The ultimate guide to Japanese knotweed - Total Landlord Insurance

May 27, 2021
The ultimate guide to Japanese knotweed - Total Landlord Insurance

Japanese knotweed may sound innocuous enough, but it isn’t. Left undiscovered or untreated, this invasive plant can cause significant and costly damage to your property.

While there’s been much debate in recent times about whether or not it truly is harmful, the fact is that it is perceived to be so. In terms of home ownership, it’s simple: Japanese knotweed is bad news.

That’s why we’ve designed this guide to tell you everything you need to know about Japanese knotweed: from spotting and successfully combating it to staying on the right side of the law.

What is Japanese knotweed?

A perennial, bamboo-like plant, Japanese knotweed (as the name suggests) was originally native to Japan, China and Taiwan. After being intentionally brought to the UK in the early 19th century due to its attractive appearance, it quickly grew in popularity with gardeners and landscapers; this was mainly because of how quickly and densely the plant grows.

It subsequently became apparent that Japanese knotweed is actually a highly invasive and destructive plant – one which is now prevalent across the UK, particularly in London, South Wales and the South West.

In the relatively mild UK climate, Japanese knotweed can grow up to 20cm a day. Its roots can reach a depth of three metres, and a horizontal spread of up to seven metres. This causes a number of significant problems: it can suppress all other plants from growing, and, alarmingly, it can break through stone, brick, and a number of metals.

What does Japanese knotweed look like?

When you’re trying to combat Japanese knotweed, the first step has to be recognising it. The most distinctive feature of the plant is its leaves which are generally described as ‘heart’ or ‘shovel’ shaped.

However, one of the reasons it can be tricky to identify Japanese knotweed is that its appearance changes depending on the season.

During winter it’s dormant and its stems are a dark brown colour. When it begins to grow in spring, the shoots tend to be red or purple in colour and you’ll see light green leaves. New Japanese knotweed, which often appears in April, is often described as having ‘asparagus-like’ shoots. At first they look like reddish knotweed crowns and they can grow at a rate of a couple of centimetres a day.

It’s during the warmer summer months that the plant starts to grow most rapidly (up to 20cm a day in a mature plant). This is when it may begin to dominate parts of your garden. Look out for stems which look like bamboo shoots with small purple specks, and larger leaves with prominent veins.

Towards the end of the summer months you can expect to see small cream-coloured flowers on the plant. As soon as the weather becomes cooler in autumn, the leaves should begin to turn yellow and wilt.

One of the main things to look out for is size; if left unattended, Japanese knotweed can grow up to three metres.

Why is Japanese knotweed bad news for your property?

Japanese knotweed is believed to cause significant structural damage to your property, which is why it’s crucial to identify it and get rid of it quickly.

While it’s widely thought that it can penetrate concrete, this is actually untrue – but it doesn’t make the plant any less destructive.

The way in which it takes hold is by taking advantage of structural weaknesses.

Japanese knotweed is highly adept at seeking out cracks and gaps to break through and grow into. Once it finds its way into infrastructure, Japanese knotweed will cause more damage as it grows, widening gaps and, most importantly, increasing the cost of its removal.

Although it can’t penetrate concrete, in some cases it has been known to cause the surrounding concrete to shatter. It can also damage tarmac and paving areas, building foundations, retaining wall structures and underground piping and drainage systems.

Of course all of these things can drastically reduce the value of your property, as seen on The BBC’s The One Show. The programme featured a couple in Hertfordshire who sadly lost £250,000 on their family home due to damage caused by Japanese knotweed.

Another thing to consider is your mortgage. If you’re buying a property which has a problem with Japanese knotweed, some lenders may decline your application.

What should you do if you find Japanese knotweed on your land?

If you think you might have Japanese knotweed growing on your land, the first thing you need to do is find out definitively. As outlined above, the plant has a number of distinctive, seasonal features which should make it identifiable.

Identification and confirmation

If you are still unsure, you can visit the GB Non-Native Species Secretariat website which provides identification sheets to help you recognise species such as Japanese knotweed. Alternatively, you can take photographs and email them to a weed control company, who should be able to verify the plant.

Spreading the word

Once you are certain you have Japanese knotweed on your land, one thing you shouldn’t do is try to remove it yourself. You also shouldn’t think of it as being only your problem. Due to the aggressive growth of knotweed, it should be considered a community issue, as it’s likely to affect your neighbours’ gardens or the surrounding area.

That’s why we recommend you let your neighbours know. If it has infested their garden, you’ll both need to treat it. Otherwise, even if you get rid of yours, their untreated knotweed is likely to spread back through to your property.

Getting rid of it effectively

Once Japanese knotweed is established, removing it – and preventing it from regrowing – can be challenging. While treating it may seem daunting, one thing you shouldn’t do is leave it. There are a number of options for effective treatment, such as excavation of the plant and its roots, and biological or chemical control.

Whichever type of treatment you use, it won’t be a quick fix – you’ll certainly need to re-spray. It takes an average of around three years to treat Japanese knotweed until the plant becomes completely dormant.

For treatment, we highly recommend that you contact the appropriate professionals. A PCA (Property Care Association) or INNSA (Invasive Non Native Specialists Association) approved contractor will be able to recommend which treatment is suitable. They should be able to carry out the treatment so that, in time, it’s permanently eradicated from your garden.

What does the law say regarding Japanese knotweed?

If you know you have Japanese knotweed, you are not legally obliged to get rid of it. However, if you allow it to spread, you could be breaking the law. Invasive non-native plants have now been included as part of the Anti-Social Behaviour, Crime and Policing Act 2014 (more commonly known as an ASBO).

Risk of prosecution

If you’re found to be knowingly allowing it to spread, you could find yourself being prosecuted. And given that Japanese knotweed is nothing if not aggressive in growth and spread, the chances of it spreading are almost certain.

Selling your property

If you’re selling your property, the legislation is clear: you are required to fill in a TA6 form (used for conveyancing), stating whether or not Japanese knotweed is present on your land. If it is, you will need to guarantee that it will be treated prior to selling.

Buying a property

If you’re buying, the presence of Japanese knotweed could impact your lender’s decision.

In most cases your mortgage lender will still grant your mortgage, as long as they have assurance that the plant will be eradicated. A management plan by a professional eradication company, backed by a transferable guarantee, will usually suffice. It is the seller’s responsibility to organise this.

Don’t leave anything to chance

On both sides, whether you’re buying or selling, we recommend that you check the property thoroughly for Japanese knotweed.

If you’re buying, a surveyor’s report from a recognised governing body, such as the Royal Institution for Chartered Surveyors, will ensure you avoid any future nasty surprises. This will also help to prevent any future disputes or claims over unforeseen costs or even misrepresentation claims.

Where are the Japanese knotweed hotspots?

There are a few Japanese knotweed hotspots in the UK.

The most notable areas are Greater London, the South West, Wales, and, in more recent years, parts of Scotland.

To find out how many reported cases of Japanese knotweed there are in your area, you can use this heatmap to search by postcode.

How can you reduce the risk and spread of Japanese knotweed?

Apart from damage to your property, one of the reasons it’s important to reduce the spread of Japanese knotweed is the threat to the local ecology and environment.

The Wildlife and Countryside Act, 1981, states that it is an offence to cause Japanese knotweed to spread.

Apart from informing neighbours and anyone who could potentially be affected, you should also think about how you dispose of Japanese knotweed once it’s been removed. This is crucial for reducing further spread.

Individuals can be fined up to £5,000 or sent to prison for up to two years if contaminated soil or plant material from any waste you transfer spreads into the wild.

How to get rid of Japanese knotweed

Can you compost it?

Unlike most other plants, Japanese knotweed is not compostable. It’s virulent nature means it’s highly likely to survive the composting process, allowing it to infest areas where the compost is used.

Can you bury it?

If you want to bury the plant, you must check with the Environment Agency to see if this is permitted.

In most cases, this is only allowed at a landfill site with a specific permit. If you are allowed to bury it, it must be buried without any other waste at a depth of at least five metres. It should also be covered with a material that cannot be penetrated by plant growth.

Can you burn it?

Burning is another option for disposal. Again, you will need to notify the Environmental Agency if you plan to do this.

While it might seem that burning is the best option, it should be noted that Japanese knotweed crowns and rhizomes can actually survive fire. Any remaining matter will need to be disposed of off-site or buried.

Off-site disposal

If you need to dispose of any plant matter off-site, you need to check that the landfill site is authorised to receive this type of waste. You should also use a registered waste-carrier to transport it.

If you’re using a specialist to treat the knotweed, they should also offer a complete excavation and disposal service. While this will be a significantly more expensive option, it does mean all the logistics and red tape will be taken care of.

What are the risks of ignoring Japanese knotweed?

The risks of ignoring Japanese knotweed fall into three distinct categories: personal, financial, and legal. All three of these factors are of course interconnected.


If left unchecked, it’s not too dramatic to say Japanese knotweed can destroy your home.

Apart from damaging the external structure of your home, it can end up invading the interior as well. It can grow through floor boards and walls – anywhere it finds a gap. The longer you leave it, the harder and more costly it will be to eradicate.

The gravity of the problem is reflected by how seriously it’s viewed by mortgage providers. As mentioned above, people have lost hundreds of thousands of pounds from their property value due to Japanese knotweed, which leads us on to...


Treating Japanese knotweed is not cheap. However, when you consider how much this plant can devalue your home – or the potential legal costs you could incur, treating it swiftly is definitely the cheaper option.

If you’re selling your home, it’s imperative you check thoroughly and act accordingly. Otherwise, you could end up with a property worth half as much as you thought. Likewise, if you’re buying, you could inherit a costly and devastating problem.


As stated previously, it is not illegal to ignore Japanese knotweed but you could still end up facing prosecution.

If you allow it to spread to your neighbours’ gardens or the surrounding areas, you could find yourself with an ASBO or a hefty fine. Failure to dispose of the plant safely could also lead to you incurring a fine or a custodial sentence.

Does my insurance cover Japanese knotweed?

In a word, no.

While there may be the odd provider who offers cover against Japanese knotweed, the vast majority do not. This is primarily because most insurance policies don’t include damage that happens gradually over time.

The fact that your insurance is unlikely to foot the bill, is another reason why it’s essential to procure a thorough surveyor’s report in order to identify the plant prior to purchasing a property.

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