Dr Paul Beckett 28 February 2017

Japanese knotweed: debunking the myths

Media interest has recently peaked around a story picked up by some of the most popular daily newspapers about Japanese knotweed affecting a family home in Birmingham. The claims are that the family are unable to sell their property due to the adjacent housing association property being overrun with Japanese knotweed.

Japanese knotweed routinely makes such headlines due to its ubiquity in urban areas and its potentially damaging effect on buildings. However, much of this is considerably overhyped and far from the truth. That being said, the perception of harm caused by the presence of Japanese knotweed in a neighbourhood does cause a significant blighting effect, which professional bodies like the Property Care Association (PCA) are trying hard to counter.

The Birmingham story claims that Japanese knotweed damages the foundations of buildings and can grow through concrete. This simply is not true. Japanese knotweed cannot grow through an impermeable surface like concrete. In fact, professionals often use thin plastic sheeting (albeit reinforced to prevent it tearing during handling) as a root barrier to prevent knotweed spreading from one area to another.

Japanese knotweed can, however, exploit gaps or joins in hard surfaces like paving or old brickwork, and grow between them. The roots and stems can then exert considerable force over years as they expand within these gaps, potentially causing heaved walkways and buckled walls.

So, in the vast majority of Japanese knotweed cases, no structural harm is done by the growth. In fact the type of damage described above is more often caused by many other plants that have deep or woody roots. Insurance claims and remedial works due to subsidence from tree roots are much more significant that any such issues caused by knotweed.

The main impact on property caused by this invasive weed is the blighting effect that results from people’s perceived fears of what the plant might be able to do their property.

As a result, its presence can significantly affect the purchasing choice of home buyers and there is debate amongst property valuation surveyors, much of this tested through civil litigation, on what this choice does to a property’s value.

Essentially, if a prospective purchaser were presented with two identical properties, save for one being affected by Japanese knotweed, they will always choose to buy the Japanese knotweed-free property. The diminution in value caused to the affected property is the amount that it would need to be reduced in order to balance the prospective purchaser’s decision between that and the Japanese knotweed-free property. This is quite a subjective exercise that is difficult to calculate fairly. However, some valuers report reductions in property values due to Japanese knotweed being between 5% and 20% of the property’s value.

So, should local authorities and housing associations disregard Japanese knotweed property risks?

The short answer is no. Unfortunately, the effect of knotweed blight is significant in term of potential impact on property value. That 5% to 20% of a property’s value can amount to tens or hundreds of thousands of pounds.

The issue is further exacerbated by the fact that knotweed located on adjacent properties can have this devaluing effect too. The RICS guidance states that knotweed can affect a property if it is located within 7m on adjacent land. Therefore a claim can be made against a landowner, such as a local authority or adjacent residences (including housing association properties) if knotweed on their land is within 7m of adjacent property and is deemed to affect the dwellings located there.

This being said, the solutions that have been put in place precisely to deal with the effect of Japanese knotweed on property are not necessarily that expensive – which is another myth busted from the recent press stories. As long as it is undertaken by an appropriately qualified PCA member, applications of herbicide to the stems and leaves of the plants is usually the most cost-effective solution to treat significant infestations.

What about the law – other than litigation?

It is not illegal to allow Japanese knotweed to grow on your land. However, it can be a criminal offence under the Wildlife and Countryside Act to allow plants to spread from your site into the countryside. This law was made in 1981 with the aim of protecting ecologically valuable habitat. As the issue with knotweed has more recently developed into one affecting people and property, laws have changed to focus on measures to deal with the unreasonableness of landowners allowing knotweed on their property to adversely affect their neighbours.

The main mechanism for this is the issuing of Community Protection Notices by the police or local authorities as part of anti-social behaviour law.

However, there are also mechanisms aimed at wider impacts, particularly where ecology is affected, through the 2015 Infrastructure Act, where Species Control Orders can be issued to force a recalcitrant landowner to effectively deal with their knotweed infestation.

So although much has been exaggerated in the recent press interest surrounding the family home in Birmingham, the take-home truth from the story is that unless you deal with any Japanese knotweed infestations effectively, you could be asking for trouble by needlessly exposing yourself to significant risk. This could be particularly galling when the remedial works to treat the problem are relatively cheap.

It is therefore vital that local authorities and housing associations take steps to identify knotweed risks and are proactive in resolving any issues they find. If they don’t, they could find themselves joining the ranks of defendants in the burgeoning knotweed litigation market.

Dr Paul Beckett is a member of the Property Care Association.

This feature first appeared in Local Government News magazine - register here for your free copy.

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