Conveyancers warned to look out for Japanese Knotweed “hotspots”

Analysis of Japanese Knotweed hotspots has identified Bristol, Merseyside and Greater London are the worst affected counties in England.

60,000 verified sightings of Japanese Knotweed have been captured through an online heatmap managed by specialists Environet which tracks the spread of the plant across the UK over the past six years. Users can enter their postcode to discover the number of knotweed sightings nearby, with hotspots highlighted in yellow, orange and, in the worst affected locations, red.

Tackling knotweed is estimated to cost the UK economy nearly £250 million a year according to Environet and homeowners are one of the groups most at risk – not only from potential damage to their property, outbuildings and gardens, which can be costly to repair, but also from the legal risks arising from encroachment or the failure to declare it on the property information form.

The firm estimates that approximately 5% of homes across the UK are currently affected by Japanese knotweed, either directly or neighbouring an affected property, typically impacting property prices by around 5% – or up to 10% in severe cases.

Using its heatmap data, and taking into account the different sizes of each county or unitary authority, Environet has calculated the number of known Japanese knotweed infestations per km² to identify the worst affected areas in 2024.

Earlier this year a court overturned a decision to award damages to a homeowner in Bridgend whose property was blighted by Japanese Knotweed. The case rested on the timeline of when any breach of duty had occurred.

Emily Grant, director of Environet, said,

“With over 60,000 known infestations of Japanese knotweed on our heatmap, we have a clear picture of its prevalence across the UK. Knotweed is easily identifiable during summer, but 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. The best way to protect your property is to be aware of the risks and ensure you can identify knotweed at different times of the year. If you do have knotweed on your property, it’s wise to act immediately to treat or remove it before it has the chance to spread further. With professional assistance knotweed can be dealt with and the value of a property largely restored.

“If you’re buying or selling a property, consider instructing a Japanese knotweed survey to check for any sign of the plant on the property and its surroundings.”

History of Japanese Knotweed in the UK

Since the first sample was shipped to Kew Gardens from Japan in 1850, via Leiden in the Netherlands, knotweed has spread widely and there is now on average one occurrence in every 3km² in England and Wales. The unitary authority of Bristol is by far the worst affected location in England with 5.18 incidences per km², followed by Merseyside with 2.49 per km² and Greater London with 2.29 per km².

The origin of knotweed sightings near ports such as Bristol and Liverpool is believed to be linked to historical shipping and trading practices lacking in biosecurity. In the 19th and 20th centuries, ships returning to the UK from various parts of the world often carried ballast, including soil from foreign ports containing non-native plant material, which was then discarded to make room for the next cargo. In traditionally industrial areas such as Greater Manchester, large quantities of soil were typically moved around through mining and other activities, aiding the plant’s spread.