Dear Barry

Ask Barry Image

If you have even a passing interest in property investment or development, then the words "Japanese knotweed" should make your blood run cold.

This invasive, bamboo-like plant is property kryptonite because it literally grows through buildings that get in its way - not even concrete floors can stop it thriving. 

Having recently assisted an investor with this issue, I thought it would be helpful to put together a comprehensive guide to this thorny issue...

It is so destructive - and so easily spread - that it is a punishable criminal offence to dispose of it unless you are a certified professional. 

What is it?

It is a non-native, highly invasive species of plant that was introduced to the UK from Asia in the 19th century as a decorative perennial plant. It was not known then how destructive it was - today, it is regarded as one of the UK's most invasive species.

Its appearance changes according to the time of year. The following information is from the NI Environmental Agency leaflet on the plant:

Spring / summer

  • Grows up to 3 metres in height forming very dense stands.
  • Small red shoots in April as the knotweed emerges.
  • Red / purple coloured stems.
  • Leaves arranged in a zigzag pattern along the stem.
  • Creamy white flowers in clusters in late summer.
  • Hollow stems.
  • Distinctive orange centred rhizome (root).

Autumn / winter

  • Brown stems lying or standing.
  • Evidence of flowers still on stems (brown coloured).
  • Distinctive orange centred rhizome (root).
  • The distinctive rhizome (root) system often extents to approximately
    7 metres horizontally from the parent plant and approximately 3
    metres vertically in depth below the parent plant.

Can you sell a house with Japanese Knotweed on site / nearby?

Our surveyor at Survey Services NI recently identified suspected knotweed on some property belonging to one of our landlords. They engaged AES Marconi, a local environmental control specialist, for a full report and treatment plan.

I contacted Alistair Bushe from the company to explore further the implications of this invasive plant for a property vendor.

He told me: "We would be involved in a number of property sales where Japanese knotweed is present. The success of the sale varies down to each individual lender's viewpoint.

"However," Alistair continued, "having a management or eradication plan in place will often result in the sale concluding. My understanding is that you should disclose all knowledge of Japanese Knotweed during a sale of the property."

So, from a legal point of view, you are allowed to own Japanese Knotweed and having it present on your property is not an offence. Knowingly allowing it to spread into an adjoining property or disposing of it improperly would, however, both count as offences.

The Wildlife Order (NI) 1985, Article 15, Schedule 9, Part II states: "Subject to the provisions of this Part, if any person plants or otherwise causes to grow in the wild any plant which is included in Part II of Schedule 9 shall be guilty of an offence."

As Japanese Knotweed is a Schedule 9 listed invasive plant species, a landowner who knowingly allowed the plant to spread from their property into another would be breaking the law.

The Controlled Waste (Duty of Care) Regulations (Northern Ireland) 2002 is the other key piece of legislation here, placing a legal responsibility on anyone who produces, imports, stores, transports, treats, recycles or disposes of waste to take the necessary steps to keep it safe and to prevent it from causing harm, especially to the environment or to human health.

How do you get rid of it?

It is possible, although it is a difficult, lengthy and expensive process and should be carried out by a professional due to the legal implications of incorrect disposal. 

Treatment is usually carried out in stages over a period of a number of years, normally involving a twice-yearly injection of herbicide, deep into the stems of the plant, over a period of around five years. Treatments usually cost around £250 a pop, so a complete course of eradication could set you back £2,500.

The effect on property values

An infestation of Japanese knotweed on a property's land is not a desirable situation, but it doesn't need to be a dealbreaker.

As a seller, you could take the cost of treatment off the asking price (so potentially losing £2,500) or present the potential buyer with the evidence that you have entered into a contract commitment to have the plant eradicated by a trusted contractor.

A savvy investor will always view a property, inside and out, and ideally get an experienced chartered surveyor to cast an expert eye around the place before thinking about buying.

If you or your surveyor suspects Japanese knotweed, get a full structural survey to see if any damage has been done to the building already. A mortgage lender will require a full report before they can make a lending decision.

How can I find out if a potential investment is affected by nearby knotweed?

Although you're not allowed to let Japanese knotweed spread, and you must dispose it properly, there is no legal requirement to notify the authorities of its presence if you find it on your land. Therefore, there is no official central 'register' you can check for hotspots.

Local knowledge can pay dividends here. Ask around - anyone you know and trust with knowledge and experience of the local property market should be able to give you a heads up on any knotweed hotspots.


I really hope this has been a helpful guide. Have you ever had to deal with Japanese knotweed? Let me know your experiences by emailing or join the conversation over on Facebook or Twitter.