Described by the Environment Agency as the most invasive plant species in Britain, Japanese knotweed is a large, non-native herbaceous perennial that fills home and land owners alike with dread.

This fearsome plant has a wide-ranging root system that extends to three metres in depth and seven metres across. At its most prolific, it can spread up to 20 centimetres a day. It can penetrate and grow through concrete, brick and tarmac – and potentially reduce the value of your property – particularly if it pops up in your living room!

When selling a property, you have to declare if there is any Japanese knotweed on your land. If so, mortgage lenders may refuse to lend on it or require a costly treatment plan to be put into place.

What does it look like?

Japanese knotweed has large heart- or spade-shaped green leaves arranged in a zigzag pattern along its hollow stem. It produces clusters of cream flowers towards the end of July.

The plant produces fleshy red tinged shoots when it first breaks through the ground in spring and summer and then dies between September and November leaving brown stems.

What can you do about it?

  • Remove it – but be careful not to leave behind any trace as even 0.8 gram of a root can generate a new plant. Once dug up, it is classed as controlled waste and can only be disposed of at licenced landfill sites. Alternatively, it can be dried out and burned.
  • Treat it – but it can take up to five years and thousands of pounds for the specialist treatment to be fully effective – and it has to be repeated several times. There are less powerful products containing glyphosate herbicides which you can use to treat it yourself but they are not as effective on a large, well-established spread.
  • Use a biological control like plant sucker Psyllid. Currently in trial stages, it is not yet available to gardeners but, if it becomes successful, it will be released more widely in the UK over the next five to ten years.

Ignore it at your peril

Although it is not an offence to have Japanese knotweed on your land and it is not a notifiable weed, if you fail to deal with it, local councils and/or the police can issue anti-social behaviour notices (ASBO) under the Anti-Social Behaviour, Crime and Policing Act 2014.

The Council of Mortgage Lenders (CML) and the Royal Institute of Chartered Surveyors (RICS) have issued guidance to address the lack of understanding about the problem with Japanese knotweed and to ensure that homeowners are fully advised when looking to buy or sell property where the plant is a potential issue.

If you have Japanese knotweed on the property, our advice is to get it checked out and treated and to disclose it in the conveyancing process. A buyer should make the necessary investigations to be fully informed and ensure mortgage funding. Factor in the cost of treatment if necessary or ask the sellers to deal with it before completion.