The latest Japanese Knotweed Heatmap from Environet shows Bristol as the area with the highest concentration of identified Japanese knotweed sightings in England, with an astonishing 33.91 verified cases per square mile - far above any other English region.
In Wales, Newport tops the 2025 list with nearly 24 incidences per square mile, while data indicates significant clusters in major Scottish urban areas like Glasgow and Edinburgh.
The heatmap, updated year‑round with contributions from experts and the public, highlights how widespread this invasive plant has become and how local environments may influence its spread.
Hotspots across England, Wales and Scotland
In England, the heatmap data for 2025 clearly places Bristol at the top, with 1,440 confirmed knotweed incidences - equivalent to 33.91 per square mile - making it the densest knotweed hotspot in the country.
Other English regions with high concentrations include Merseyside, Greater London, and Lancashire, but none approach Bristol’s density. Environet UK In Wales, Newport recorded the most sightings with 1,747 total incidences and around 23.81 per square mile, followed closely by Cardiff and Bridgend.
Scottish data from the heatmap similarly highlights heavy infestations in urban areas such as Glasgow, Edinburgh and Aberdeen, where high numbers of verified sightings also points to a significant presence of knotweed.
How the heatmap data is gathered
The Japanese Knotweed Heatmap is built from more than 100,000 verified sightings across the UK, making it one of the most comprehensive tools for tracking this invasive species.
Members of the public can contribute directly by submitting potential sightings - often with photographs - which are then verified by Environet experts and added to the database.
The map visualises local densities of Japanese knotweed using a colour‑coded system (yellow for low, orange for moderate, and red for high), helping homeowners, buyers and surveyors assess risk at a postcode level and show which homeowners should expect to pay to kill or remove Japanese Knotweed.
Data suggests waterways increase likelihood of Japanese Knotweed
Analysis of the heatmap hints that Japanese knotweed is often found along major transport corridors and waterways, which act as conduits for spread - including riverbanks, canals, and urban edges where soil and plant fragments can travel.
Invasive knotweed thrives along watercourses where rhizome fragments are easily transported downstream during floods or heavy rainfall, establishing new infestations along riverbanks and adjacent land.
This evidence is supported by advice from the Royal Horticultural Society, who say: “Japanese knotweed often colonises the banks of rivers, streams and ditches; this can contribute to erosion and increase the likelihood of flooding".
The Pembrokeshire Coast National Park conservation team state this is because: "Fragments of rhizomes, or pieces of cut stems, are washed downstream forming new plants.”
So to further avoid the potential issue of Japanese Knotweed it is advisable to steer clear of buying homes near waterways, whereever possible so you don't have to worry about Japanese Knotweed removal costs.
