Japanese knotweed is the one plant that can genuinely make a property unsellable. Mortgage lenders will not touch a property with an active infestation, and even a treated one needs a management plan and an insurance-backed guarantee before most lenders will consider it. So when it starts spreading from your neighbour's garden into yours, the stakes are high.
Can I Make My Neighbour Deal With It?
There is no law that requires a homeowner to remove knotweed from their property. That surprises a lot of people. It is not illegal to have knotweed in your garden. What is illegal, under the Wildlife and Countryside Act 1981, is causing it to spread in the wild. And under common law, if your neighbour's knotweed spreads onto your property, that is a private nuisance and you can take legal action.
The landmark case is Network Rail Infrastructure Ltd v Williams (2018), where the Court of Appeal ruled that allowing knotweed to spread from your land to a neighbour's property is actionable as a nuisance, even if you did not plant it and even if it was there when you bought the house. The court said that the loss of amenity and the interference with the quiet enjoyment of the neighbouring property was enough. You do not have to wait until it actually damages the foundations.
What Can I Actually Do?
Start by writing to your neighbour. Explain that you have identified knotweed growing in their garden and that it is spreading, or at risk of spreading, into your property. Ask them to instruct a specialist contractor to treat it. Be polite but put it in writing, because you may need the letter later.
If they ignore you or refuse, your options are to treat the knotweed that has encroached onto your side of the boundary (at your own cost, which you can then claim back) or to take legal action. A solicitor's letter often does the trick. If it gets to court, you can claim for the cost of treatment on your land, any diminution in your property value, and potentially the cost of treatment on their land if the infestation cannot be dealt with from your side alone.
What About the Council?
Councils have powers under the Antisocial Behaviour, Crime and Policing Act 2014 to issue community protection notices requiring landowners to deal with knotweed. In practice, councils are reluctant to use these powers and will usually suggest you resolve it privately. But it is worth reporting it, particularly if your neighbour is a social landlord or a housing association, because they tend to take council involvement more seriously than a letter from next door.
Selling a Property With Knotweed
If your property has knotweed, or has had it in the past, you must declare it on the TA6 property information form. You will need a professional treatment plan, typically a programme of herbicide injections over three to five years, and an insurance-backed guarantee. Some specialist firms offer guarantees that satisfy mortgage lenders. Without this, you will struggle to sell to anyone who needs a mortgage, which rules out most buyers.
If the knotweed originally came from a neighbour's property, document that. Your buyer's solicitor will want to know whether the source has been dealt with, because if it has not, the problem will come back.
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