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

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Coronavirus, community conservation and the green recovery

Can a community and volunteer based approach to conservation projects lead the green recovery after coronavirus?

Callum Sinclair, Project Manager, Scottish Invasive Species Initiative
May 2021

As we emerge from the 2021 lockdown, we reflect on the impact the coronavirus pandemic has had on the Scottish Invasive Species Initiative. As a project we weathered the Covid storm surprisingly well. Although activities were constrained, we kept invasive plant species in check and maintained our mink monitoring and trapping network. We credit this to taking a community-based approach to invasive species management – and so we pondered what we have learnt and whether other projects could take a similar approach to help deliver a ‘green recovery’ after the pandemic.

Community control vs Covid

At the heart of the Scottish Invasive Species Initiative are the local people who care for their local environment. An initial and ongoing programme of awareness raising and education activities raised the profile of invasive species and their impacts. This generated significant buy in and enthusiasm to get involved from land managers, local groups and organisations, businesses, and individual volunteers. Having recruited and enthused, we support and enable people to actively volunteer by providing training and helping them gain qualifications and experience. This network of committed people is gaining the skills needed to make a difference to their local places – in this instance controlling invasive non-native species – both now and in the future.

When the spring 2020 lockdown occurred we weren’t, like everyone else, able to carry on as usual. It was unclear if we were about to lose a year and give up the gains made by the project since 2018. Rather than regress, however, what we witnessed instead were the investments in embedding skills and training with local communities paying back. Farmers and land managers used their staff (trained by us) to carry on giant hogweed treatment when we couldn’t, and local people headed out to pull Himalayan balsam at sites we’d worked on together previously. Mink raft monitoring volunteers used their daily exercise to visit and check their rafts – telling us that this small activity gave them a purpose and made them feel they were making a difference during an immensely difficult time. The project didn’t grind to a halt – far from it – in fact we perhaps were able to snatch a glimpse of what might happen when the Scottish Invasive Species Initiative is no more.

We believe that this approach – investing in communities, training volunteers and engaging people locally can provide effective and sustainable benefits – as endorsed by the enthusiasm of our volunteers and partners to carry on invasive species control when we weren’t around.

To date volunteers have contributed a total of 2,181 days to control invasive plants and 119 volunteers have been put through their pesticide application course.

Building community resilience

Taking a community and volunteer-based approach – rather than just employing contractors on an ongoing basis – also provides benefits to the people involved. Volunteering, pretty much any volunteering, is much more than just the activity itself. In our case people are making a real contribution to their local environment. But the benefits are much wider than that. Being involved within their community, meeting other people and socialising, being physically active and engaging with nature – these all contribute to improved physical and mental wellbeing. We believe these benefits have helped give people greater personal resilience to the individual and collective stresses we have all felt during the coronavirus pandemic.

Our engagement work in communities is wide and varied. It ranges from providing education resources for remote use and our direct education work in schools, to partnerships with farmers and universities to find alternative and innovative techniques for invasive species control. Engaging and enabling land managers to be a central part in the development of potentially more sustainable and affordable invasive control methods – such as sheep grazing to control giant hogweed – helps these businesses to see alternative control strategies and be more resilient to future change.

Our Macduff sheep grazing trial in partnership with a local farmer is investigating if and how managed sheep grazing can be used as a form of chemical-free, low intensity giant hogweed control.

Environmental benefits

This community-based approach, as well as empowering people, also has environmental benefits.

Invasive non-native species require long term management and commitment. They often produce large numbers of seeds or have persistent rhizomes and require treatment year on year. The American mink needs ongoing control to continue to depress population numbers and counter inward migration. These animals travel great distances to secure new breeding territories and can quickly repopulate previously managed areas. Having an upskilled, enthusiastic and committed local community group willing to, for example, ‘adopt’ a section of river to deliver annual plant control and/or mink control can be effective through their collective efforts – both providing sustainable and economically effective approaches to management.

Thanks to the monitoring and trapping work of 360 volunteers maintaining 670 rafts and traps, 388 mink have been removed. This provides benefits to the local native wildlife populations, particularly water voles and ground nesting birds.

Reaping the rewards

Restored river habitats and, in turn, landscapes that are free from invasive species give so much back to us. Our rivers are important for recreation and provide brilliant opportunities to exercise and have fun or places for quiet reflection to boost mental wellbeing. The connection between people and nature – widely recognised as providing significant health benefits – is further enhanced when people have directly contributed to the care and conservation of the environment.

The importance of connecting with nature has been widely recognised throughout the pandemic. Many of us discovered, or rediscovered, our local green spaces and enjoyed, and needed, the serenity these places and being outdoors brought.

As we carefully step into what, we hope, will be our recovery from the pandemic, we have the opportunity as a society to reshape and rethink the way we do things. We can find new and sustainable ways to work which provide long-term benefits both to people and the environment – the “green recovery”. For us at the Scottish Invasive Species Initiative we’re not sure that is actually such a new thing – it’s the ethos by which we’ve worked since we started.

Our rivers are important places for recreation and reflection.

Is the model sustainable?

The approach we take – working locally with partners, communities and individuals – undoubtedly benefits both people and the environment and fosters strong connections between the two.

Volunteer input to the project – 9,794 volunteer days contributed, equal to 46 full time equivalent staff from March 2018 to March 2021 – is incredible and encouraging. The work completed through these contributions, given by 650 people, would simply not be affordable to the project otherwise. Each and every contribution – large or small – is important to us but, perhaps more significantly, important to the person donating their time.

The Scottish Invasive Species Initiative continues to build this community network so the work we have started can be a self-sustaining after we have gone. The work continued by our volunteers and community network during the 2020 coronavirus lockdown suggests this can be the case. But we recognise that to sustain, connect and coordinate a volunteer network to a common and shared purpose will need support and resources. Maintaining enthusiasm, preventing ‘volunteer fatigue’, continually recruiting and supporting volunteers, training and providing local co-ordination are all essential to the success of this approach. However, the scale of return we find would suggest these investments are both extremely cost effective and environmentally beneficial.

The scale of achievement in 2020, despite working restrictions due to the pandemic.

So, while the coronavirus pandemic has reached every part of our society and touched so many of us personally, we believe there are lessons from the period that can help us in the future. Investing in people and communities is always worth it to build support, capacity and commitment. When investments are maintained the benefits continue. If those investments cease, then the goodwill and benefits generated from all that has gone before are lost.

As a project we have emerged relatively unscathed from the coronavirus lockdown – we know many have not been so fortunate. We believe the community-based approach we have adopted and the hard work and dedication of our many, many volunteers have been central to this. We simply can’t thank these people enough.

To find out more about our work and how to get involved visit our website, follow us on social media or contact us on sisi@nature.scot

Back to basics: grey squirrels in Scotland

Guest Blog – Gill Hatcher, Saving Scotland’s Red Squirrel Project, May 2019

It’s ‘Invasive Species Week’: an opportunity to talk about the threats non-native species can pose to our special wild places, our natural heritage, and global biodiversity; as well as what people can do to help. But often the first challenge in raising awareness is simply explaining what ‘invasive’ means. It’s not a household term, and it can be a complex – and sometimes controversial – subject to discuss.

Perhaps none more so than the grey squirrel. The grey squirrel is equally one of the most familiar examples of an invasive species in Scotland, and the most clouded in confusion, misinformation and emotion.

The eastern grey squirrel is a North American species that was first brought to Britain to decorate the gardens of Victorian stately homes. With an abundance of food and fewer competitors to contend with, they quickly settled in their new surroundings and began to spread across the country.

As the only squirrel species native to the UK, red squirrels struggle to compete with grey squirrels for food and living space. Some grey squirrels also carry squirrelpox, a virus that doesn’t harm them but is deadly to reds. As grey squirrels have moved into an area, reds have gradually disappeared; and today greys have completely replaced reds throughout most of England and Wales, as well as Scotland’s Central Belt and the city of Aberdeen.

In many of these places grey squirrels have been present for well over a hundred years, with the native red squirrels barely a distant memory. So for many people in the UK today, grey squirrels are the norm. In cities and towns, they are often one of the few wild mammals people come into regular contact with, providing a rare connection with nature. It’s not surprising that many don’t realise that grey squirrels are not native, that red squirrels were once widespread or even that the two are different species of squirrel. Before we can even begin to explain the term ‘invasive species’, we often have to start with the basics if we want widespread understanding and support for red squirrel conservation action in Scotland.

While raising awareness of the threat from invasive grey squirrels, it’s also important to emphasise that there are still many places in Scotland where red squirrels are thriving. Home to 75% of the remaining UK population, there are healthy populations in the Highlands, the Central Lowlands and parts of South Scotland. Red squirrels are under threat, but they are still very much worth saving.

Saving Scotland’s Red Squirrels is a partnership project led by the Scottish Wildlife Trust. Established in 2009, its aim is to protect Scotland’s red squirrels by combating the spread of grey squirrels. With the help of landowners and volunteers, grey squirrels are being controlled through live trapping and humane dispatch in the areas where this action will have a positive impact on core red squirrel populations.

In 2017 the project was awarded a £2.46 million National Lottery Heritage Fund grant to begin a new phase called ‘Developing Community Action’. Recognising that landscape-scale community involvement will be the key to the long-term protection of Scotland’s red squirrels, the project is now focussing on building a strong network of local volunteers, providing them with the skills and resources they need to carry out red squirrel conservation work in priority areas. The first step is community engagement: building a basic understanding of why red squirrels need our help and why, at this time, control of grey squirrels is the only way to ensure red squirrels will continue to have a home here.

Community involvement in red squirrel conservation

One way to start this conversation, and one of the simplest ways people can help the project is by reporting red and grey squirrel sightings. The sightings map contributes to our understanding of squirrel distribution across Scotland, and it could act as a stepping stone to getting more involved. There are lots of ways to take action for your local red squirrels, from survey work, to grey squirrel control, to helping spread awareness by volunteering at an event.

You can report a sighting and find out more about volunteering with Saving Scotland’s Red Squirrels at scottishsquirrels.org.uk

Photograph credits; Grey squirrel -Bob Coyle, Red squirrel & community action – Jo Foo

Invasive Species Week

Vicky Hilton, SISI Volunteer & Comms officer, May 2019

This year Invasive Species Week is being held from 13-17 May, the purpose of the week is to raise awareness across Great Britain about invasive species and their impacts.

Being a project all about invasive species, here at the Scottish Invasive Species Initiative, we’re quite excited about having a whole week of all things invasive and we’d love everyone to share in our enthusiasm and get involved!

There are lots of ways to get involved in Invasive Species Week, you can join in with a practical hands-on volunteering session to remove invasive plants, adopt a mink raft, go on a guided walk, drop in on one of the pop-up stalls or follow our social media posts throughout the week to learn more about invasive species and what you can do to make a difference.

Here’s a list of events we and our partners are running during the week – click on each event to follow the link to our website where there are more details.

If you can’t make any of the events, don’t worry you can still stay involved in Invasive Species Week! Follow us on social media on Facebook, Twitter and Instagram, we’ll be posting lots of information all week about invasive species, what to look out for, what to do about them, alien facts, what things everyone can do to help, gardening tips etc.

Or why not take the initiative and do your bit in your local area? Learn how to identify Himalayan balsam, (download an ID guide here), an invasive plant found growing widely along river banks and head out with a friend (always advisable not to work alone near a river) and go and pull some up! It has a really shallow root system so its easy to pull by hand, put it in a pile away from a path to compost or hang it roots up from a fence/tree (so it doesn’t re-root). It might still be quite small at this time of year but it’s great to pull it and remove it before it flowers and spreads more seeds.

Don’t forget to share how you’ve got involved in Invasive Species Week with us on social media – tag us into your posts and let us see what you’ve been up to!

Invasive Species Week is coordinated by the GB Non Native Species Secretariat. Read more about the week here.