The “top ten” locations in Wales affected by Japanese knotweed have been identified. As the growing season for Japanese knotweed is in full swing, this invasive species can cause damage to properties, patios, paving and decrease the value of land. Invasive plant expert
Environet has now disclosed the Welsh areas most affected by knotweed in 2025, using the latest figures from its online heatmap tool, Exposed: The Japanese Knotweed Heatmap. In total, there are 31,338 confirmed sightings of knotweed across Wales, equating to 3.9 instances per square mile, which is significantly higher than England’s rate of 1.1 instances per square mile.
Now featuring more than 100,000 recorded instances of the UK’s most pervasive plant, the map is updated with new confirmed sightings throughout the year, offering the most thorough online record of Japanese knotweed in the country.
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The public can input their postcode to find out the number of sightings in their vicinity, with high-risk areas marked in yellow, orange or, in severe cases, red.
This helps them gauge the potential risk to their property or one they’re considering purchasing.
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They can also contribute to the heatmap by reporting knotweed infestations using the ‘Add Sighting’ function and attaching a photo of the plant for expert verification.
The data shows the worst places in Wales are Newport city, Cardiff city and Swansea city.
Wales’ Top 10 Japanese knotweed hotspots for 2025 are:
Japanese knotweed hotspots in North Wales in 2025, with red indicating higher concentrations of the invasive plant -Credit:Environet
Incidences
1. Newport city – 1,239
2. Cardiff city – 949
3. Swansea city – 936
4. Llanelli, Carmarthenshire – 693
5. Betws-y-Coed, Conwy – 421
6. Neath, Neath Port Talbot – 396
7. Dolwyddelan, Conwy – 298
8. Llanafan, Ceredigion – 289
9. Rogerstone, Newport – 268
10. Tanygrisiau, Gwynedd – 254
Japanese knotweed made its debut in Britain during the Victorian era, arriving in a collection of plant specimens sent to Kew Gardens and was swiftly embraced by gardeners and horticulturalists who remained oblivious to its destructive potential.
The plant starts sprouting in early spring, rapidly developing into dense green bushes featuring pink-speckled stems, heart-shaped foliage and bamboo-style canes.
However, it can create significant headaches for property owners.
Its subterranean root system can extend up to three metres downwards and stretch up to seven metres sideways, pushing through gaps in concrete, tarmac drives, walkways, drainage systems and cavity walls.
Whilst severe property damage remains uncommon, mortgage providers demand knotweed treatment before property sales, potentially triggering neighbourhood disputes and reducing property values by roughly five per cent, according to Environet.
A 2023 DEFRA study estimated the expense of addressing knotweed costs the UK economy almost £250 million annually. Environet Director Emily Grant commented: “By mapping verified sightings, we’re helping people understand the level of risk in their local area and encouraging early identification and treatment.
“Vigilance is the best way to protect your property from Japanese knotweed, making sure you tackle it as early as possible before it becomes well established. At this time of year, knotweed is in full growth mode and easy to spot with its bright green heart-shaped leaves which grow in a zigzag pattern up the stems.
“Mature plants flower around August, when they produce clusters of tiny white blooms.”
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