Learning from Japanese knotweed Research & Development

In the midst of the coronavirus pandemic many of us are taking advantage of new opportunities for learning. The PCA themselves are providing webinars on a range of subjects and this has been well-received.

So it was great to hear the other day that the ICL were hosting a webinar on my favourite subject – Japanese knotweed (JKW) – and that it was to be delivered by Dr Daniel Jones of Advanced Invasives.

Daniel did his PhD on Japanese knotweed and the culmination of much of the fieldwork was published in 2018 to great fanfare and media attention. Any scientific research which illuminates the response of this problematic invasive plant to various management strategies is welcome. So, whilst acknowledging that this is really only a few of the many facets of Dan’s presentation, here are the highlights we wanted to share from his research.

Rhizome fragments – HOW SMALL?

The first eye-opening revelation was that there is some new research in a lab in Canada which suggests that rhizome fragments as small as 0.02g were able to regenerate! Some 30x less than the oft-used benchmark of 0.7g, which is already very low.

We need to wait for the full source details of this research to clarify if we are comparing eggs with eggs, but the point that Dan Jones wants us to take away from this is that it highlights the potential risks associated with soil screening, or indeed any kind of soil excavation as part of a Japanese knotweed management plan.

These pieces of rhizome are unlikely to be captured in sieving machines nor visible to the naked eye. Perhaps, but this seems to be rather academic as any such soil removal or improvement projects would always normally be followed-up with routine site inspections for at least two years. Also, the energy reserves in these pieces of root/rhizome are so small that even minor residues of a herbicide spray treatment should preclude successful germination (hence the recommendation in the Environment Agency Code of Practice to spray before excavation where possible).

Is ‘Eradication’ the right word?

Dan reminded us that one of the main conclusions of their fieldwork, and one that certainly caught the imagination of the news media at the time, was that even the BEST herbicide spray treatments failed to achieve full control after 3 years continuous management. They were dealing with large/mature stands of Japanese knotweed and most of our PCA members would have reacted “we know!”. Nevertheless, within the sector as a whole, it is quite common to find companies offering to ‘eradicate’ Japanese knotweed within 3-4 years. Dan’s research suggests this is rather too optimistic as, in their experience, the energy reserves in a stand of Japanese knotweed(and limited translocation of glyphosate even when applied in the late summer) preclude such black-and-white outcomes.

We have to say we agree and our guidance emphasises the importance of explaining to clients’ that each and every stand of Japanese knotweed will need more-or-less years to bring it under control, especially if there has been a history of Knotweed management prior to the start of the new Management Plan.

As for whether we should ever use the term ‘eradicate’ following herbicide treatment, it seems this all depends on how you define it! It is certainly essential to qualify in reports/proposals what ‘eradication’ means and this must include the explanation that re-growth is always possible after some years – hence the importance of guarantees! But then that seems contrary to the definition of eradicate which is “to destroy completely”!

Post-treatment management

Work has been continuing at the field sites reported on above and we all hope there will be new publications soon regarding further management strategies. But the one area Dan was able to update us on was their change of focus towards the long-term fate of these large sites once the herbicide applications have ceased. In short, new invasive weeds (Himalayan balsam and Solidago – Goldenrod – seem to be the main culprits) will colonise the bare ground almost immediately once the knotweed has been removed.

Let’s remember, these are very unusual ecological conditions. The seed bank of most native weeds will have been exhausted by the dominance of Japanese knotweed over several decades and, in this case, the above species appear best adapted to take advantage (they do not give the ‘natives’ a chance to get going). Can this be prevented to encourage a stable re-colonisation by more native species?

We believe there are some positive lessons to come from the trials and we look forward to seeing the published research very soon. Clearly the message here is that our work is not complete until we have considered the long-term fate of the site, not just in relation to re-establishing a stable sward of native plants, but monitoring for re-emergence of Japanese knotweed or whatever INNS was originally present.

A cradle-to-grave approach is something we can and should be offering clients ‘as appropriate’ for each site and we are confident that the work continuing at these field sites will help us formulate optimum strategies.

Are all glyphosate spray products the same?

If you visit the Health & Safety Executive (HSE)/Chemicals Regulation Division (CRD) website and type ‘glyphosate’ into the herbicide products search box, you get an awful lot of products popping up!

At the PCA, we do not feel qualified to say too much about the different products you can get, but Dr Dan Jones explained that not all are equal and some care and thought needs to go into selecting products which have adjuvants to optimise uptake and distribution. With that being the message, we encourage all of our PCA invasive weed members to contact their distributor to request what guidance can give on the subject.

To emphasize the point, one thing that Dan said made me sit-up…the biomass of Japanese knotweed is nearly 80% below ground so we need to make sure the 20% we can see absorbs enough herbicide to travel over 6m to all parts of the rhizome network. Worth thinking about optimising the impact of those herbicide droplets!

Thanks to ICL and Dan Jones for providing this webinar and we look forward to more of the same in the future!

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