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 – that's one occurrence in every 3km². To help Brits fight back, the company created an interactive online heatmap tracker (pictured), designed to inform homeowners and house hunters of the local presence of knotweed.

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. Read on to find out if you live near a Japanese knotweed hotspot...

But first, what is Japanese knotweed? A herbaceous perennial, it's 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 canes which grow to 2.1m (7ft) tall”. These canes have characteristic purple flecks while the leaves are “heart or shovel-shaped”. Aggressive and fast-growing, the plant can grow up to 10cm a day. 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 the UK where Japanese knotweed is most pervasive. Is your city or town on the list? 

In 20th place is the Welsh town of Aberystwyth in Ceredigion. Currently, it's estimated that the beautiful coastal region is home to 467 cases of Japanese knotweed. In fact, South Wales is among the worst affected areas in the UK for the invasive plant, with four other places in the region named in the top 20. Read on to find out more...

In 19th place is Sheffield, South Yorkshire. According to Environet's heatmap tracker, there are currently 517 confirmed cases in the city. This figure is shocking when you consider that last year Sheffield was in 10th position (alongside Nottingham) with just 225 infestations within a four-kilometre radius of the city. 

This image shows a site in Sheffield that was covered in Japanese knotweed. Environet removed 756 cubic metres of infested soil using a track-mounted excavator, then processed it in its Xtract™ machine, which segregates the stems from the soil so that the clean, processed earth can be re-used.

One of the UK's largest and most densely populated cities, Manchester took the 18th spot on Environet's list, with 532 confirmed cases across the city. According to the Manchester Evening News, there have been more than 2,800 infestations across the region of Greater Manchester in the past year and even the urban heart of the city isn't exempt.

This image was taken by Environet prior to tackling a bad case in a residential corner of Manchester. 

Next up is Blackburn in Lancashire. This industrial town, and the surrounding authority of Blackburn and Darwen, currently has 541 known cases. According to the Lancashire Telegraph, Blackburn had 407 cases of Japanese knotweed last year, when Environet revealed its 2022 hotspot list, which could be reassuring to homeowners in the area, since the spread seems to be fairly slow. 

16th place goes to another South Yorkshire locale, Rotherham. This industrial town has 543 confirmed cases right now, which is 237 more cases than this time last year. In 2022, Environet named Rotherham as the eighth-most infested part of the UK, with 306 confirmed sightings. The Yorkshire Post suggests that of the almost 60,000 known cases of Japanese knotweed in the UK, more than 3% (2,000 cases) are in Yorkshire.

The beautiful, remote village of Betws-Y-Coed in Conwy, Wales, landed at 15th position, with 543 confirmed instances of knotweed. This figure is even more shocking when you consider that the village is home to only 564 people, which means there are almost as many cases of the weed as there are residents.

Next up is Chesterfield in Derbyshire. Environet revealed that the quaint market town has 563 cases of Japanese knotweed. Amazingly, the town stretches just 10 square miles, making the statistic even more surprising. According to the Derbyshire Times, the village of Brimington is the area with the most cases – 251 to be precise. 

In 13th place is Dolgellau, a town in Gwynedd, north-west Wales. Currently, the town has 564 confirmed sightings of knotweed. In fact, the problem is so extreme in the area that a local man has quit his job and set up an eradication service, to help residents fend off the destructive plant.

Next, it's the maritime city of Liverpool, in England's northwest. Coming in at number 12 on Environet's list of the top 20 places affected by Japanese knotweed, Liverpool currently has 604 confirmed cases. Liverpool didn't feature in the top 10 in 2022, but the weed has clearly been an issue 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.

Just outside the top 10, in 11th place, is the Welsh market town of Neath, which lies to the east of the major city, Swansea. Right now, Neath is home to 631 known cases, however, the plant has been problematic here for years.

In fact, 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. Now, let's explore the top 10 most affected parts of the UK...

Taking the 10th spot with 664 cases is Bridgend, a town in the south of Wales. The cities of Nottingham and Sheffield were in 10th place in 2022, with 225 cases apiece, highlighting how many more sightings of knotweed are now in the UK.

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 weed took three men five days to complete.

Llanelli in Carmarthenshire, Wales, finds itself in ninth position with 706 identified knotweed sites. In 2022, Llanelli came in at number six with 389 cases, highlighting an 82% increase in cases and therefore the speed with which the weed has spread across the town in a single year.

Another Welsh locale on Environet's list, Caernarfon in Gwynedd comes in eighth place, with 708 confirmed cases of Japanese knotweed. New to the top 10, this pretty royal town has a population of around 9,827 people, meaning almost one in 7 (or 7%) are affected by the plant. 

In 2020, Gwynedd was named the seventh-most affected place in Wales, with 220 infestations, highlighting significant and rapid growth.

At number seven is the city of Derby, with 711 cases. The Derby Telegraph confirmed that there have been "178 occurrences within four kilometres of Alvaston and Allenton, 167 occurrences around Breadsall and 120 within four kilometres of Little Eaton." There have also been 83 occurrences within a four-kilometre radius of Belper, one of Derby's most attractive and sought-after residential spots.

The East Midlands city of Nottingham lands in sixth place, with 803 known sightings of Japanese knotweed. Last year, the city came in joint-tenth place, with just 225 cases, revealing the extent of the growth across the city with an enormous 257% increase.

This image from Environet shows the weed overtaking a local resident's backyard, encroaching on their front door.

We've reached the top five! Next up, it's Preston in Lancashire, which currently has 857 known cases of Japanese knotweed. Preston wasn't included in the 2022 top 10, showing a huge increase in cases across the city. Three other Lancashire spots have significant problems with the weed, including Blackburn, which we've already covered, Chorley with 365 cases and Burnley with 310.

In 2022, Cardiff was named the seventh-most affected location in the UK, with 361 known sightings of the invasive plant. This year, the Welsh capital takes fourth place and now has 872 cases across the region.

This image from Environet highlights the extent of the problem. 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. 

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 with 986 cases. The figure is more than double the number recorded in 2022. Last year, Bristol came second on the list with 475 infestations. Central Bristol is the most-affected place, with 318 cases, followed by Frome Valley with 105.

With 1,010 known cases of Japanese knotweed, Bolton in Greater Manchester finds itself in second place. Pictured is the state of one homeowner’s garden, before Environet stepped in.

With two small children, the client wanted to find a non-chemical treatment, so Environet used its Resi-Dig-Out method, protecting the property from future encroachment from adjoining land by installing a root barrier along the boundary line. 

Now, it's time to reveal the place in top position...

The top spot in 2023 goes to Swansea, the second-largest city in Wales. With 1,350 known cases, this coastal city is slowly being overtaken by Japanese knotweed. In fact, 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.

Shockingly, Swansea didn't appear on Environet's top 10 in 2022, which highlights just how quickly the plant is spreading in the area. Homeowners here are suffering, too. One client told Environet that they had attempted to sell their property, but were told it was worth almost half its original asking price, due to the amount of Japanese knotweed in the garden and adjoining properties.

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. These will grow into tall, bamboo-like stems with distinctive, shield-shaped leaves.

In summer, clusters of tiny cream or white flowers will bloom. "Knotweed is easily identifiable during summer, but as we head through the autumn and into winter, the above-ground growth dies back and it becomes much harder to spot," explains Nic Seal, founder of Environet.

If you think you have spotted Japanese knotweed at home, or perhaps in your neighbour's garden or local area, it's best to ensure you have correctly identified the plant before taking any action. It's advisable to consult with an invasive plant removal specialist, like Environet, who can assess the situation, confirm the presence of knotweed and develop a treatment plan.

Luckily, there are various ways that Japanese knotweed can be treated or removed. "The type of treatment or removal will primarily be determined by the desired outcome and future land use," Environet explains. 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.” This is a good option if you're planning on keeping the property long-term.

However, excavation and disposal at landfill (also known as Dig & Dump) will – hopefully – eradicate the plant for good, but this method can “be between four and ten times the cost of herbicide treatment.” It is always recommended that you hire a qualified, professional company to deal with knotweed.

So, how much does it cost to remove Japanese knotweed? Well, unfortunately, there’s no fixed answer. The cost will be affected by various factors, including the quantity of infested land, the site conditions and the treatment method required.

Environet suggests that “variations in knotweed maturity, rhizome depth, lateral spread and ground type can impact costs.” Depending on the method chosen, 5 square metres of knotweed will cost between £3,000 and £12,000, Environet estimates. The cost of treating a 50-square-metre-area could be as much as £60,000.

According to Environet, tackling knotweed costs the UK economy nearly £250 million a year and homeowners are one of the groups most at risk – "not only from potential damage to their property but also from legal risks arising from encroachment or when properties are sold."

Typically, affected properties are worth between 3% and 15% less than their unaffected counterparts due to several factors, including the cost of treatment or removal and the ‘knotweed stigma’ deterring buyers. “There are lots of horror stories out there but with professional help, knotweed can be successfully treated and a property’s value can be largely restored,” Nic Seal explains. 

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. “Mortgage companies are concerned about Japanese knotweed because, over the course of a mortgage’s life, it could cause substantial damage if not controlled,” Environet explains.

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. OnTheMarket suggests having a professional treatment plan in place before marketing the property, ensuring you get an insurance-backed guarantee for the work and informing the estate agent of the situation.

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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