A knotty problem: who pays to treat Japanese knotweed?

Japanese knotweed arrived on our shores as a Victorian garden ornament. It now has its own Select Committee report, its own question on the residential property information form, and a strong claim to being the most litigated plant in the UK.

The latest instalment is Sateeshkumar v Wigan MBC [2026] EWCC 39, decided in June. A garden in Atherton backed onto a park where knotweed had flourished for at least 20 years. It did what knotweed does and moved in next door. The homeowner won £2,635 to treat her own garden. On appeal, she lost it again because the court found that the treatment costs were not caused by the council's breach of duty.

Although the facts arose from a domestic garden dispute, the decision is likely to be of interest to developers, portfolio owners and investors assessing environmental liabilities on larger sites.

The rule that has not changed

Key to the invasive success of Japanese knotweed are rhizomes, root-like stems through which the plant spreads and produces new plants.

The Court of Appeal in Williams v Network Rail [2018] EWCA Civ 1514 accepted that rhizomes lurking under someone's land are a genuine burden, because they make developing it harder and more expensive. That means you have a good claim in nuisance if knotweed spreads from your neighbour's land when they knew (or ought to have known) that it was there and posed a risk.

For a landowner with a large or long-held portfolio, the duty is a live one, and Williams was itself a case about land that had harboured the problem for 50 years.

The rule that has moved

Timing is now key. By the time the council knew, or should have known, about the knotweed in 2018, it had already encroached into Mrs Sateeshkumar's garden and would need to be treated. Applying the reasoning on causation in Davies v Bridgend [2024] UKSC 15, the judge held that she could not show she would have avoided the treatment costs "but for" the council's inaction.

The interesting part is the botany. In the classic tree root cases the offending tree stays firmly on your neighbour's land, quietly drinking your soil dry, so the nuisance genuinely continues. Knotweed is not so loyal. Once the rhizomes are established on your land, the plant sets up on its own and carries on regardless of what your neighbour does on its side of the boundary. The judge's point, in effect, was that it stops being their plant and becomes yours.

What this means in practice

• Landowners:
  • You are not on the hook for encroachment that predates your knowledge (i.e. before you knew or ought reasonably to have known about the knotweed).
  • Once you know your property has a knotweed problem, the clock starts, and your neighbour can recover losses flowing from your inaction after that point. Log and act on knotweed sightings.
• Buyers and Investors:
  • If you acquire a site with an adjoining knotweed problem of long standing, your ability to recover treatment costs from your neighbour is now materially weaker. Price it in rather than litigate it later.
  • Where treatment has been carried out, make sure the guarantee is insurance-backed and assignable.
  • If you plan to let the land, expect tenants to want knotweed carved out of the repairing obligation and the service charge.
• Developers:
  • Treatment by chemical spraying can take more than three years, so this is a programme issue, not just a budget line.
  • Rhizomes can lie dormant for long periods (up to 20 years) and reappear once you disturb the ground, so be alive to the issue even if you think it has been dealt with.
  • Contaminated soil is controlled waste: it cannot be sold as topsoil, cannot be mixed with other spoil, and if it cannot be dealt with on site it must go to a facility permitted to take it. On a large earthworks package that changes the numbers.
  • Land Remediation Relief is available for knotweed removal (though not for disposal to landfill), so it is worth involving your tax advisers early.

A County Court appeal binds nobody, but until the next chapter the winner in the continuing knotweed saga is the landowner who keeps the better records. Photograph it, date it, and tell your neighbour about it in writing. Knotweed may be impossible to eradicate, but a paper trail is remarkably easy to grow.