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Japanese Knotweed Inspections

For professional surveyors and property or construction specialists that suspect or come across Japanese Knotweed, requesting an inspection to establish the presence (or otherwise) of this invasive weed is a critical step. 

Having such inspections carried out by a suitably qualified specialist is critical; they can give not just a confirmation of what is present (including, if requested, other invasive species too), but also a detailed description of the japanese knotweed management options available, something which depends on the client’s own plans for their property or site.  

On development sites in particular, invasive weed inspections can also help to establish, very early in the planning process, the optimum approach which satisfies all the needs of the client but at ‘best cost’ and lowest environmental impact.

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Japanese Knotweed Identification - Oct to Nov growth

Key inspection considerations when it comes to Japanese Knotweed 

So what are the ‘key’ features of a professional site inspection for Japanese knotweed? For property professionals and homeowners alike, listed below are some key points you should expect to discuss with the qualified specialist you bring on site assuming you are unfortunate enough to have Japanese knotweed on your land!

+ Identification and Confirmation

Qualified Japanese knotweed surveyors are aware of their duties and responsibilities to their client and others who may rely on their report (i.e mortgage lenders). So the first thing they are likely to tell you (somewhat counter-intuitively) is what they CAN’T see (usually within a section of their report called ‘Limitations’).  Japanese knotweed is a plant that (to the trained eye) should be ‘easy’ to see and identify.  But there are a multitude of reasons why it may be ‘present’ above ground, but not visible, for example; through prior works such as. cane removal, ground clearance or lack of access on overgrown sites.  

Crucially, unless otherwise specified on instruction, most PCA members will assume the site inspection is by visual means-only.  This means that dormant knotweed rhizomes in the soil can be missed (e.g. a winter survey or knotweed not growing due to past treatments or, worse, deliberately hidden). We recommend clients discuss the value/need for some trial pits when giving instructions.  In certain situations, a hybrid approach can work well identifying critical areas on the site and conducting soil investigations ‘as required’ rather than a scatter-gun approach.

+ If Japanese Knotweed is found during the inspection

If Japanese knotweed has been found during the inspection, it needs to be clearly and accurately mapped.  Such ‘plans’ are an important marker for future reference by a variety of stakeholders and other property related professionals!  We recommend such plans show not just the Japanese knotweed visible, but indicate the likely and ‘worst-case’ spread of the rhizome network in the soil too. 

These so-called ‘buffer zones’ are always indicative.  No assumptions should be made about boundary encroachment liability until some soil inspections are conducted... and the best time to do this is during the initial inspection.  On development sites, the volume of soil excavated can be significantly reduced by having a qualified CSJK clerk of works directing the dig until all the rhizome is found and stockpiled or removed.

+ Knotweed Management Plan

The optimum Knotweed Management Plan (KMP) may not seem to be relevant to the site inspection but any qualified CSJK surveyor will want to know before or during their site visit what factors may limit or determine the options available for remediation of the knotweed.  This could be something as simple as budget or the precise planned location of buildings; issues of timing (you can’t spray knotweed in January!), but really, everything is pertinent!

+ Japanese Knotweed Control or Eradication

Control or Eradication?  By way of an example to support the last point, every Japanese knotweed site inspection which is conducted with a view to evaluate site management options should have a context i.e. what does the client need to, or want to, achieve?  

In domestic or commercial settings, it may be acceptable to manage the Japanese knotweed in situ, e.g. by herbicide spraying; especially in large gardens/plots, no development plans and where this options enables the issue of a Guarantee/IBG .  The client may even give specific instructions to prevent disturbance of surrounding vegetation.  But eradication (normally meaning physical removal of all soil/rhizomes) will usually entail a different approach. This approach may include considerations of access for large machinery and also careful planning to limit pathways for spread during remediation.

Japanese knotweed Site Inspections - what are the things to look for?

Having thought a little about key inspection considerations, context & client requirements; what is it that Japanese knotweed surveyors are likely to do when on site?  

The below is not an exhaustive list and may not apply to all situations/sites; but for surveyors, property & construction specialists (as well as homeowners), hopefully the points below help to explain the process.

+ Biosecurity Considerations

Often the first thought is to review/inspect the overall site in the context of vector routes for Japanese knotweed and/or other invasive plants.  This is necessary to advise the client to avoid spreading propagules in the short, medium and long term. It also helps the client to avoid becoming a vector themselves!

+ Knotweed Identification & Mis-identification

Fortunately, Japanese knotweed is quite a distinctive plant, to the trained eye at least.  But there are plenty of look-a-likes. Japanese knotweed specialists have to be on their guard for mis-identification.  

+ Impact and Risk

In a residential setting, a CSJK surveyor may be asked to confirm the Risk (or management) category according to RICS Guidance Note “Japanese knotweed and residential property”. This will entail professional judgements about whether the Japanese knotweed is causing damage to built structures including garden walls, drives/paths etc. and/or whether it is impacting on ‘amenity use’ (meaning any normal leisure activity such as use of a lawn etc.)

+ Knotweed Size and Location of the Stand 

Sounds simple enough, but things are not always quite what they seem to be.  Several small ‘bits’ of Japanese knotweed can actually be a remnant of one larger stand which has been unsuccessfully ‘managed’ in the past.  A qualified Japanese knotweed specialist will use their knowledge and experience to assess these possibilities which can impact on future remediation plans.

+ Property Boundaries and Spread

Japanese knotweed spreads through the soil so when it is found near a boundary, special attention should be given to establishing, where possible, the extent to which it may have grown onto adjoining land (or vice versa)

+ Environmental Impact and Risk Assessments

Certain features of the site will be critical to conducting both the survey and any subsequent works arising safely and without harm to non-target plants etc. so these need to be considered even though the initial survey may be a simple instruction to ‘determine the presence of Japanese knotweed’.

+ Questionnaires

This may be done before the appointment/inspection is made, but a qualified japanese knotweed surveyor will want to collect and review any and all information available regarding previous site history

Visit the Invasive Weeds Technical Document Library

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Technical support documents for knotweed inspections

The full advice the PCA gives to its members and other professionals is available in our Japanese knotweed Code of Practice.  We have also produced a specific guide for valuation & property based surveyors (link to recent GN) which sets-out the potential pitfalls of conducting surveys during the winter or e.g. when recent groundworks have been completed.

View the Code of Practice >> 

Guidance for valuers & surveyors >>

Videos you may be interested in

Want to improve your inspection techniques & knowledge?

For those interested in learning more about inspecting and investigating Jpanese knotweed, there is a variety of PCA training options for surveying professionals as well as technical/trade professionals. 

Use the search tool below to find available Japanese Knotweed & invasive weed related training courses or simply go to our training & qualifications section.  Alternatively, if you want to chat to someone, contract our training team on 01480 400 000 or contact them online.

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More about PCA Membership 

Interested in taking part in the CPD scheme but you are not a PCA member?  Find out more about membership and why it will benefit you!

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Professional Guidance

For those professionals looking for information, technical help and guidance towards and variety of property related problems, why not check out our 'professional guidance' pages.

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