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Japanese Knotweed

Japanese knotweed not as bad as feared, according to surveyor institution

“…knotweed isn’t a death sentence for home sales…it’s certainly not the ‘bogey plant’ that some make it out to be.” – Philip Santo FRICS

There has recently been good news for homeowners affected by Japanese knotweed, following the issue of updated draft guidance by the RICS (the institution that governs professional building surveyors). This is the first update in almost a decade, and marks a shift towards a more reassuring attitude towards this invasive plant.

Instead of writing off properties that are affected, the latest guidance suggests that in many cases, Japanese knotweed can be effectively managed. Importantly the author of the RICS report has stated that finding Japanese knotweed should not necessarily derail house transactions.


The true impact of Japanese knotweed: reality vs fear

The RICS report suggests that a lot of previous reports of the risks posed by the plant are disproportionate, even amounting to scaremongering.

The report points out that it is now widely accepted that Japanese knotweed cannot grow through intact concrete. New evidence has been used to redefine the distance from which Japanese knotweed is considered to be risk to structures – this has been revised down, from 7 metres to just three.

That is not to say that Japanese knotweed is not a problem. It grows very aggressively, and like other plants it takes the path of least resistance. While it will not break intact concrete, it can exploit cracks and gaps in structures, worsening existing damage. If allowed to grow unchecked, the large root system can also block drains and damage lightweight structures.

There is also a legal obligation for affected homeowners to prevent Japanese knotweed spreading to the wild or to neighbouring properties, and to declare it to prospective purchasers.

The impact of Japanese knotweed will vary depending on where the plant is. As an invasive species, it needs to be controlled or removed. If it is in one corner of a large plot, excavating it or treating it with herbicides (which can take several years of treatment) might have almost no impact on the use and enjoyment of the garden. If it is the middle of a small garden, however, ongoing herbicide treatment will have a large impact on the garden’s use over the course of the treatment.


Effect on property value

Here, there is more good news. The latest guidance strongly suggests that RICS surveyors should be recommending that control and management of Japanese knotweed, rather than costly removal, is best option for most property owners. This in turn reduces the costs associated with a knotweed discovery, and will hopefully reduce its impact on market value.

The latest RICS draft guidance also provides a framework that they hope lenders will adopt to make knotweed-related mortgage decisions more consistent across the market.

The guidance suggests that if Japanese knotweed is not causing visible damage and is unlikely to prevent or restrict use of a space, then lenders should not require specialist reports and it should not affect lending. The RICS point to the fact that lenders routinely accept the risks posed by large trees and other plants.

While there will always be some effect on property value, it is hoped that this new guidance will give people a more balanced, realistic view of the risks that Japanese knotweed poses. If this advice is taken on board, the reduction in property price will, in time, become less dramatic and more in line with the actual costs of Japanese knotweed management or removal.

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