Zapping invasive plants – a trial on Exmoor

Holly Moser, Exmoor Non-Native Invasive Species Project Officer
September 2021

As many of you may be aware invasive non-native species (INNS) control can take many years and here at Exmoor National Park Authority we have been going at it for the last sixteen! A major part of our work, our knotweed control programme, has been running since 2005 and has resulted in a large reduction of the knotweed species, particularly Japanese and Himalayan knotweed across the National Park.

Each year we seek permission from landowners to treat knotweed using the conventional method of spraying glyphosate. Currently, thanks to the funding, we can offer this service free of charge. Our contractor completes the treatments between September and November when the herbicide is most effectively drawn down into the rhizome system.

This autumn we will be visiting 95% of the 850 plus knotweed sites we can treat using glyphosate, probably the highest percentage ever achieved on the programme! This may sound a lot but many of these sites are being monitored and may no longer need treatment because there is no visible growth showing. It is vital that we continue to monitor these sites because knotweed rhizomes can stay dormant underground for something like up to ten years, which means shoots can sneakily reappear!

On Exmoor there are several knotweed sites we cannot treat with herbicide because they fall on organically certified land and we have not found a suitable “organic” alternative to glyphosate treatment. This land provides a serious reservoir for invasive species to flourish, particularly where a plant is situated close to a watercourse allowing them to spread easily. There is also a lot of controversy surrounding the use of glyphosate as a herbicide and the future of its application is uncertain. So it is crucial that we expand our toolkit of methods to control INNS and find solutions for control on organic land.

In 2017, we began trialling Rootwave Pro which has proved to be very effective on annual weeds and was first trialled on Japanese knotweed in 2016 in Gloucester, by the Environment Agency. This technology forces an electric current of up to 5000 volts down through the plant, raising the temperature and boiling the plant cells. It’s very dramatic to watch – lots of steam and popping and banging! The plant is left to decompose, and you can see the effects of the treatment within a few weeks. I have to admit it is satisfying watching a plant which is causing so many problems and is so difficult to get rid of, turning yellow and wilting without the use of herbicides to treat it.

How are the trials going so far?

In 2017 and 2018 the National Park commissioned Ubiqutek, the manufacturers of Rootwave Pro, to carry out one treatment a year on five knotweed sites. It soon became clear that one treatment a year might not be enough because the knotweed continued to throw up lots of little shoots.

Luckily the project was able to use our new funding to purchase our own Rootwave Pro so that we could continue these trials and increase the frequency of treatments. We were also able to train up several local contractors to operate the machine. So we can now carry out up to three treatments a year on 28 different sites including other INNS: American skunk cabbage, giant hogweed and montbretia.

Although we have made great progress carrying out multiple treatments a year it has not been smooth sailing…

There are several factors to consider when using Rootwave Pro:

• First the location. The machine needs to be transported to site on the back of a 4×4 or ATV due to its size and weight and you are also limited to a 27 metre long treatment cable from the vehicle. Although we would not expect to find knotweed on some of the most remote parts of Exmoor this has been a limiting factor for us. The technology is however adapting quickly and who knows how long it will be until Rootwave can be carried on your back…

• Secondly, it cannot be operated if it is raining. On Exmoor this is a challenge, especially with the wetter summers we are now experiencing. However wetter ground is better for conducting electricity so if your operator is able to drop everything and fit in a couple of hours when the rain has stopped then rainy days don’t have to be a write off.  Having the flexibility to change plans and carry out a treatment when the time is right is definitely a bonus!

Of course, we have also had to deal with the elephant in the room which is affecting everyone. Busy schedules and short time windows impacted by the pandemic and combined with the British weather have not made it easy for our contractors. The result is that we have struggled to complete three treatments a year at all of our sites as we had first planned.

What results have you seen since you started the multiple treatments?

It’s important to point out that we are only in our second year of multiple treatments and the very early stages of this trial, but despite all the challenges things are looking quite promising.

It’s clear that every site has reacted differently to each treatment as you would expect when there are so many factors like competition from other species, ground substrate and moisture content in play. In our first year of multiple treatments, some of our knotweed sites continued to show vigorous growth. This is to be expected – invasive species, by their very nature, are tough to control.

Other sites looked more positive with less regrowth appearing. At this site (image below) our contractors reported only a handful of stems after its first ever treatment.

We have also seen similar results over the years at the sites that were first treated in 2017. Below you can see a comparison of two sites that received a single treatment in 2017, 2018 and 2019 and two treatments in 2020. These photos were taken in September. Both sites are yet to receive a treatment this year which makes a good comparison to September 2017 when the sites hadn’t been treated at all. Again, some sites have continued to show quite vigorous regrowth (top two images); while at others the difference is more obvious (bottom two images).

For our other species it is a similar story that multiple treatments for several years looks like it could be the way forward. We were however delighted to see that one of our skunk cabbage sites, where there was a single plant, does seem to have disappeared for the time being after its first year of multiple treatments.

So we are seeing some interesting results in the early stages and these trials are playing an important role in broadening our options for INNS control. Like any trial, there are always lessons learnt to be passed on and we have definitely had our fair share. Check our website for updates as our trials progress and please do get in touch if you have any questions.

Tel: 01398 322259

Email: ennis@exmoor-nationalpark.gov.uk

The Exmoor Non-Native Invasive Species (ENNIS) project is trialling innovative approaches to invasive species management and working with the local community to map and control invasive non-native species such as Himalayan balsam, American skunk cabbage and American signal crayfish. If you would like to find out more about the project please follow this link to our ENNIS website.

^{The ENNIS Project is funded by the European Agricultural Fund for Rural Development and Defra.}