Four locations in south Wales have been found to be among the worst in the UK for Japanese Knotweed. 

Japanese knotweed is an invasive plant widely recognised as a pest species that outcompetes native plants and causes damage to the surrounding environment.

New data has found there are around 58,000 known infestations of the invasive plant in the UK.

Environet uses an online map to chart the spread of Japanese Knotweed to help alert homeowners and homebuyers to the risk level in their local area.

For homeowners, the plant can pose serious problems if left unchecked, with the potential to grow up through cracks in concrete, tarmac driveways, pathways, drains and cavity walls.

Whilst it isn’t illegal to have knotweed growing on your land, if it is allowed to cross a boundary into an adjacent property, legal disputes can arise between neighbours.

Penarth Times: Japanese Knotweed is recognised as a pest species in the UK.Japanese Knotweed is recognised as a pest species in the UK. (Image: Getty/Newsquest)

When a property is sold, sellers are asked a direct question about whether knotweed is present, even if hidden beneath the ground or within 3 metres of the boundary, and those who fail to declare it can be sued by their buyer for misrepresentation.

Environet estimates that approximately 5% of homes across the UK are currently affected by Japanese knotweed, either directly or through a neighbouring property, impacting property prices by up to 10% in some cases.

Founder of Environet, Nic Seal, 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.

“There are lots of horror stories out there but the with professional help, knotweed can be successfully treated and a property’s value can be largely restored.”

The top 10 worst places for Japanese Knotweed in the UK

Swansea is the worst spot in the UK for Japanese knotweed in 2023 according to Environet with 1350 recorded infestations. 

This is over 300 more than the next worst place - Bolton, with 1010. 

Cardiff (872 infestations), Llanelli (706) and Bridgend (664) also feature in the top 10 Japanese Knotweed hotspots in England and Wales.

Penarth Times: South Wales have some of the worst areas for Japanese Knotweed in the UK.South Wales have some of the worst areas for Japanese Knotweed in the UK. (Image: Environet)

The worst places for Japanese Knotweed in the UK, according to Environet are:

  1. Swansea
  2. Bolton
  3. Bristol
  4. Cardiff
  5. Preston
  6. Nottingham
  7. Derby
  8. Caernarfon
  9. Llanelli
  10. Bridgend

While the worst affected areas in Wales are:

  1. Swansea
  2. Cardiff
  3. Caernarfon
  4. Llanelli
  5. Bridgend
  6. Neath
  7. Dolgellau
  8. Betws-y-Coed
  9. Aberystwyth
  10. Ffestiniog

How to identify Japanese Knotweed?

The appearance of Japanese knotweed changes with the seasons, so Environet says it is important to consider the time of year when you are checking for the invasive plant.

Key traits of Japanese knotweed include:

  • Red shoots emerge in spring that look like asparagus
  • Leaves which are shield or shovel-shaped
  • Stems that resemble bamboo canes with purple speckles
  • Small, cream-coloured flowers developing towards the end of summer

Environet added: "In the autumn, the leaves will start to go yellow and drop as winter approaches.

"Knotweed can grow to about two or three metres if left unattended. The stems will change to a darker brown before the plant becomes dormant in winter."

What to do if you spot Japanese knotweed 

There are five things to do if you spot Japanese Knotweed, says Environet:

  • Confirm the identity of the plant by contacting Environet’s free ID service
  • Commission a professional Japanese knotweed survey to confirm the extent of the infestation, where it originated and the best way to treat it
  • Organise professional treatment

If you’re selling your property, inform the estate agent at the outset and even if the infestation is removed, be sure to declare it to potential buyers.

If you’re unsure whether your property or one you wish to buy is affected by knotweed, you can order a survey to check the property and its immediate surroundings for any sign of the invasive plant.