New RICS Guidance on Japanese Knotweed  

Japanese Knotweed sends shivers down the spine of home owners, fast spreading, a blight on property and a red-flag to lenders. However the RICS have issued new guidance to re-balance the perception of the species and control outbreaks. Chris Drake from our Building Surveying department explains.

New RICS Guidance on Japanese Knotweed  

image above - Japanese Knotweed in Spring, showing new shoots resembling asparagus emerging from the ground, the shoots are reddish in colour

Japanese Knotweed is a hardy perennial plant that can grow up to 10cm per day and can reach a height of up to 2 metres.  The plant spreads through underground Rhizomes that form thick clumps just below the surface.  One of the difficulties faced by landowners who find Japanese Knotweed on their property is that the plant can spread easily and establish quickly from just a small portion of Rhizome and will quickly outcompete all other native plant species in the area.  This propensity to spread means that material containing Japanese Knotweed is likely to be classified as controlled waste under the Environmental Protection Act 1990.

Summer – Distinctive shield shaped leaves with purple zigzag stems. 

Japanese Knotweed has been the cause of problems within the residential property market due to the plant’s ability take over garden areas and disrupt patios and other hard landscaping and even disrupt lightweight structures like garden walls.  However, the reputation of Japanese Knotweed as a wrecking ball that will cause substantial damage to a residential dwelling may have been exaggerated out of proportion in the past. 

Flowers appear during mid-summer.

Recent studies have concluded that Japanese knotweed rarely causes structural damage to substantial buildings such as dwellings. Large clumps of Japanese knotweed, if left uncontrolled, can damage lightweight garden walls, paths, drains and other landscaping features but, even in close proximity to a dwelling, Japanese knotweed is not typically associated with major issues such as subsidence, heave or impact damage.

The RICS have this year released a new Guidance Note “Japanese Knotweed and Residential Property” to try and re-balance our perception of the species, and encourage a more 'management-based' approach to control an infestation, rather than eradicate it all together which can be both costly and disruptive.  The Guidance Note replaces the previous Information Paper released in 2012, which introduced a measuring metric to assess the risk to property if Japanese Knotweed was present within 7 meters of a dwelling or boundary.  Up to now this has been the basis for mortgage lending against properties found to have Japanese Knotweed nearby.

The recent 2022 Guidance note considers recent research and experience to re-classify the previous 4 “Risk” categories used in 2012 into 5 new “Action and Management” categories with emphasis on outbreak management.

Management Category A: Action

Visible damage has been caused which is likely to affect mortgage lending. A specialist survey should be undertaken and works completed under a Japanese Knotweed management programme. The value of a property may be affected.

Management Category B: Action

Japanese Knotweed has affected the use of an amenity space, i.e. a garden which is likely to affect mortgage lending. A specialist survey should be undertaken and works completed under a Japanese Knotweed management programme. The value of a property may be affected.

Management Category C: Manage

Japanese Knotweed is present at the site, however, no damage to a structure or affect to amenity space is observed.  It is unlikely that mortgage lending will be affected.  The value of a property should not be impacted other than for the cost of remedial management works.

Management Category D: Report

Japanese Knotweed is noted within 3 meters outside of the site boundary.  It is unlikely that mortgage lending will be affected.  The value of the property should not be affected.  It would be good practice to engage with a Japanese Knotweed specialist to suggest preventative measures to limit the spread of an infestation onto the property.

Record, but no action required

Japanese Knotweed is more than 3 meters away from a site boundary.  The location of the knotweed should be recorded.  Mortgage lending should not be affected.  The value of a property should not be affected.  It would be good practice to engage with a Japanese Knotweed specialist to suggest preventative measures to limit the spread of an infestation onto the property.

Autumn / Winter – Leaves turn orange and then eventually brown

By acknowledging when Japanese Knotweed is present within the boundaries of a residential dwelling and assessing the impact to the property and amenity space, remedial management plans can be introduced so that the plants presence need not be an immediate deal breaker when it come to the sale or purchase of a property.

We suggest that if Japanese Knotweed is suspected within close proximity to your property or boundary that specialist advice is sought from an improved inspector.  Currently the Property Care Association (PCA) and Non Native Specialists Association (INNSA) accredit approved Japanese Knotweed inspectors who can provide advice on management plans for controlling an ongoing or potential outbreak.

Symonds & Sampson's Building Survey & Design department offer a wide range of services to property purchasers, developers and owners across the region, contact Chris or his colleagues at our Wimborne office on 01202 882103 or find details here

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