Bristol is the Japanese knotweed 'capital of England', new data from Environet shows.
In the year to date, there have been 1,440 reported sightings of Japanese knotweed in the South West city, according to the findings.
Merseyside is another hotspot for Japanese knotweed, with 2,963 sightings reported, but spanning a less dense area, at 11.87 episodes per square mile compared to 33.91 in Bristol.
Greater London is the third most prevalent Japanese knotweed hub, with 4,687 reported sightings and 7.74 incidents per square mile.
Lancashire came out in fourth place, seeing a hefty 8,612 sightings spanning 7.25 episodes per square mile.
Residents in Greater Manchester also need to be on high alert for Japanese knotweed, with the area seeing 3,558 Japanese knotweed reports, translating to 7.22 incidents per square mile.
Spanning further afield, the Isle of Man, a self-governing British crown dependency, also reached the top 10, as did the Channel Islands, Isle of Wight, Derbyshire and Nottingham, followed closely by the West Midlands and Surrey.
What is Japanese knotweed?
According to the Royal Horticultural Society, Japanese knotweed is a persistent, clump-forming herbaceous perennial, which was introduced to Britain as an ornamental garden plant but has since become an invasive weed subject to legal control measures.
Japanese knotweed first arrived in Britain in the Victorian times in a box of plant specimens delivered to Kew Gardens and was quickly adopted by gardeners and horticulturalists, who were unaware of its invasive nature.
Since then it has spread widely, with an average of 1.1 occurrences in every square mile of England and 3.9 in Wales, according to Environet.
Japanese knotweed can grow in most soil conditions found in Britain, particularly in man-made habitats, such as roadsides, waste ground, railway embankments and cuttings and spoil tips. It is also commonly found along rivers and streams.
Why is Japanese knotweed common in locations like Bristol?
Speaking to This is Money, Emily Grant, a director at Environet, said: 'It’s likely that Japanese knotweed is most prolific near ports such as Bristol, Liverpool and Cardiff, because of historical shopping and trading practices.'
She added: 'In the nineteenth and twentieth centuries ships would return from all over the world carrying ballast, which often included soil containing non-native plant material like knotweed.
'That was then dumped to make way for the next cargo, and it quickly took hold. It may also have been planted deliberately to stabilise railway embankments and slag heaps.
'In industrial areas like Manchester, Lancashire and the Midlands, extensive soil movement from mining and construction accelerated its spread.'
How can I spot Japanese knotweed?
Japanese knotweed begins to emerge in early spring, quickly growing into lush green shrubs with pink-flecked stems, heart-shaped leaves and bamboo-like canes.
In August it starts flowering with bunches of tiny creamy-white blooms that hang in tassels.
It can pose serious problems for homeowners, with underground rhizomes that can grow up to three metres deep and spread up to seven metres horizontally, emerging through cracks in concrete, tarmac driveways, pathways, drains and cavity walls.
While serious damage to property is rare, mortgage lenders require knotweed to be dealt with when a property is sold, meaning it can cause legal disputes between neighbours and reduce a property’s value.
Environet estimates that the presence of Japanese knotweed at a property can reduce its value by 5 per cent, though this is variable and will depend on the scale of the problem.
A 2023 Department for Environment, Food & Rural Affairs report put the cost of tackling knotweed to the economy at nearly £250million per year.
What should I do if I suspect Japanese knotweed?
The government's website states: 'You must stop Japanese knotweed on your land from spreading off your property.
'Soil or plant material contaminated with non-native and invasive plants like Japanese knotweed can cause ecological damage and may be classified as controlled waste.
'You do not legally have to remove Japanese knotweed from your land unless it’s causing a nuisance, but you can be prosecuted for causing it to spread into the wild.'
If you do notice knotweed, you do not need to report it, but if it is on your property you should act sooner rather than later.
'The more it grows, the more it will cost to remove', Grant told This is Money.
You will not be able to sell your property until you have had any Japanese knotweed professionally dealt with, and you could be at risk of a legal claim from a neighbour if you leave it to spread.
Grant added: 'From there a treatment plan can be put in place, whether it’s herbicide, removal, or a combination of both.
'Whatever option you go for, be sure to get an insurance-backed guarantee so if it starts to regrow you know you’re covered – and that’s what mortgage lenders will expect when you come to sell your property.'
Japanese knotweed must be carefully disposed of carefully. If you cannot dispose of it suitably on site, you must send it to a landfill site or incineration facility that has the correct type of permit.
It is best not to try and tackle a Japanese knotweed invasion yourself, unless you have appropriate training, skills and equipment.