Japanese knotweed – what do landlords need to know?

Japanese knotweed is now considered to be one of the most dreaded and invasive species of plant you can find in the UK. And awareness of this unwelcome garden guest is growing – Hamilton Fraser’s Total Landlord Insurance has seen an increase in queries relating to the weed in recent weeks.

This raised awareness of the impact of Japanese knotweed amongst landlords is good news: Disputes with neighbours when this invasive plant spreads across boundaries, along with the impact on your property’s value, can take their toll on your peace of mind, as well as your wallet, so it pays to be informed.

While it isn’t illegal to have Japanese knotweed growing on your land, if you allow it to encroach onto a neighbouring property you could be sued for the cost of treating it, as well as diminution, which could represent on average 10 per cent of the property’s value.


The state of Japanese knotweed in summer 2019

A mix of unseasonably warm weather and heavy downpours of rain last winter, followed by one of the hottest springs on record in 2019, combined to create super growing conditions for Japanese knotweed, the results of which we are now seeing this summer.

While some reports accuse Japanese knotweed of infesting Britain and wreaking havoc by ruining homes, others suggest that the plant is wrongly wiping value off property because mortgage lenders rely on ‘flawed science’. Read more about the controversial status of Japanese knotweed in our article, Is Japanese knotweed really a problem for homeowners, along with more recent coverage from Parliament that questions whether the UK approach to Japanese knotweed is ‘overly cautious’, following the recent conclusion of The Science and Technology Committee that more academic research is needed into the effects of Japanese knotweed on the built environment.

Future guidance may well change in response to ongoing research, but for now it is important that landlords can identify Japanese knotweed, are aware if their property is located in a ‘hotspot’ and know what to do if they discover that it is growing on their land.

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What is Japanese knotweed, where did it come from and how do I identify it?

Japanese knotweed is a fast-growing invasive plant with bamboo-like stems and small white flowers. It is native to Japan and considered to be an invasive species in the UK, having been introduced in the mid-nineteenth century as an ornamental plant, principally to line railway tracks in order to stabilise the soil.

The plant’s underground structures can be more extensive than the above-ground portion of the plant. Since its introduction, Japanese knotweed has become known as one of the most problematic species in the UK as it strangles other plants, often killing them off and in really severe cases taking over entire gardens. It also causes damage to building structures by targeting weak points such as cracks and attempting to grow through them.

Japanese knotweed – what do landlord’s need to know?

Japanese knotweed is relatively easy to identify, once you know what the characteristics are. These include:

  • Asparagus-like spears emerging in spring
  • Bright green shield shaped leaves that form a zig-zag shape on the stem
  • Tall green canes, with purple speckles
  • Clusters of creamy white flowers in late summer
  • Brown, brittle canes left standing in winter

Environet has produced a Japanese knotweed Identification Guide and a three minute video on How to identify Japanese knotweed to help you identify the plant in situ.

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Is your rental property in a Japanese knotweed hotspot?

An online heat map created by Environet in July 2019 shows the degree of Japanese knotweed infestation across the UK. Designed to inform homeowners and homebuyers of the local presence of knotweed and the potential risk to their property, the map has already been populated with thousands of infestations.

The map marks the most concentrated to least concentrated areas from yellow through to red to show how prolific the plant’s presence is in certain areas.

Epicentres have been located across outer London, south and west Wales, in particular between Newport and Swansea, outer Manchester, Sheffield and Nottingham.

Wales is particularly badly affected, with four towns and cities appearing in the top ten, alongside locations with a strong industrial heritage such as St Helens in Merseyside and Blackburn and Preston in Lancashire.

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

For in depth advice on what to do if you discover Japanese knotweed on your property, visit the Department for Environment, Food and Rural Affairs guidance on  how to identify, prevent spread and dispose of Japanese knotweed

The Royal Horticultural Society (RHS) advises that cultural control methods (non-weedkiller methods) pose problems when it comes to disposal, since Japanese knotweed is classed as ‘controlled waste’ under the Environmental Protection Act 1990. Due to its classification, it requires disposal at licensed landfill sites.

Specialist Japanese knotweed contractors must be registered waste carriers to safely remove the weed from a site. Most importantly, Japanese knotweed should not be included with normal household waste or put out in green waste collection schemes.

It usually takes at least three to four seasons to eradicate Japanese knotweed using weedkiller. The Royal Horticultural Society (RHS) advises that professional contractors will have access to more powerful weedkiller that may reduce this period by half. The RHS provides guidance on seeking help from professionals.