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15 Aug 2019 < Back

We Need to Talk About Japanese Knotweed

We Need to Talk About Japanese Knotweed

For those of you that may not know, Japanese knotweed is now a well established and increasing known invasive plant that can be commonly found in residential surveys.

For those wondering “what the heck am I talking about“, a simple ‘Google’ search will instantly throw up a wide variety of webpages, press and other articles highlighting three key impacts:

  1. Damage
  2. Diminution
  3. It is very difficult to get rid of!

Separating knotweed fact from fiction!

All three aspects are a major concern to homeowners and mortgage lenders, but does the rhetoric about Japanese knotweed truly reflect its real impact on buildings? Following recent discussions at a ‘Commons Select Committee,’ our Technical Manager Dr Peter Fitzsimons tries to separate fact from fiction!

Why does knotweed damage get ‘special treatment’?

Many plants are capable of causing damage to masonry yet none of these plants ever get the headlines and attention of Japanese knotweed.  Why exactly is this???

Apart from large trees that are also a matter of concern when close to foundations, Japanese knotweed gets ‘special treatment’ by mortgage lenders? Japanese knotweed is not a woody plant but does produce shoots which, by sheer force of cell pressure (turgidity), are capable of breaching hard surfaces such as tarmac and paving and this includes shallow raft foundations too.

Let’s be clear on the knotweed threat!

Now let me be clear. We are not trying to say this is a real-life version of ‘Day of the Triffids’.  All the plant is doing is exploiting existing weaknesses in these materials.

However, once the Japanese knotweed has established itself, the stems will become semi-woody/fibrous and expand to a diameter of up to 35 mm (1.5”). At the same time, the knotweed root (or ‘rhizome’ as it’s known) that produced the shoots also expands and hardens and causes local ground heave to occur, leading to more disruption and nuisance.

Left for long enough each group of stems will produce a ‘crown’ in the soil that is a dense structure capable of pushing over boundary walls and other similar obstacles. Existing buildings are usually resistant to direct displacement, but any extensions or new buildings over Japanese knotweed infected soil will usually get invaded by knotweed shoots growing up through cavities in the search for light.

Japanese knotweed and property values

The key reason why property value is diminished by the presence of Japanese knotweed is due to the various elements of damage that can arise.

The plants invasive tendencies make it very difficult and expensive to control. A new factor has emerged recently following a High Court judgement in favour of a plaintiff who claimed damages against Network Rail for failing to control Japanese knotweed on their land.

The ruling was based on ‘loss of amenity’ which in plain language means the plaintiff was unable to enjoy the use of their land without disturbance and if they chose to build, could only do so after incurring significant additional costs to remediate the soil. As a consequence, any property owner (or prospective buyer) is potentially likely to be ‘on alert’ for liability issues that may arise where Japanese knotweed is present near any property boundary.

Is knotweed just not another stubborn plant?!?

Japanese knotweed has a reputation for being intransigent and persistent however, some would say knotweed is ‘just another plant’ and is therefore relatively straightforward to control. Who is right?

Well, the shoots/and leaves can be cut down and are susceptible to herbicide treatment which helps. However, there are two fundamental problems that stop most gardeners and even some contractors getting Japanese knotweed under control:

  • Firstly, as it is a perennial plant, cutting is never effective (even when repeated year-on-year) and gives a disposal problem (the green waste is a potential new source of infection).
  • Secondlyherbicide selection is critical – a systemic herbicide is always needed to ensure translocation to the roots too and the treatment needs to continue over many years. Long-term planning is required to complete a JKW management plan!

It is the root which gives Japanese knotweed its reputation

The features of Japanese knotweed that really underpins its reputation is the rhizomatous root system. In a ‘nut-shell, this means the plant can spread through the soil and produce new growth to expand the overall canopy.

In effect, each part of the root network is a potential new plant. When we say ‘do not disturb’ we really mean it! Contractors that specialise in soil excavation have to operate tight biosecurity protocols to ensure every last bit of the knotweed rhizome is taken offsite, rather than dropping off the shovel of the digger to start a new colony in future years.

PCA Members offer Professional Advice

The ‘Commons Select Committee’ have now published their report and for those that wish, you can read the summary and the in-depth report via our recent blog – Science & Tech Committee Knotweed Report.

Various stakeholders were asked to look at their guidance and processes to ensure that the advice being given to professionals like valuation surveyors is reasonable and proportionate. The PCA and its members have always advocated an approach to Japanese knotweed control which is based on science and experience and, of course, legal responsibilities. Unfortunately, the one thing we cannot change is the genetic pre-programming of the plant that is the focus of so much anxiety.

For those that are affected by Japanese knotweed or work on behalf of individuals affected by knotweed, our advice is simple! Always get professional advice, use experienced and qualified contractors and ensure all works are covered by warranties underwritten by appropriate insurance. In the end, with this approach, we may yet win the battle.


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