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First time buyer

12 August 2016

Japanese Knotweed and Your Property

Japanese Knotweed first graced British soil in the late 1840s, with farmers originally buying the plant as feed for their animals. But by the turn of the 20th century its relentless nature soon became regarded as a nuisance, and it’s fair to say the plant has been wreaking havoc ever since. A fundamental valuation element in the buying and selling of a house, Japanese Knotweed has become every home owner’s nightmare. Here, Principality Building Society’s Mortgage Product Manager, Christopher Johnson, shares the need-to-know information – from mortgages to potential fines - when tackling this dreaded weed.  

What is Japanese Knotweed? 

It’s more than likely you’ll have heard of Japanese Knotweed before, but unless you’re faced with this plant in your home, or prospective home, you might not have taken much notice. Japanese Knotweed, in short, is a hugely invasive plant that can grow at the rate of a yard per week. The usual growing season for Japanese Knotweed is from April to October but mild winters and warm damp summers have seen the growing season extended. Leaves are heart shaped and flower a red or purple bud in the spring time. The plant can stand up to three metres high.  

Why is it so bad for your property?  

The wide-ranging root system, and the rate it can grow, poses a serious threat to the overall construction of any building, and specifically its foundations and drains. In fact, the damage caused by Japanese Knotweed roots and stems costs the economy around £166 million every year in weed control and property devaluation. What’s worse, the plant cannot be dug out of the ground as it is so difficult to effectively eliminate all of its roots. However spraying herbicides into its stems will eradicate the plant over the course of time – but at a price. A small patch of Japanese Knotweed can cost £1500.  

How to find out if you have it 

If you’re uncertain whether you have an outbreak on your property, there are plenty of websites which will help you identify Japanese Knotweed during different seasons, such as 

When applying for mortgage 

If found on a property, Japanese Knotweed can seriously devalue the price of the house, and in some extreme cases, may even be the reason a sale doesn’t go through. If a suspected outbreak of the plant has been flagged up on a mortgage survey, a further inspection will take place to identify the risk to the property.  

Some lenders may refuse to lend where Japanese Knotweed is present on a property, or even in the neighbouring garden. However, more often than not most will lend under the agreement that there is a treatment or management plan in place undertaken by a trained professional from the Property Care Association (PCA). This will usually require an insurance-backed guarantee. Most lenders, like Principality, will look at each case individually taking into account the valuer’s comments. A word of warning however, if you don’t actively seek the removal of Japanese Knotweed on your property, you could face receiving an ASBO (Anti-Social Behaviour Order), with a potential fine of up to £2500. 

When selling your house  

Similar action must be taken if you’re selling a house with a suspected outbreak of Japanese Knotweed. The official documents to be filled in when selling your house will include a section on Japanese Knotweed, and you must disclose whether the property is affected by the plant. As with buying a house, you must say if there is a management plan in place to deal with it and provide any necessary proof.  

What to be aware of  

There are various legal restrictions that surround Japanese Knotweed, so the safest option is to hire professional help in the treatment of the plant. When treating an outbreak, it’s incredibly important to have a guarantee that it will never return. Even after treatment, roots can remain dormant for many years but still invade even the smallest cracks in a building. With this in mind, it’s highly recommended to get an insurance backed guarantee. This will ease your mind in the long run and help satisfy your lender at the same time.  

While Japanese Knotweed is something that we’d all like to avoid in our lifetime, but at the rate it grows – there is no guarantee that you’ll never be affected. Crucially, it’s important to be honest about its presence on your property and take an active role in its treatment. If you’re unsure of the right move when it comes to treating the weed, ask the advice of your lender or a professional weed treatment company. No matter how long the process may be to completely remove it, it will be worth it knowing you won’t ever have to deal with Japanese Knotweed on your property again.  


  • Japanese Knotweed is so prevalent in the UK that records show there is not a single six-mile square patch of the country where it is not present 
  • The potential cost of trying to eradicate the plant in the UK has been estimated at more than £1.25 billion 
  • Japanese Knotweed affects more than just houses – the combined costs of removing Japanese Knotweed across the 10 acres of Olympic grounds in London cost £70 million, and four years, to do so

Published: 12/08/2016