Japanese knotweed is wiping an estimated £21.4bn from UK residential property values, according to new research that suggests more than 1.5 million homes are affected and that buyers still heavily discount properties where the invasive plant is present.

More than 1.5 million homes affected by knotweed stigma

Research from removal specialist Environet, reported this week, found affected homes lose around 5% of their value on average, or roughly £13,500 per property. It also found 32% of would-be buyers would not buy a knotweed-affected home at all, while a similar share would only proceed if there was a professional treatment plan and a lower price.

Emily Grant, director at Environet, said a significant share of buyers still will not touch an affected property, while Andrew McColl, chairman of the Residential Property Surveyors Association, warned that treatment and management plans can take five to ten years to complete.

For landlords and investors, that is not just a sales issue. Knotweed can hit refinancing, inheritance planning, portfolio valuations and the ease of disposing of weaker stock. Even where the plant has been treated, buyer caution can linger long after the immediate physical problem has been addressed.

This follows Residential Landlord’s recent coverage of tighter HMRC scrutiny on property valuations, which highlighted how much now rides on getting values right. It also revives a long-running issue first flagged in Residential Landlord’s earlier reporting on knotweed and investment values. The latest figures suggest the stigma has not faded nearly as much as many in the market hoped.

There is a practical warning here for buy-to-let investors hunting for value. A discounted purchase is not automatically a bargain if the exit route narrows, lenders stay cautious and treatment drags on for years. In some cases, the price cut is simply the market pricing in a headache that will not go away quickly.

Why landlords need better due diligence before they buy

Any landlord buying older stock, edge-of-site plots or homes with unmanaged gardens should push harder on surveys and disclosure. Knotweed is a specialist problem that can quickly become a portfolio problem if it affects refinancing, sale timing or neighbouring properties.

• Ask directly about historic or current knotweed treatment before exchange.
• Check whether a funded management plan or insurance-backed guarantee exists.
• Price in slower resale and weaker lender appetite, not just treatment costs.

The latest report suggests buyers remain far less relaxed about knotweed than some sellers assume. That is bad news for anyone banking on a quick refurb and exit.

Opinion

Knotweed is one of those problems the market keeps claiming it has learned to live with, right up until a valuation falls short or a buyer walks away. Landlords should treat it as a real pricing risk, not a minor gardening issue. A cheap purchase can turn expensive very quickly when stigma lasts longer than the treatment plan.