Japanese knotweed Research Around the World

We could be forgiven for thinking we are masters of our universe when it comes to managing Japanese knotweed. The Environment Agency’s and our own Code of Practice, based on text-book biology and years of collective practical experience, are referred to as ‘best practice’ guidance. The very term is prejorative; “there is no better way than this”!

But of course we all know that best practice is really only a reflection of what we know today. Research is the mechanism that leads to new insights and novel ideas and this drives the evolution of ‘best practice’. So we thought we would take a look at what is going-on around the world and how this may help us develop improved strategies or broaden the scope of options available for controlling Japanese knotweed. Plot spoiler – some do, some don’t!

Canada is monitoring soil treatment methods

A research group based in Quebec, with extensive experience studying invasive plant communities, has been monitoring the effectiveness of soil treatment methods (we would call it riddling or screening, their process involves crushing too) designed to physically destroy and/or remove Japanese knotweed rhizomes from soil.

Drs Claude Lavoie and Elizabeth Groeneveld kindly agreed to share some of their results although this is very much an on-going experiment/field-trial. They have found that their process was not completely effective at screening out rhizomes and the reason for this is that fragments as small as 0.02g could regenerate! This is 35-times smaller than the oft-quoted Environment Agency benchmark of 0.7g so is a significant observation!

To be fair, this was just one fragment among many that regenerated; all the smallest pieces growing after 10 weeks (some only 2mm in diameter) were found within 6 cm of the surface of the stockpile but, more typically, viable fragments from that zone were 0.1 – 0.5g (5 out of 14 viable fragments recovered so far from 90 m3 of soil).

What exactly can we take away from all this?

So what do we conclude? Clearly the 20 mg rhizome fragment was an outlier – a ‘freak’ – but this does not take away from the very clear evidence that Japanese knotweed is a remarkably well-adapted plant which survives (thrives on?) disruption/disturbance. That is the clear take-away message for me from this study. Call it a ‘weed’ if you like but what a weed!!

The other message we would convey is that it demonstrates the importance of using a systemic herbicide before excavation and the longer the time period between spraying and excavating the better (to maximise translocation throughout the rhizome network). We wait keenly for their further reports and contributions.

France sets up knotweed research group

The French Government agricultural research institute (INRAE) have recently set up a Japanese knotweed research group. In fact, this was only a few months ago so no experiments to report, but it is significant to note that they have already started gathering data from the knotweed community regarding the effectiveness or otherwise of ‘tarping’ i.e. covering the ground with a root/shoot-proof barrier that stops light reaching the soil.

It is an intuitive assumption made by many that if you starve a plant of light and water it ought to be possible to ‘kill’ it. This is often true, especially for annuals but what about a perennial like Japanese knotweed which has large underground energy reserves? Well fortunately, this has been the subject of some fairly systematic research and the results have not been great (Jones et al., 2018; “Geomembrane covering was the least effective control treatment ….”).

New beginnings, but not setting the world alight with research!

To be honest, the French group know that the results of tarping are variable at best; the debate here is about control rather than eradication and it is definitely something they will go on to explore further. They have welcomed the responses from PCA members who have reported on their own experiences with tarping and we hope to have further opportunities to share our knowledge and experience with Dr Francois-Marie Martin and her team in the near future.

Netherlands turns to CABI for help

Municipal boroughs in the Netherlands are waking-up to the fact that Japanese knotweed is damaging their built environment and discovering that it is difficult to control (‘eradicate’), so they have turned to the UK-based laboratories of CABI for help. With funding coming from a consortium of local boroughs and NL research institutes, they are evaluating the possible use of insects collected from the very North of Japan to achieve control of Japanese knotweed colonies which would otherwise need to be treated with herbicides or excavated.

This has been tried before in the UK, but the released populations of insects did not survive overwinter successfully. It is hoped that this trial, using insects with greater over-wintering potential, will re-kindle interest in this ‘ecological’ approach to knotweed control.

Wales focuses on more targeted treatments

Dr Dan Jones (Advanced Invasives Ltd and Honorary Researcher at Swansea University) recently shared news of on-going work at Taffs Wells (the site of most of their research work on Japanese knotweed – find out more via our blog). We can be sure that the restless pursuit of better, more targeted and effective treatments for Japanese knotweed continues together with insights to the best re-vegetation strategies.

PCA members are the experts!

Japanese knotweed has always been the subject of much interest from academics who wanted to study its biology, its impact on ecological systems and its unique physiology. However, now we are seeing an increased focus on control strategies; not just by universities, but Government departments too. Even the USA is rapidly waking-up to the commercial implications of Japanese knotweed contamination in the soil, especially in urban areas.

Rest assured the PCA will be scanning the horizon looking for any and all reports which we think can help us to evolve our own ‘best practice’ but at the moment it seems we and the UK as a whole are the experts!

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