Merseyside has been identified as the second worst affected area in the UK for Japanese knotweed, according to a heatmap based on verified sightings of the invasive foreign species. The plant is now present in every county of the UK, with an average of 1.1 occurrences per square mile in England and 3.9 in Wales, impacting an estimated 1.58 million homes across the country.
Merseyside's knotweed hotspots
Data provided by Environet shows Merseyside comes second only to Bristol, with 3,393 incidents — equivalent to 13.45 per square mile. If left unmanaged, knotweed can cause damage to buildings, hinder property sales and lead to legal disputes if it spreads across property boundaries. Research conducted by Environet in partnership with Censuswide in January 2026 indicates that 7% of the UK’s homes — more than 1.58 million properties — are impacted by the invasive plant. Property values are typically reduced by around 5%, equivalent to £13,500 per home, highlighting the significant impact on the nation’s housing wealth.
How the rankings are compiled
The rankings are based on invasive plant specialist Environet’s own enquiries, public sources and verified sightings submitted by members of the public using the map’s ‘Add Sighting’ feature. Homeowners and homebuyers can enter their postcode to discover the number of known sightings within a five-mile radius, with hotspots highlighted in yellow, orange, or in the worst cases, red.
Historical spread of knotweed
Introduced to Britain as an ornamental garden plant in the Victorian era, Japanese knotweed has since spread across the country through the movement of contaminated soil, waterways and more than 150 years of industrial activity, construction and infrastructure works. Former docklands and industrial areas dominate the national hotspot rankings, highlighting the role these activities have played in aiding its spread. Knotweed does not reproduce by seed in the UK since all the plants are female, but its root system can extend up to 7 metres laterally beneath the ground and once established, even tiny fragments of rhizome can regenerate into new plants.
Economic impact
After the fungus Hymenoscyphus fraxineus, which causes ash dieback disease, Japanese knotweed is the UK’s second most costly invasive non-native species, responsible for an estimated £246.5 million in annual economic costs according to CABI.
Top ten Merseyside locations affected by knotweed
Environet director Emily Grant said: "Merseyside is consistently one of the worst-affected locations in the country for Japanese knotweed, which is probably due to its industrial past and the mass movement of soil over time. The port is also likely to be a factor, as in the 19th and 20th centuries, ships bound for other parts of the world would return loaded with ballast containing plant material, to help improve their stability in the water. It’s suspected that this ballast was contaminated with Japanese knotweed rhizomes."
Grant added: "Japanese knotweed is now firmly established right across the country, which means homeowners everywhere should know what to look for. My advice to anyone living or buying a home in a hotspot is to check your garden regularly during the growing season. At this time of year, an established knotweed plant can be identified quite easily by its hard bamboo-like canes and distinctive bright green shield-shaped leaves which grow in a zig-zag pattern along the stem. The earlier it’s spotted, the easier and cheaper it is to deal with. Acting quickly with professional advice will prevent it spreading further and help protect both your property and its value."
Anyone who suspects they may have knotweed on or near their property can email a photo to Environet at expert@environetuk.com for free identification. If confirmed, a survey will determine the extent of the infestation, the risk of damage or encroachment and the most effective way to treat it.
How to spot Japanese knotweed during summer
- Tall, upright canes which can reach 2 to 3m high
- Dense clusters of stems emerge from a central crown in the ground
- Large green shield-shaped leaves with a pointed tip grow alternately up the stem
- By late summer, cone-shaped tassels of small creamy-white flowers emerge on the upper canopy
