Japanese knotweed has a reputation. Say the name and people start imagining foundations being torn apart, mortgage offers being withdrawn, or homes becoming unsellable. But how much of that fear is based on fact, and how much is just myth passed from one survey report to the next?
Knotweed is an invasive perennial plant, originally introduced to the UK in the 19th century as an ornamental import. It grows fast, spreads underground, and is very difficult to kill off without the right approach. In fact, its rapid growth and dense coverage once made it a popular choice for stabilising railway embankments. It was widely planted along train tracks for precisely that reason. The problem is, it worked a little too well.
The plant’s underground root system, or rhizome, can spread several metres in all directions. If even a small piece is left in the soil, it can regenerate and continue growing. That is why management and removal is so important.
That said, knotweed is not the horror story some people make it out to be. It does not eat through concrete, it does not destroy every house it touches, and it does not mean a property is automatically unmortgageable. But it is a legal and practical concern, and it should not be ignored.
Under the Wildlife and Countryside Act 1981, it is an offence to plant or cause the spread of Japanese knotweed in the wild. While it is not illegal to have it in your garden, if it spreads onto neighbouring land, you can be held responsible. Most mortgage lenders also take knotweed seriously. If it is found within seven metres of the property, many lenders will require a professional management plan before they approve a loan.
That seven-metre rule came from a single report in 2012 and has since been revised. The latest RICS guidance encourages a more measured view, based on risk, proximity, visible damage, and whether any steps have been taken to control it. Residential building surveyors now classify knotweed risk using four categories, not just one blanket rule.
Knotweed is unlikely to damage a sound structure, but it can exploit weak points. Cracks in garden walls, paving, outbuildings, and retaining structures are the most common areas where it gets a foothold. The real issue is its persistence. Left unchecked, it can dominate a site and complicate access, drainage, or future works.
Treatment usually falls into two main approaches. Chemical herbicide treatment, which takes around three to five years and should include a guarantee, or excavation and disposal, which is faster but far more costly. In both cases, a specialist contractor should be used, and the plan should be documented. Many lenders will want to see evidence that treatment is underway or completed, along with an insurance-backed guarantee.
From a buyer’s perspective, the presence of knotweed is not a reason to abandon a purchase, but it is a reason to pause and get the right information. Ask whether a treatment plan is in place, whether it has been completed, and whether it includes a guarantee. Also ask how far the knotweed is from the main dwelling and whether any part of the structure or boundary shows signs of damage or obstruction.
It is also important to acknowledge the stigma. Even when knotweed has been treated and signed off, it can still affect a property’s perceived value. Two otherwise identical houses, same street, same layout, but one has a history of knotweed and the other does not. Most buyers will prefer the one without. That difference in perceived desirability carries a cost. Personally, I would not buy a property that had suffered from knotweed unless there was a financial incentive to do so. For that reason, we always recommend that buyers or owners of affected properties seek a full Red Book valuation from a chartered building surveyor, so any impact on value is properly understood and factored into negotiations.
Other invasive plants also raise concern. Giant hogweed, which causes severe skin reactions, is a health hazard rather than a structural one. Bamboo, while not illegal, spreads aggressively and is increasingly flagged by surveyors for its ability to damage garden walls, drains, and paving. Like knotweed, these plants become an issue when they are not managed.
At Survey Shack, we do not test for or treat invasive plants, but we do guide users on how to recognise the signs. Our app highlights visual indicators of knotweed, bamboo, and other species where present, helping homeowners and buyers alike take the next steps with confidence. The goal is not to alarm anyone or trip up a sale. It is to make sure the facts are on the table and that the right people are consulted when needed.
Japanese knotweed is a nuisance, and it carries responsibilities. But whether it becomes a catastrophe or just another job to deal with depends on how it is handled. Some will see it as manageable, others as a reason to walk away. What matters most is not the name of the plant, but the clarity of the advice that follows.
If you are looking for a “surveyor near me”, it doesn’t get any closer than your own pocket – Survey Shack is the perfect tool to get that crucial first read on a property’s condition. From helping you identify invasive plants like Japanese knotweed to spotting early signs that may warrant a full building survey, home buyers survey, or even a consultation with a residential structural engineer, our app puts clear, actionable insight in your hands.
