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.

Spraying Himalayan knotweed

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.

The Rootwave machine. You rub the electrode up each stem for approximately 30 seconds. It is a very dramatic treatment with lots of steam, popping and banging!

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.

Left: 2017 – Japanese knotweed before any Rootwave treatments took place.
Right: 2018 – after a single treatment in 2017.

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.

Left: May 2020 – young Japanese knotweed appearing before Rootwave treatments took place.
Right: September 2020 – two months after the second treatment of the year.

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.

Left: August 2019 – before any Rootwave treatments.
Right: August 2020 – two months 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).

Two Japanese knotweed sites treated using Rootwave – single treatment in 2017, 2018 and 2019 and two treatments in 2020.
Top left: Site 1 before (Sept 2017), Top right – Site 1 after treatments (Sept 2021)
Bottom left – Site 2 before (Sept 2017), Bottom right – Site 2 after treatments (Sept 2021)

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.

Left: June 2020 – a single skunk cabbage plant before any Rootwave treatments.
Right: June 2021 – after two treatments in 2020.

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.

Flying, Flinging and Floating – Seed dispersal by Giant hogweed and Himalayan balsam

Vicky Hilton, Volunteer & Communications Officer, Scottish Invasive Species Initiative
September 2021

Here at the Scottish Invasive Species Initiative our invasive plant control seasons for giant hogweed and Himalayan balsam have now drawn to a close and thousands of plants have been sprayed, cut or pulled – removing these pesky plants for another year and critically preventing seeding and spread.  However, that’s not the end of the story.  In the soil lie dormant seeds: snoozing silently but waiting their chance to germinate.  And beyond our control areas are plants which are now about to do what they do best – scatter their seeds for future germination.

When it comes to invasive plants seed production and dispersal is usually significant and impressive – the ability to reproduce quickly, and in numbers, is one trait that makes these invaders spread successfully and be worthy of the status of ‘invasive’.

Plants have limited mobility and rely upon a variety of dispersal methods to transport their seeds, and so reproduce, away from the parent plant.  Seeds are more likely to survive away from the parent plant – they can reach habitats favourable to survival and with less competition and larger distance seed dispersal can allow plants to colonize altogether new geographical areas.

Giant hogweed and Himalayan balsam are both highly mobile species – they are good at seed dispersal and spread readily to new areas.  While this makes good evolutionary sense for the plant in ensuring its survival and success as a species – for those of us working to stop the spread of these species it certainly brings challenges.   

Understanding how these species disperse their seeds and how long these seeds persist in the soil helps enormously with the planning of control work.  For example, both these species use flowing water as a dispersal mechanism – meaning that control work needs to start with the most upstream plants in a river catchment. Treating plants downstream first would be futile as seeds from those upstream plants will wash down and re-infect cleared areas. 

Giant hogweed

Giant hogweed plants flower only once after growing for 2-5 years (longer if conditions aren’t ideal).  After seeding the plant dies.  Although that might give hope of successful control, in reality if the plant sets and spreads seed then the damage has already been done. A single flowering plant will produce between 10,000 – 50,000 seeds, with 20,000 seeds per plant being typical – and each seed can remain viable for many years. 

The dry flattened oval seeds are produced at the end of August.  Each is approximately 1cm in length with brown lines extending about three quarters of the length.

As the seeds are shed from the towering 2m high seed heads their winglike membrane allows them to be caught by the wind and dispersed over short distances (2-10m).  As they drift away from the parent plant they often fall into a river or burn where the water can carry them long distances to new areas.  Studies have shown that giant hogweed seeds can float for up to three days – in that time seeds could be moved huge distances in normal flows and even further in flood events.

Humans can also give a helping hand and be responsible for spreading giant hogweed seeds.  This can be accidental e.g. when seeds are caught and transported on vehicles or in shoe treads – or via transportation of seed contaminated soil.  

If seeds land on a suitable area of ground (there is usually plenty along a riverbank) they will settle into the soil and overwinter to allow maturation and breaking of dormancy.  Germination occurs from January to March with approximately 90% of seeds successfully germinating. Those seeds that don’t grow in the first year can lay dormant in the soil – the seed bank – for many years.

The consequence of this seedbank means that control works will need to be repeated on the same site for several years until the seedbank is depleted and the site truly clear of hogweed. Viable hogweed seeds have been recorded up to seven years after seeding occurs.  This reflects what we witness in the field – we have sites new to the Scottish Invasive Species Initiative where after control for four consecutive years, we still see significant regrowth of seedlings each year.   However, at sites where control has taken place for six or seven years (by previous and the current project) we see significant decreases in annual seedling reoccurrence giving hope that we are closer to achieving hogweed eradication in some locations.

Himalayan balsam

Himalayan balsam is an annual plant with a cycle of growing, flowering, seeding and dying all in one year. As such its survival from one year to the next depends entirely on successful seed production – so prevent the seed production and you remove the plant. 

Balsam seeds are produced in a seed pod, or capsule, which hangs on a long stalk alongside the flowers. The kite-shaped green pod is 2-3cm long and 8mm wide and contains on average 6 seeds per pod (between 4-16 seeds can be present).  The small round seeds are initially white, turning black as they dry.  On average 700-800 seeds per plant are produced.

When it comes to seed dispersal – balsam is the master.  It uses ballistic seed dispersal – exploding seeds pods – to forcefully fire the seeds up to 7m from the plant.  The seed pods are dehiscent – meaning that when mature they will spontaneously explode at the slightest disturbance. The capsule splits along the longitudinal edges – which are built in lines of weakness in the pod – firing out the seeds as the sections curl back.

These expelled seeds are often carried by humans – unintentionally caught on clothes and picked up in shoe treads – and there have also been observations of seeds being transported locally (up to 10m) by small rodents.  But mostly the spread of balsam seeds is aided by flowing water – fresh seeds can be transported within the sediment of riverbeds – particularly during the high flows present during flooding.  The dry seeds are buoyant and can float over large distances in the water currents – before being dropped into new areas to successfully colonise and continue their spread.

Himalayan balsam seeds also have a high germination rate – approx. 80% – but the good news is that balsam does not have a persistent seed bank.  Seeds only survive in the soil for around 18 months.  This means two to three years of control work at a site can be all that is required for successful balsam removal – as long as new seeds are not being brought into the site to replenish stocks!

A note on Japanese knotweed

Japanese knotweed (one of our target species in the project) is not of concern when it comes to seeding – it spreads solely by root and stem fragments.  While it does produce tiny white flowers each year it doesn’t set seed.  In Britain Japanese knotweed plants are all female with male sterile flowers. Its vigorous powers of vegetative reproduction mean that it has been able to spread to all parts of the British Isles without the aid of sexual reproduction – therefore it is classed as a single, exceptionally widespread clone.

Stopping the Spread

Everyone can help in stopping the spread of giant hogweed and Himalayan balsam seeds by practising good biosecurity – checking boots, equipment and vehicles for seeds and carrying out regular cleaning of these items between visits to different countryside sites.  You can find more information on good biosecurity practice on our website.

Autumn is the season

Of course, although we’ve covered giant hogweed and Himalayan balsam specifically here as problem and target species of the Scottish Invasive Species Initiative, do remember that not all plants are invasive.  Autumn is the main season for the setting and dispersal of seed of the majority of plants in Britain so now is a great time to see plant reproduction in action. 

As you’re out and about in the coming weeks take a second to appreciate the evolutionary genius of our plants as they spread seeds by wind, water, explosion, adhesion, using hooks to attach to passing animal traffic and as part of enticing fruits and berries for animal helpers to enjoy and transport.  You’ll be amazed at how many different tactics and methods there are – tell us how many you see.  

For more information about the work of the Scottish Invasive Species Initiative please visit our website – www.invasivespecies.scot

Buttery Business

James Symonds, Project Officer, Scottish Invasive Species Initiative
March 2020

White butterbur (Petasites albus) is a rather lovely plant in many ways. Its white, Hyacinth like flowers are, along with the Snowdrop, some of the first to be seen in the new year and can brighten up those winter walks along our rivers. In summer its plate-sized leaves create an overlapping mat of terrestrial lily pads that catch the breeze and flutter to and fro in a shimmer of white and green, very striking indeed.

White butterbur in flower in late February

However, its delicate appearance belies an insidious nature.

Native to the mountainous areas of Central Europe and the Caucuses it was, along with most invasive non-native plants species, introduced to Britain as an ornamental plant. Its introduction was earlier than most of the headline grabbing exotics like Japanese knotweed, 1683 to be precise, perhaps due to its natural range being so much closer to home. It was recorded in the wild by 1843 and is now firmly established and naturalised across much of Britain, with the North East of Scotland seeing the highest concentrations of plants.

White butterbur is a rhizomatous plant, meaning the plant has a network of roots (the rhizome) underground that are interlinked and plants grow from this root system. When the leaves die back in the late summer the plant puts energy back in to the rhizome where it is stored over winter ready for its early flowering and rapid growth in the new year. This is where its insidious nature starts to become evident.

Exposed rhizomes on the edge of a river bank

Because of the substantial energy stores in the plants rhizome it can grow rapidly as soon as conditions allow. By mid-April early May, just when our native woodland plants are really wanting to get going, it already has a dense mat of foliage covering the woodland floor. This mat completely shades the ground meaning no other plants can access the sunlight so essential for their growth. With little or no competition, the rhizome has enough energy to spread laterally potentially, or eventually, taking over entire woodlands.

Dense carpet of White butterbur leaves on the River Fiddich

As White butterbur prefers deciduous wet woodland, it is very common along the river networks of the North East of Scotland. When the rivers flood and erode their banks, fragments of this rhizome are broken off and washed down stream where they settle and create a new stand of White butterbur and so the plant spreads, eventually taking over entire woodlands along water courses to the detriment of our native flora.

Nowhere have I seen this more evident than on the River Fiddich, a sizeable tributary of the River Spey. From its confluence with the Dullan Water in Dufftown to where it meets the Spey at Craigellechie, White butterbur has become the dominant species along its course. Anywhere that is prone to flooding is a mass of White butterbur and very little else, hectares upon hectares of ecological desert.

White butterbur dominating the river banks

In these areas even young trees struggle to break through the darkness of the butterburs low lying canopy. Trees younger than perhaps 20 years are hard to find raising the question of the woodlands very existence. So what can be done?

There is, in fact, very little research out there on the control of White butterbur so here at the Scottish Invasive Species Initiative we have set up some trial sites to see what works and what doesn’t, you can read more about the details of our trial and findings here.

Early indications suggest, perhaps unsurprisingly, that application of a systemic herbicide is the most effective way of treating the plant. The most effective time to spray it seems to be once the plant is in full leaf, June onwards. At this point the biomechanics of the plant shift and instead of directing energy in to growth, energy is being transferred back in to the rhizome for over wintering, this greatly improves translocation of herbicide from the leaf that is sprayed to the rhizome which is where it is needed! That being said I have treated White Butterbur effectively in late April but these were small poorly established patches.

If your daily exercise is currently taking you for a walk along a river bank keep an eye out for this pretty little invasive (below left) and its cousins, the purple flowered native Common Butterbur, Petasites hybridus (below centre) and the formidable Giant Butterbur, Petasites japonicus (below right) with its more elaborate flower spikes and creamy flowers, the leaves of this one can get to 90cm across!!

Finally – how do you think the Butterburs got their name? Answers on the back of a postcard…

Hogweed munching sheep at work

Al Reeve, Project Officer, Scottish Invasive Species Initiative, June 2019

In an era where people are increasingly environmentally aware and there is a move away from the over-use of chemicals we want to investigate a solution to invasive species control that can be done inexpensively, at low intensity, and with minimal environmental impacts….so, enter the humble sheep!

This trial project by the Scottish Invasive Species Initiative is a practical experiment to discover if a land manager could use sheep to control a substantial giant hogweed invasion. A previous sheep grazing trial, by the Deveron, Bogie and Isla Rivers Charitable Trust at Auldtown (near Huntly) which began in 2013, shows that in a controlled environment, sheep can make light work of giant hogweed infesting a field. (Read the Auldtown Grazing Report). However this new trial is not just about measuring the impact of the sheep on the giant hogweed and woodland environment, but of the real life example of managing the site and sheep from the farmers perspective, recording the successes, failures and lessons learned from the outset.

The new trial site is at Kirkside Farm, Macduff, where we are working in partnership with the Gordon family who farm here. It is a 1km stretch of woodland sandwiched between arable fields and the River Deveron. Giant hogweed has begun dominating the woodland, particularly around the sunnier edges, and along the numerous streams that flow through the wood to the river. In the past, attempts to control the giant hogweed has been via pesticide application using a knapsack sprayer, which proved very time consuming and costly. Due to the scale of infestation and the difficult terrain of the site so far little progress has been made in restricting the giant hogweed’s expansion.

After fencing in the site, in April this year we released 25 sheep into the woodland area, asked dog walkers to keep their dogs on leads and waited to see what happened… and for the first month, nothing did! The sheep lazed around the entrance to the site, maybe due to the new environment or maybe because they had never eaten anything other than grass before.

During this time we started mapping the extent of the hogweed through the site both though ground surveys as well as using drone footage (courtesy of TAHR Media) to visualise the infestation. Working in partnership with The University of Aberdeen we will be monitoring the site and have developed a base map of the hogweed which, after an annual survey, we can compare to. We have also identified 40 monitoring spots where we will measure the effect of the sheep on the rest of the woodland vegetation.

As time went by though, we started to pick up more indications of the sheep growing in confidence with clear trails developing through the wood showing where the sheep had been walking as well as a few nibbled hogweed stems.

Fast forward a couple of months and the sheep have well and truly gained a taste for the hogweed. There are now large patches of much-munched hogweed, a huge change from previous years. It’s worth mentioning here that sheep don’t actually kill the hogweed plants just by chewing on them but by restricting the plants from flowering they are slowly removing the amount of energy stored in the hogweed’s tuberous roots. Hogweed take 3 or 4 years to mature before finally sending out a flower spike to release seeds but by continually grazing the plants sheep can stop the older plants from flowering and mow down new seedlings as they emerge from the soil.

The alternative control method of using a broad spectrum herbicide like Glyphosate results in the killing of the target plant but, however carefully applied, also results in some collateral damage of the other species surrounding it. This has the effect of opening up the soil which enables more hogweed seedlings to germinate and fill the newly cleared space. One of the many benefits of grazing as a control method is that the ground flora is not killed off and this acts as a barrier against hogweed seedlings emerging.

Despite the great work of the sheep this year, there are still some patches of hogweed that have grown too big and dense for the sheep to get on top of and so this first year has not been a complete success, but we’ve learnt a valuable lesson – next year we need to get the sheep on site earlier in the year. However, on a positive note we’ve been delighted by the response of the site users and dog walkers in acting responsibly and the local awareness raising and signage has been a success.

The sheep will stay on site for a few more months and hopefully continue their good work and we can get a fair idea of what their impact has been so far. Next year, we can act on our lessons learned and improve our methodology.

This is a multiple year project and one of the outputs will be to develop practical and useful advice to empower other land managers to use sheep – or other livestock – as a means of controlling invasive plants on their land. This will include public open days and site visits etc. as well as written findings.

This project truly is a work of collaboration and the Scottish Invasive Species Initiative would like to thank all the partners for their continuing support and input; the Gordon family of Kirkside Farm, The University of Aberdeen, the Deveron, Bogie and Isla Rivers Charitable Trust, the Macduff Distillery and the local people of Macduff.