Japanese Knotweed Can Cut House Prices by Up to 15%

There is a plant growing in gardens across Britain that can shave tens of thousands off the value of a house. It is not poisonous. It will not harm you, your children or your dog. It just grows, at up to ten centimetres a day in peak season, straight through tarmac, drains and the weak points in a wall.

The Victorians thought it was rather elegant. Introduced to Europe in the nineteenth century as an ornamental garden plant, Japanese knotweed had no natural predators here and simply kept going. Understanding what Japanese knotweed does to a property is now a standard part of buying and selling a home in Britain. Affected properties can lose somewhere in the region of 10% to 15% of their value, and mortgage lenders frequently refuse to proceed without a professional management plan in place.

How an ornamental became a liability

The problem is underground. What you see above the soil is a fraction of the organism. Below it sits a network of rhizomes that can extend several metres in every direction, storing enough energy to send up new shoots after years of apparent dormancy.

A fragment the size of a fingernail can start a new colony. Which means a well meaning gardener with a strimmer and a green bin can turn one problem into six, and has.

The UK government put the cost of controlling it at £1.25 billion, and that estimate is now more than a decade old.

Telling it apart from things that are not it

Half the panic about knotweed involves plants that are not knotweed. Before you ring anybody, check what you are actually looking at.

• Spring. Red or purple shoots pushing up like asparagus spears, with a waxy surface, growing visibly week by week.

• Summer. Hollow stems that look like green bamboo with a reddish tinge, reaching two or three metres. Leaves are heart shaped and grow in a distinct zig-zag along the stem.

• Late summer. Clusters of small creamy white flowers.

• Winter. Brown, brittle, apparently dead canes still standing. The roots below are very much alive.

• Commonly mistaken for bindweed. Bindweed twines and climbs on thin stems and has white trumpet flowers. Knotweed stands upright on rigid canes.

• Commonly mistaken for Himalayan balsam or Russian vine. Balsam has pink orchid-like flowers and serrated leaves. Russian vine climbs rather than standing on its own.

The reliable combination is hollow bamboo-like stems, zig-zag leaf arrangement and creamy flower clusters. If you have all three, take it seriously.

The seven metre rule

This is the bit that catches buyers out. Surveyors and lenders often work to a guideline that knotweed within seven metres of a building poses a risk, because the roots can travel roughly that far horizontally from whatever is visible above ground.

It is not law. It is a risk convention, and it is widely applied. Which means knotweed at the bottom of a long garden, nowhere near the house, can still cause a mortgage application to stall. It can also be growing on the neighbour’s land and still be your problem.

Why it is so difficult to kill

Cutting it back does nothing except spread it. Bleach does nothing. Most of what people try at home does nothing.

Serious treatment means either repeated glyphosate applications over a period usually measured in years rather than months, or full excavation, which typically involves digging to around three metres and removing the contaminated soil. Knotweed waste is classed as controlled waste in the UK, so it cannot go in your bin, your compost or a skip. It has to go to a licensed landfill site.

You are legally obliged to mention it

Having knotweed on your land is not illegal. Allowing it to spread onto neighbouring property is a different matter, and can land you in court.

When selling a home in England or Wales, you are required to declare knotweed on the TA6 property information form. People occasionally decide not to, which is a poor idea. Non disclosure of a known infestation has produced successful misrepresentation claims after completion, and the buyer’s remedy tends to cost considerably more than the treatment would have.

It is not the catastrophe it sounds like

Worth ending on this, because knotweed has acquired a reputation slightly out of proportion to reality. It does not devour foundations. It exploits existing cracks and weaknesses, which is a meaningful difference.

A professionally documented treatment plan with an insurance backed guarantee satisfies most lenders, and once that paperwork exists the property becomes mortgageable again. The plant is stubborn rather than unbeatable. The expensive mistake is pretending it is not there.

About the author

Paul Gibbens – Property Expert at Housebuyers4u

He is a property expert at Housebuyers4u, advising UK homeowners on quick house sales, realistic valuations and how to avoid common pitfalls such as unclear fees, hidden option agreements and last-minute price changes.