Experts have revealed the worst hotspots in Wales for Japanese knotweed. Environet released the latest data from its online tracker, which has a record of 58,000 infestations of "the UK's most invasive plant".

Japanese knotweed usually emerges in March or April and grows rapidly to reach up to 2.5 metres in height by mid-summer. An Environet spokesperson said: "It is identifiable by its hard, bamboo-like canes and distinctive shield-shaped bright green leaves which grow in a zigzag pattern along the stem."

According to the list Swansea has been the area of Britain worst-hit by Japanese knotweed in 2023, and four other parts of Wales feature in the top 10 British hotspots — Cardiff in fourth position, Caernarfon in eighth, Llanelli in ninth and Bridgend in tenth. Environ also published a Wales-only top ten list for this year:

  • 1) Swansea: 1,350 (number of infestations)
  • 2) Cardiff: 872
  • 3) Caernarfon: 708
  • 4) Llanelli: 706
  • 5) Bridgend: 664
  • 6) Neath: 631
  • 7) Dolgellau: 564
  • 8) Betws-y-Coed in Conwy county: 543
  • 9) Aberystwyth: 467
  • 10) Ffestiniog in Gwynedd: 455

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Environet users can entering their postcode into an online map to discover the number of verified knotweed sightings within a 4km radius. They can add a sighting by uploading a photo to be verified by experts. In Wales, there are on average eight occurrences of Japanese knotweed every ten square kilometres. It is usually spread accidentally through the movement of soil or gardening waste, or via rivers and streams when pieces of rhizome break off and take hold in new locations.

This Environet map shows hotspots for Japanese knotweed (the worst-affected areas are coloured red)
Environet map shows the hotspots for Japanese knotweed (worst-affected areas coloured red)

According to Environet, tackling knotweed costs the UK economy nearly £250million a year. Here is its 2023 list of the worst ten hotspots across Britain for the invasive plant:

  • 1) Swansea: 1,350 (number of infestations)
  • 2) Bolton: 1,010
  • 3) Bristol: 986
  • 4) Cardiff: 872
  • 5) Preston: 857
  • 6) Nottingham: 803
  • 7) Derby: 711
  • 8) Caernarfon: 708
  • 9) Llanelli: 706
  • 10) Bridgend: 664

Nic Seal, founder of Environet, said: “Vigilance is the best way to protect your property from the risks posed by Japanese knotweed. Make sure you know what knotweed looks like and how it differs from other common garden weeds like ivy and bindweed, so you can keep an eye out for it in your garden and neighbourhood. Knotweed is easily identifiable during summer, but as we head through the autumn and into winter, the above-ground growth dies back and it becomes much harder to spot. It’s also easier to conceal, so anyone viewing a property to buy should be extra careful."