Japanese knotweed hotspot map shows worst affected places - what to look for
Japanese knotweed can be a homeowner's nightmare. If it is left untreated it grows at a phenomenal rate and can quickly cover gardens - as well as outbuildings and homes when it spreads.
Invasive plant specialist Environet has revealed the Japanese knotweed hotspots for England and Wales in 2024. And they have come up with advice on how to spot it and what to do if you think you are affected by it.
The analysis is based on the latest data from its interactive online tracker, Exposed: The Japanese Knotweed Heatmap. This is updated with new verified sightings reported by Environet’s team and members of the public throughout the year.
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Now populated with almost 60,000 known infestations of Japanese knotweed, Exposed has monitored the spread of the invasive plant across the UK over the last six years, helping homeowners and homebuyers stay informed about the risk level near their property or one they wish to buy. 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.
People can also add sightings to the heatmap if they suspect a plant may be knotweed by uploading a photo to be verified by experts. 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 England the unitary authority of Bristol is the worst affected location with 5.18 incidences per km², followed by Merseyside with 2.49 per km² and Greater London with 2.29 per km².
South Wales is the worst affected area of Wales, home to eight out of 10 Welsh hotspots. Cardiff topped the table with 6.74 incidences of knotweed per km², followed by Swansea with 3.58 and Bridgend with 2.9.
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.”
Japanese knotweed usually begins to appear in March or April, swiftly growing to reach up to 2.5 metres in height by mid-summer. Its hard, bamboo-like canes and distinctive shield-shaped bright green leaves which grow in a zigzag pattern along the stem make it easy to spot.
The mature plants flower in August, becoming covered in clusters of small creamy-white flowers. During autumn and winter they start to die back, as temperatures cool and light levels drop, sending the plant into its dormant winter phase.
Its green leaves die and fall from the branches, the stems will turn brown and brittle, and after the first few frosts the plant will have completely died back, leaving only the dead canes remaining. This makes it an ideal tie to excavate it when the garden is not in use and before it starts to grown once again.
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, outbuildings and gardens, which can be costly to repair, but also from legal risks arising from encroachment or when properties are sold. Whilst it isn’t illegal to have knotweed growing on your land, legal disputes can arise when it’s allowed to spread across property boundaries and when sellers fail to declare it to buyers.
Environet 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.
What to do if you spot Japanese knotweed
According to the government people are responsible to stop any Japanese knotweed from spreading from their land, It says: "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."
It warns: "Do not treat knotweed yourself unless you have the appropriate skills and experience. You can find companies that specialise in treating knotweed."