What is Japanese knotweed and how it does it damage property?
Japanese knotweed is back in the press – this time for the part it plays in increasing flood risk. Increased press coverage over the years has led to many homebuyers asking us about its presence when conducting building surveys in London and property surveys in Kent. In this blog, we will delve into Japanese knotweed, what it is, how it impacts homeowners, and why it needs to be managed.
What is Japanese knotweed?
Japanese knotweed is non-native invasive species of plant. It was brought to the UK in the 1800s as a decorative plant and is described by the Royal Horticultural Society as a persistent, clump-forming herbaceous perennial. Whilst it looked pretty with its shield shaped leaves, in a distinctive zig zag growth pattern, small flowers, and bamboo-like shoots, it would soon become problematic.
For more information, take a look at how to identify Japanese knotweed.
Why is Japanese knotweed bad?
Japanese knotweed forms long and vast networks of roots (rhizomes) under the ground. Once established, these can sprout above ground anywhere, and are extremely difficult to get rid of.
In the natural environment it outcompetes native plants for space, light, moisture and nutrients, reducing diversity. It will often be found on riverbanks and the edges of streams, contributing to erosion and increasing the likelihood of flooding.
As such it is listed in Section 9 of the Wildlife and Countryside Act 1981.
Does Japanese knotweed damage property?
Yes. It can cause wide ranging problems for a property, as these roots cause damage to hard surfaces and take advantage of any areas already a little worn. It will grow into cracks and weaknesses, making these worse. In a wall it can cause structural weakness (which is often why garden walls lean and collapse), whilst on patios, tarmac, or resin driveways, for example, it will cause uneven surfaces.
Is Japanese knotweed dangerous?
Japanese knotweed is non-toxic. In fact, there are several recipes for cooking and eating it. It’s not dangerous to humans or animals. However, you do not want it anywhere near your home because of the problems it can cause to your property.
Is Japanese knotweed found anywhere in particular?
Japanese knotweed is found across the UK, particularly in urban areas. It thrives on waterways, rural properties and wetland habitats. If your house has a stream or river running close by you might want to take a look to see if there is knotweed growth.
Equally, if you are close to wasteland or heavily disrupted ground, or agricultural land, there could be garden waste that has the roots of knotweed within it and this could become a source for the plant to start to regrow near your own property.
What are signs of Japanese knotweed in a house?
Unless you can see the plant itself and are able to identify it, there are no real signs of Japanese knotweed. Take a look at the various plant identifier websites to help you understand what it looks for, but it is always best to get a professional opinion.
In the winter it may not be visible at all, as it dies back to ground level, although dead shoots (often looking like bamboo) may be an indicator. In early spring, very small shoots just above ground level may be the only indication.
Is it law to declare the history of knotweed on a property?
If you think you have knotweed on your property, our surveyors in Kent and property surveyors in London will help you understand the extent of the knotweed problem, if it could be affecting your house’s foundations, or potentially into a neighbouring property, provide advice on treatment, and estimate the cost of remediation.
What if my neighbour has Japanese knotweed?
If Japanese knotweed is growing within three metres of your boundary, it will usually impact your ability to sell or mortgage your house. In this case, you should contact the landowner to ask them to take action. If you do not know who owns the land, you may be able to find the information by carrying out a land registry search.
Whilst it is not a legal requirement to remove knotweed from property, landowners are bound by law to prevent the spread of knotweed. So if your neighbour has knotweed in their garden and it is ‘under control’ then you can’t do much – especially if it is not within three metres of your property.
However, if it is spreading towards your property, they must do something about it or face legal consequences. There have been several legal cases where claims have been raised, and in some cases damages awarded for diminution in value.
It’s worth considering that they may not even know they have knotweed, so talking to them about the problem in the first instance is always the best policy.
Bear in mind that the neighbour may not be an adjoining homeowner who you can have a chat with fairly easily about it. It could be agricultural land, waste land or a construction site. If that is the case, you need to get in touch with the landowners to make the problem known. They have a duty of care to put the issue right.
Do surveyors check for Japanese knotweed?
Yes – as RICS surveyors we have a duty of care to identify and report on any visible signs of knotweed on or near a property we are surveying.
Does a level 3 survey check for Japanese knotweed?
A level three home survey is a comprehensive report and includes an in-depth analysis of the property’s condition, including advice on defects, repairs and maintenance options.
We will check for signs of knotweed within the perimeter of the property, but be warned it may be outside and there is potential that if it is not managed, it could spread to your property. Our surveyors do check the surrounding area and neighbouring properties, particularly if there is a stream or other type of land where there is greater risk of the presence of Japanese knotweed.
What if Japanese knotweed is found in a house survey?
If during our house survey we find Japanese knotweed present, you will be able to negotiate on the asking price, or indeed choose not to purchase that property.
As the risks of knotweed we have highlighted here show, unless it is removed properly, it could be a recurring problem years after you have purchased a property.
Does Japanese knotweed devalue property?
In most cases, yes. The extent of growth and the risk to property can impact the mortgage valuation, and there remains a risk of action from neighbouring owners.
In addition, buyers may want to negotiate on that lower valuation too. So if you have knotweed and you haven’t removed it, potential buyers are unlikely to want to pay as much for your house as they would if it had no knotweed. Not only that, the potential for it to cause damage to foundations will be a concern to potential buyers – as an owner, you should consider this too. Removal of knotweed entirely should always be considered rather than simply preventing spread.
How do you get rid of Japanese knotweed?
Whilst it is not dangerous, removing knotweed is almost as challenging as removing asbestos in houses. To remove it properly, you should have a professional company who understands the best ways to remove it and how to dispose of it properly.
You could use a herbicide yourself to try to calm it, but the only guaranteed way of getting rid of knotweed altogether is excavation. When this method is chosen, there are laws around the disposal of the waste. Make sure you use a reputable firm and they follow the Government regulations for disposal.
Can I remove Japanese knotweed myself?
Whilst Japanese knotweed is not dangerous, like asbestos, for example, we would still recommend that a professional service is used to get rid of knotweed. Not least because it also has to be disposed of through official routes – you can’t just take it to your local tip. You could try herbicides, but this will not be a long-term solution.
Can I sell my house if there is Japanese knotweed?
Yes, however, it is a legal obligation to declare the presence of Japanese knotweed on a property when selling it. When you complete the relevant forms (TA6 form) during the selling process, you must include the presence of knotweed if you have it. If you don’t, and it is later discovered, the buyer could take you to the civil courts. We can do a single defect inspection to look for Japanese knotweed if you are concerned.
If you’ve had Japanese knotweed in the past and had it removed, you can’t absolutely guarantee it is not present. So we suggest you answer ‘not known’ to the question “Is the property, or an area adjacent to or abutting the boundary, affected by Japanese knotweed?” That’s because if you answer ‘no’, you must be certain (Law Society emphasis) that no rhizome (root) is present in the ground of the property, or within 3m of the property boundary, even if there are no visible signs above ground.
More detailed legal information about Japanese knotweed can be found on the Environet website.
It’s worth noting that Japanese knotweed is not illegal in itself – it is just illegal not to declare it when selling your home.
Will I get a mortgage on a house with Japanese knotweed?
Typically yes, but there will be caveats. Always have a survey conducted and get specialist mortgage advice. Most lending companies will insist that you have a formal management plan with an insurance backed guarantee in place if Japanese knotweed is present.
Get expert advice from Collier Stevens
Our experienced building surveyors in Kent and building surveyors in London are here to assist you with anything you may need. For more information on Japanese knotweed, get in touch with our team or take a look at our advice hub.
Steven Way
Steven Way BSc MIHEEM MFPWS MRICS is the Practice Principal and is a Chartered Building Surveyor. With over 35 years as a practicing surveyor, Steve has a wealth of experience and expertise in both residential and commercial building surveying, party wall advice and disabled access auditing. He has also built his reputation as an expert witness.