Does Japanese knotweed cause property damage?

Environet's Managing Director, Nic Seal, comments on the latest study conducted by AECOM and the University of Leeds. 

I’ve been asked to comment on a research paper “Japanese knotweed (Fallopia japonica): An analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension”  which has resulted in some sensationalist reporting suggesting that Japanese knotweed’s capacity to cause damage to buildings is a myth. As an experienced specialist in the field of Japanese knotweed I’ve seen many a case of damage and, whilst I agree with some aspects, I have to disagree with the overall conclusion.

The document is available by following this link.

The final paragraph of the conclusion states:

While F. japonica is clearly a problematic invasive   non-native species with respect to environmental  impacts  and  land  management,  this  study  provides  evidence  that F.  japonica should not be considered any more of a risk, with respect to capacity to cause structural damage in  urban  environments,  than  a  range  of  other  species  of  plant,  and  less  so  than  many.   In this context, although the impacts of F. japonica on biodiversity and other ecosystem services remain a cause for concern, there is no evidence to support automatic mortgage restriction based on the species’ presence within 7 m of a building.

I agree that Japanese knotweed is unlikely to cause significant structural damage to buildings indirectly by subsidence, or by collapse, as in a tree-fall. Japanese knotweed does however cause damage, both by its above ground canopy exerting pressure on adjacent walls/fences, but also by its expanding network of underground rhizomes and roots, and, for mature stands, its crown.

In our experience the distance that the rhizome may be found from the visible plant (above ground) is on average 2-3m, but could be more or less depending on site conditions. Rhizome tends to reduce in diameter with distance from the crown. However these rhizomes do have the ability to exploit weaknesses in built structures, and will then expand exerting pressure on the element to cause damage. The damage will range from cosmetic, to minor and if left unchecked to significant damage which could prove costly to repair.  I agree that damage that undermines the structural stability of a property is rare, and would only occur where the problem has been ignored and as a result been exacerbated.

I have seen many cases of rhizomes growing into underground drains. They not only block the drain but eventually expand and crack them, meaning that the drain run needs to be replaced.

I’ve seen knotweed growing within cavity walls of buildings, with canes growing out through air bricks to find sunlight. The woody mass of rhizome in the cavity pushes the inner and outer skins of the wall apart resulting in a costly repair.

The most extraordinary case I’ve seen is knotweed growing out of a chimney pot on a 2 storey building. Knotweed was growing within the building’s façade, a solid old stone wall. It had then grown into the chimney flue. When inspected from above it could be seen that the entire flue was blocked with dense woody mature rhizome and crown material. The chimney and wall had to be taken down to remove the rhizome material and the wall had to be rebuilt. If that is not significant structural damage tell me what is!

The study states that knotweed does not grow through concrete, which I would agree with. However, knotweed is very successful at finding weaknesses in concrete and exploiting any cracks. It can then cause damage. I’ve seen it growing through a concrete floor at the intersection of a service duct. I’ve also seen it growing up behind skirting board where a new concrete floor for an extension abutted the original floor.

There are many cases of it growing up through and lifting asphalt. Whilst there are many plants that may be able to do same, it clearly can cause damage, an unwelcome sight on a newly laid driveway.

We created a short video to show the damage that knotweed can cause which is on the Environet website.   

I have seen reports in newspapers claiming that a property needed to be demolished as knotweed was growing under it. This is plainly not true, but in my view to say that Japanese knotweed does not cause significant damage to buildings is also not true. As usual the truth lies between these two extremes of views.

Due to the fast growing invasive and damaging effects of Japanese knotweed I believe that mortgage lenders are right to restrict lending criteria on affected properties. Their policies have probably resulted in the largest deployment of resources to eradicate the plant, described by the Environment Agency as “indisputably the UK’s most aggressive and destructive weed” – that is surely a good thing, and is more than any government body has been able to achieve to protect consumers from the knotweed menace. 

Share this post