Why does Japanese knotweed affect a property sale?

Because it is an invasive plant capable of causing damage, is extremely difficult to eradicate, carries legal obligations, and — most consequentially — many lenders will not lend on an affected property without a treatment plan.

What it is. Reynoutria japonica, introduced to Britain in the nineteenth century as an ornamental. It grows extremely fast in summer, spreads through an extensive underground rhizome system, and can regenerate from a fragment of rhizome a few centimetres long.

What it actually does — with a caveat. It exploits existing cracks and weaknesses in hard surfaces, damaging drains, drives, patios and boundary walls. However, research published in recent years found no evidence that it causes significant structural damage to buildings, and less than many other common species including trees. The reputation has been more damaging than the plant, and industry guidance has been revised to reflect a more proportionate assessment based on proximity.

Why it still matters commercially:

Mortgage availability. Many lenders require a professional treatment plan with an insurance-backed guarantee, typically 5 or 10 years, before lending. Without one, a sale can fall through.

Cost. Professional treatment runs to thousands of pounds, and excavation and removal considerably more.

Time. Herbicide treatment typically requires several years of repeat application and monitoring.

Legal obligations:

Sellers must disclose it. The TA6 property information form asks directly, and answering "no" when you knew is misrepresentation, which has been litigated successfully.

Allowing it to spread onto neighbouring land can give rise to a private nuisance claim.

It is an offence to plant or cause it to grow in the wild, and the waste is controlled — you cannot put it in household waste or compost it.

If you find it: get a professional survey, obtain a treatment plan from a company offering an insurance-backed guarantee, and do not attempt to dig it out yourself.