Japanese knotweed – It’s what you can’t see you should worry about!

For those who sometimes wonder ‘what’s all the fuss about’ with Japanese knotweed, hopefully this blog will go some way to explaining it further. We are used to hearing all about the damage its been causing and we usually refer to its amazing growth characteristics – self-evident to anyone who has ever had this plant on their land. Who would ever have this on their land!

This is ONLY part of the story…

But this is only half the story. In this article we want to say a few words about what’s going on in the soil….and how this has a crucial impact on understanding recent legal cases regarding neighbour disputes under the common law of ‘nuisance’ (Loss of Amenity). Also, the desirability/importance of contractors, their clients and their client’s neighbours embracing collective responsibility for dealing with Japanese knotweed when it sits on or close to boundaries.

Most of our members will be familiar with these issues, but there is something here for all professionals involved in Japanese knotweed management and hopefully will also help non-professionals gain a better understanding of the iceberg principle that applies to Japanese knotweed – there is more below the surface than above it!

The notoriety of Japanese Knotweed

Japanese knotweed has become notorious for its impact in the wild and in urban settings through nuisance impacts and damage to masonry/tarmac/concrete. ‘Notorious’ is a word we choose carefully at the PCA as the press certainly like to write about it, and why not! It is a brute of a plant, growing faster and taller than any native species and with shoots and roots/rhizomes capable of growing through cracks in masonry which are almost invisible to the naked eye (in search of light).

Given the degree of alarm these features have caused in the domestic and professional property sectors it is a source of pride that the PCA and its members have, since March 2012, been in the ‘vanguard’ of Japanese knotweed control responses – through professional, reliable, guaranteed knotweed management plans that when first introduced, resolved many of the complex conveyancing issues prevailing at the time and still do today.

OK…but why Knotweed buffer zones?

So…getting back to the original point…what has that got to do with buffer zones?!? Japanese knotweed has evolved to rapidly colonise new sites and has a radiating rhizome (underground shoots) network which can spread several meters away from the edge of a ‘stand’ of knotweed canes. The likely soil area impacted by rhizomes is usually called the buffer zone (or ‘impacted area’). From a PCA perspective, this is defined within the Code of Practice “Management of Japanese Knotweed as:

“The radius of an appropriate buffer zone will depend on the site and the types of properties present and is intended to help identify areas with potential rhizome growth or the area within which precautions are warranted, e.g. erection of fencing.”

We hope the following will help to emphasise the importance of marking buffer zones on survey reports and management plans, especially when/where the knotweed is on or close to a site boundary.

Why bother mapping out the knotweed area?

One of the most important parts of a Japanese knotweed management plan is a clear, scaled drawing or map/plan of the site showing the location of the knotweed. Most do a good job either using off-the-shelf drawing software packages or sometimes a marked-up aerial photo (although this usually limits the definition/detail). What is sometimes missing is the supplementary detail regarding so-called buffer zones – the soil area likely to be impacted by knotweed rhizome extension. We need to review the reasons why this is important but first let’s just ask the question “how big should the buffer zone be?”

So…what is the Buffer Zone Guidance

Since the publication of the Environment Agency’s Code of Practice in 2006 (‘withdrawn’ but still accessible) and the development of a formal knotweed impact Risk Assessment framework (RICS Information Paper 27/2012), a buffer zone of 7 metres has been typically used as a ‘rule-of-thumb’. This was challenged in 2018 (published research) and 7m found to be something more akin to a worst-case-scenario leading to a Commons Select Committee Inquiry Report and a recommendation that the 7m-rule be reviewed.

Some progress has been made towards achieving a new consensus (7m+ may well be uncommon but not unknown) but many companies have already responded to the above publications and implemented their own buffer zone guidance. Typically, this may be a ‘high probability’ zone from 0 to 3 m from the edge of the stand and a ‘low probability’ zone from 3 to 7 m. This seems a good compromise; continuing to provide guidance within the existing framework for lenders/valuation surveyors whilst providing the client with an appreciation of the most likely extent of rhizome extension (particularly useful if they are contemplating an excavation programme where costs vary closely in relation to the volume of soil to be removed or processed).

Including a 3m buffer zone also has another use in plans/drawings. The new TA6 ‘Home Information’ form used by the Law Society for conveyancing purposes now asks the home owner to state if knotweed is affecting the property AND/OR the adjoining land within 3m.

Why do Buffer Zones matter?

It is stating the obvious to some extent, but indicating buffer zones on a plan/map gives us the opportunity to explain to the client that Japanese knotweed is like an iceberg and the bit we can see above ground level may only represent 20% or so of the total biomass of the plant. Given that each tiny fragment of the rhizome network (Knotweed Research Around the World) can lead to a new plant growing elsewhere, the client needs to be made aware that the area of knotweed they can see is only a part of the overall picture.

What are the Practical Implications?

  1. The ‘Treatment Area’ of a Japanese knotweed Management Plan. When taking in to account the buffer zone the extent to which a residential garden may have to be segregated can be considerable. This is something that needs to be discussed with the client so they fully appreciate the co-operation required to ensure that any knotweed shoots emerging within the buffer zone are not disturbed and the area remains free from other vegetation (as far as possible) to facilitate detection.
  2. The Guarantee area. The standard Guarantee default (mirrored in IBGs) is that the client is assured that any re-growth of knotweed within the 7m zone is covered even though treatments may never have extended beyond the original green footprint. By extrapolation this means that (for herbicide treatment plans) the client is asked to avoid digging the area for approx. 15 years (during the treatment plan and for 10 years after)! If they are keen gardeners this may be an unacceptable restriction so make sure these matters are discussed before you start.
  3. Boundaries and neighbours. It is always desirable to engage through your client with neighbours where knotweed is straddling or growing close to fences etc. If this can be achieved amicably then the Management plan can proceed normally albeit with some logistical complications (and two, three or four or more copies of paperwork if each client wants a Guarantee/IBG). But it seems quite common to find that a neighbour may not co-operate even when not required to contribute to costs. Clearly this represents a major restriction to you and your client and this MUST be addressed.

The simple solution (but not really a solution at all) is to restrict the scope of the guarantee so that future encroachment is not covered but this is hardly the ‘certain outcome’ that mortgage lenders seek when they request evidence of a professional/insured Guarantee.

Better is to discuss the use of vertical root barrier membranes (Root Barrier and Japanese Knotweed Remediation) and/or complete excavation which for small stands of Japanese Knotweed can sometimes be achieved with mini diggers plus some manual spadework. More expensive of course but perhaps the only viable alternative when neighbours are determined to be uncooperative.

Final thoughts

For many of our PCA members, a lot of the advice given above is guidance you will recognise from our “Code of Practice “Management of Japanese Knotweed” but we are bringing focus to these issues as they are crucial to understanding the new TA6 form Guidance Notes and will inform the up-coming review of the RICS Risk Assessment protocols (we hope to bring you news on this before too long).

To summarise, Japanese Knotweed surveyors need to use their ‘Spidey senses’, a bit of imaginary ground-penetrating radar as-it-were, when mapping knotweed stands and do all that they can to convey the rhizosphere knotweed map in their minds to their client!

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