most cases of Japanese knotweed begins with the Isle of Wight, where there are 0.85 incidences per square kilometre and a total of 325.
Next, in 19th position is the county of South Yorkshire, which covers areas like Rotherham, which took the 16th slot when Environet did a similar reveal in 2023, and Sheffield, which sat in 19th place last time round.
This photos shows a site in Sheffield undergoing treatment for the dreaded plant, warning people to leave the site alone as Japanese knotweed spreads very easily.
The County Borough of Conwy in North Wales, which includes the tourist town of Conwy and the remote village of Betws-Y-Coed (pictured) is next on our list.
The region reported a total number of 1,064 cases, which equates to around 0.94 cases per square kilometre, which does dampen the appeal of this pretty rural region a little.
Another Welsh locale on Environet's list, Gwynedd, comes in 17th place, with 2,514 confirmed cases of Japanese knotweed.
Caernarfon (pictured) took the 8th slot in 2023 in Environet’s ominous list with 708 infestations, and in 2020, Gwynedd was named the seventh-most affected place in Wales, with 220 infestations, highlighting significant and rapid growth in the area.
With more than one incident of Japanese knotweed per square kilometre, the West Midlands takes the next spot in our list.
Encompassing places like Coventry, Dudley, and Wolverhampton as well as the UK's second largest city, Birmingham (pictured), although there were a total of 908 cases of knotweed reported across the West Midlands in 2024 (a lot less than in Gwynedd) when you break it down into incidents per square kilometre it comes out worse.
With 1.01 reported cases per square kilometre, the county of Derbyshire matches the West Midlands in our methodology, so really this should be shared 15th place.
However, when you look at the overall cases – 2,651 – it does sound quite alarming, and as this picture of the Peak District National Park show, even Areas of Natural Beauty are not immune to the horrors of Japanese knotweed.
Another Welsh region to make our top 20 of Japanese knotweed hotspots is Monmouthshire, with 1,185 total incidences. The lush, tranquil region borders England, and here you can see knotweed in full flourish along the banks of the River Usk.
According to the local conservation group, the Wye Valley National Landscape, riverbanks are one area where the plant can be found, but its appearance is detrimental to the local ecology, altering the habitat structure, which it says has a direct impact on salmonoid fisheries by hindering conservation efforts of rivers.
In 13th position is the Channel Islands with 281 reports – the archipelago that lies in the English Channel that is best known for its beautiful beaches, wonderful wildlife, and unique culture that includes both English and French influences.
Made up of the two Crown Dependencies of the Bailiwick of Jersey and the Bailiwick of Guernsey, these isles nevertheless seem to have an issue with Japanese knotweed, as in this example found near Fort Tourgis on Alderney.
Next up is Lancashire. This scenic county currently has 4,556 known cases of the invasive species.
In 2023, Blackburn (pictured) was named as one of the worst-affected locations in Lancashire for Japanese Knotweed with 541 reported infestations of the plant, behind only Preston, which had 857.
Coming in just outside the top 10 is yet another Welsh region: Rhondda Cynon Taf. Encompassing five valleys in the south-east of the country – Rhondda Fawr, Rhondda Fach, Cynon, Taff, and Ely valleys – there were 689 cases reported here in 2024.
The valleys are known for their mining heritage, as celebrated in the Rhondda Heritage Park (pictured), but it sounds like a bit more attention must now be paid to the eradication of this invasive plant, with as many as 1.62 cases reported every square kilometre.
It’s more bad news for Wales as we enter our top 10, as with 505 Japanese knotweed cases reported in the town and community of Caerphilly, covering a relatively small area, it actually has quite a high propensity of cases, with 1.82 incidents per square kilometre.
According to news service Nation Cymru, the issue has even been found in the grounds surrounding the town’s historic castle.
Next up is the Welsh county of Neath Port Talbot, which lies to the east of Swansea. Right now, Neath is home to 944 known cases, however, the plant has been problematic here for years, with 631 reported cases in Neath 2023.
In 2018, Neath Port Talbot Council set up a Japanese knotweed treatment service to create extra jobs locally. At the time, it was thought the service would bring in around £40,000 a year, a figure that has no doubt increased substantially.
One of the UK's largest and most densely populated cities, Manchester took the 18th spot on Environet's list in 2023, with 532 confirmed cases across the city, but according to the Manchester Evening News, there were 2,807 infestations across the wider region.
This year, the situation has worsened but only slightly, with 2,846 cases across Greater Manchester, including terrible residential outbreaks such as this one captured in a garden in a corner of the city.
As a major city with a population of 9.8 million, it's perhaps no surprise that there are a lot of cases of Japanese knotweed in London – 3,592 at the last count.
While a big part of the problem is found in private gardens, often the issue is more pronounced in parks and in public spaces such as on this pathway in Leytonstone in the city’s East End.
Next, it's the country that includes the maritime city of Liverpool, in England's northwest. Coming in at number 6 on Environet's list of the top 20 places affected by Japanese knotweed, Merseyside currently has 1,611 confirmed cases.
Liverpool seems to have had a bit of a problem with the plant for a while: in 2021, Liverpool law firm Angelus Law secured a £27,500 settlement for a homeowner whose property was being encroached upon by the hardy plant, which was growing in the garden of a neighbouring property.
This borough north of Cardiff, whose main town earned the moniker Iron Capital of the World during the Industrial Revolution, has a high incident rate of Japanese knotweed with 2.57 cases found per square kilometre and 286 in total.
The most famous knotweed case in recent years involved a Mr Churchill who made a claim against Merthyr Tydfil County Borough Council regarding Japanese knotweed that he said encroached on his property from council-owned land, with the case even making it as far as the Court of Appeal.
Taking the 4th spot with 826 cases is Bridgend, a town in the south of Wales, which in 2023 ranked 10th place with 664 cases, so it looks like the issue here is spiralling and needs to be dealt with.
In early 2023, a homeowner was awarded £4,900 from Bridgend Council, after weeds from council land encroached on, and devalued his property. In addition, this image shows a disused landfill site, which was being converted into a solar wind farm. Upon surveying the site, Environet found 11 strands of Japanese knotweed. The task of removing the weeds took three men five days to complete.
Last year's worst-hit area, Swansea, drops two places to third position but this is a list in which nobody wants a podium position. With a slight increase on last year’s number of cases – from 1,350 known cases to 1,355 – it seems this coastal city hasn't improved its position by doing anything to fix the issue, it just seems other areas have got even worse.
The issue in the area is so bad that Swansea City Council has even appointed a Knotweed Officer, Sean Hathaway. This remarkable image shows Sean at a site close to Swansea Marina, which is blanketed by the weed, but it seems he still has a lot of work to do.
Straddling Gloucestershire and Somerset, Bristol lies on the south-west coast of England. In 2023, it became the third-most infested place in Britain for Japanese knotweed sightings, but in 2024 it has moved into second position with 710 incidences, which works out at 5.18 cases per square kilometre.
In 2023, Cardiff took fourth place in Environet's list with 872 cases across the region but this year a rise to 946 cases – 6.74 every square kilometre – has catapulted it into the top position.
This image from Environet highlights the extent of the problem in Cardiff. After struggling to sell his property due to Japanese knotweed, the homeowner was forced to take it off the market. A two-man team spent five days removing the weed, processing between 15 and 20 cubic metres of soil.
The best way to protect yourself and your local area from Japanese knotweed is to ensure you know how to identify it. In spring, you might notice asparagus-like shoots sprouting from the ground that grow tall with bamboo-like stems and shield-shaped leaves, while in summer, clusters of tiny cream or white flowers will be in evidence.
However, it's not always so obvious to identify. Emily Grant, Director of Environet says: “It's a perennial plant so as we head through the autumn and into winter, the above-ground growth dies back and it becomes much harder to spot.”
If you think you have spotted Japanese knotweed at home, you need to get it verified by the experts before taking any action. Invasive plant removal specialists, like Environet, can assess the situation, confirm the presence of knotweed and develop a treatment plan.
There are various ways that Japanese knotweed can be treated or removed. Herbicide treatment is normally the cheapest method but is typically regarded as a form of control rather than removal, due to the risk of regrowth.
Excavation and disposal at landfill (also known as Dig & Dump) will – hopefully – eradicate the plant for good, but is a lot more expensive. The overall cost of treatment depends on many factors, but Environet says the average cost of a Japanese knotweed management plan in the UK is £5,000.
According to Environet, tackling knotweed costs the UK economy nearly £250 million a year and homeowners are one of the groups most at risk from both damage to their property and being sued by neighbours.
Typically, affected properties are worth between 5% and 10% less than their unaffected counterparts due to several factors, including the cost of treatment or removal and the ‘knotweed stigma’ deterring buyers.
However, Emily Grant says: “With professional assistance knotweed can be dealt with and the value of a property largely restored.”
As well as affecting property prices, the presence of an invasive plant like Japanese knotweed can also make the process of obtaining a mortgage very complicated.
Getting a mortgage with Japanese knotweed on the property isn’t impossible, but most lending companies will insist that you have a formal management plan with an insurance-backed guarantee in place. Additionally, if you live in a knotweed hotspot, then some mortgage companies will require a dedicated Japanese knotweed survey before granting you a loan.
It isn't just house buyers who need to be cautious, either. Since 2013, anyone selling a property has been required to check their garden for Japanese knotweed and declare its presence on the TA6 Property Information form. Sellers “must now be certain there is no knotweed present, including rhizome beneath the ground or within three metres of the boundary, even if there are no visible signs above ground”, explains OnTheMarket.
Sellers who aren't aware of any infestation are still advised to respond “Not known” to the Japanese knotweed question on the TA6 form, leaving the buyer to "undertake their own enquiries." If knotweed is present, the property may be harder to sell, but not impossible.
In 2018, a homeowner in Streatham, London, was sued for £32,000, for not declaring that his property contained Japanese knotweed, before selling it. You have been warned!
You can join the fight against Japanese knotweed by reporting any sightings on the Environet website. Simply visit the Exposed™: The Japanese Knotweed Heatmap and click “Add Sighting”. Zoom in on the satellite view and click the central position of the knotweed location. You can also add sightings by uploading a photo, which will then be verified by the experts. Wherever possible, Environet verifies all new sightings to keep the heatmap current and accurate. Together, we can track and defeat Japanese knotweed!
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