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

Deploying a natural enemy for one of the UK’s most invasive weeds

Wayne Coles, CABI
Jan 2020

For 180 years Britain’s waterways have gradually become clogged and choked with what the Victorian gardeners thought in 1839 was a pretty addition to the nation’s landscape.

But Himalayan balsam (Impatiens glandulifera), also known as Touch-me-not Balsam and Policeman’s Helmet, is no longer regarded as a visual delight but rather a blight upon Britain’s riverbanks, streams, ponds, lakes, damp woodlands, roadways and railways.

Himalayan balsam was brought to the UK by the Victorians as an ornamental plant but it has since become the scourge of the nation’s waterways – Photo: CABI

In fact, the plant – whose native range is the foothills of the Indian and Pakistani Himalayas – is considered one of the UK’s most widespread invasive weed species competing with native plant species for space, light, nutrients and pollinators, thereby reducing local biodiversity.

Though it dies back in winter, Himalayan balsam is – amongst other places – causing a nuisance on the River Tweed in Scotland as part of a nationwide invasion which in 2003 the Environment Agency (EA) has estimated would cost £300 million to eradicate. The plant has since continued to invade new areas.

Nevertheless, science is trying to fight back against the Himalayan balsam ‘menace’ with a biological weapon in its arsenal – the fungal agent Puccinia komarovii var. glanduliferae– that will be released into Scotland in 2020.

Dead plant material from Himalayan balsam can also enter rivers and increase the risk of flooding.

It was back in 2006 when CABI was asked by the EA, the Department for Environment, Food and Rural Affairs (Defra) and the Scottish Government to find a reliable. effective and safe natural enemy to help control Himalayan balsam – a journey which has seen scientists adopt a ‘classic approach’ and return to the foothills of the Himalayas for a natural solution.

After sourcing a rust fungus from the Puccinia species and bringing it back to CABI’s quarantine facilities in Egham, Surrey, for further analysis and rigorous testing scientist now believe they have a tangible biological solution for the invasive weed which will be released with funding from Scottish Natural Heritage’s Biodiversity Challenge Fund in partnership with the Tweed Forum.

Kate Pollard, a Research Scientist at CABI, said, “The highly damaging fungus was identified by CABI in the foothills of the Himalayas, where it infects leaves and kills seedlings of the plant, helping to maintain population levels.

Extensive laboratory testing was undertaken under quarantine conditions to confirm that the fungus was highly host-specific, infecting only Himalayan balsam, and safe for release.

“Approval to release the rust into England and Wales was granted in 2014 and since then, the rust has been released at 47 sites across 19 counties. Field results are promising with high levels of infection, spread and overwintering recorded at a number of sites.”

The rust fungus Puccinia komarovii var. glanduliferae could go a long way to helping to solve the River Tweed’s Himalayan balsam invasion – photo: CABI

Prior to the ‘appliance of this natural solution, Himalayan balsam on the River Tweed has been tackled using a combination of hand-pulling the weed, thanks to an army of volunteers, and spraying.

The fungus has previously been released at a site along the River Tweed on the English side of the border. It is hoped that the release later this year will help step up the fight against Himalayan balsam on the waterway further as part of a concentrated longer-term solution.

Reference
Environment Agency. (2003). Guidance for the control of invasive weeds in or near fresh water. Environment Agency, London, UK

Find out more about CABI’s work to release biological controls to fight Himalayan balsam from the dedicated website.
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Did you know?

Himalayan balsam is Britain’s tallest annual plant with each plant tending to be around 1-2 metres high, although they can reach a height of 2.5 metres in some cases!

According to Section 14 of the Wildlife and Countryside Act 1981, it is an offense in England and Wales to allow Himalayan Balsam to spread into the wild. Therefore, it is highly recommended that you remove Himalayan Balsam from your grounds or garden.

It is important to make sure that when disposing of Himalayan balsam, the waste disposal site has a permit to accept and dispose of invasive species. As GOV.UK explains, you can be fined up to £5,000 or be sent to prison for 2 years if you do not properly dispose of Himalayan balsam and other non-native invasive plants.

The Good, the Bad and the Balsam

Karen Müller, Project Officer, Scottish Invasive Species Initiative
September 2019

Himalayan balsam season has drawn to a close. On our river banks, our staff and volunteers have downed tools for another season. The time has come for the last of the pink-petaled invaders still left standing to be pinging their seeds from ripe pods in an endless effort to increase their range. But with thousands of plants degrading in piles on the banks and tens of thousands of seeds prevented from ever reaching nurturing soil we are happy to call this a job well done, for now.

It’s time to reflect on the last few months – our highs and lows, ayes and nays, lessons learned and casting our eyes to the future.

Getting to grips with balsam

This year’s waves of heat and rain appear to have been particularly favourable for balsam, with many areas dazzling with shades of pink. To counter this, a continuous thwacking of weed whackers, scythes and strimmers against balsam stems, the sound of soil releasing roots and the popping of hollow stems under the weight of more balsam being added to piles has filled the air over the last few months. There has been a flurry of volunteer activity across the project area, making it possible that river banks from Tayside to Sutherland have decidedly less of a pink hue to them.

Volunteers come in all shapes and sizes and the one lovely thing about balsam is that its control is suitable for a wide range of ages – from primary school children to people living out their best life in retirement – and can be done in larger groups. It’s the perfect way to meet new people, partake in a hands-on activity with your kids or spend some relaxing time in nature while trying something new and helping the environment. Whether it is anglers, dog walkers, bird watchers pulling a few plants when they are out and about or individual people, schools or organisations that specifically join us project officers for a balsam bash – many a mickle makes a muckle in the battle against balsam.

Compared to other invasive plant control, balsam provides instant gratification when you look back on the area you are working on – nothing makes your chest swell with pride like knowing you have just built the highest balsam pile yet of the season. And let’s not stop there, optical illusions, balsam snowmen, a spot of friendly competition who can build the biggest pile or who can scale the finished pile the fastest and stand victoriously at its summit – the pile is your oyster.

Unquestionably, balsam control is not always all sweetness and light –the plant is willing to make use of many kinds of growing spaces and so we found ourselves thigh deep in ‘bogs of eternal stench’ or crawling around in gorse on more than one occasion. Did it stop us having fun? Of gorse not!

Through the good times and the tough, plenty of opportunities to whip out the Kelly kettle, freshly brewed tea and coffee (not the instant kind, mind!) and, most vital, chocolate biscuits, support us in keeping our volunteers in top motivation.

Time to reflect and perfect

So, what have we learned over the past two seasons through trial and error? For one, we have stocked our armoury well with strimmers, scythes, weed whackers and gloves since last year – something to counter anything the balsam might throw at us.

Selectively pulling balsam from amongst other vegetation seems to have the least impact on non-target species, if compared to mowing down stretches of bankside vegetation at more badly infested sites. But, if you’re working on borrowed time, you sometimes have to face the hard hitting questions. Mowing is effective in terms of time and effort and, thus, usually prevents a much larger proportion of seeds from dropping. So, do we mow in the hope that after a couple of seasons we have adequately reduced the density and can move on to pulling the balsam up? Or shall we pull regardless of the fact that much less can be achieved in the same amount of time? It is important to ponder these quandaries and often we have to make decisions on a case by case basis, depending on the existing vegetation, how dense the balsam has become and its potential to spread from there.

As it gets later in the season the balsam is really out to impress, growing up to two meters in height. At times the stems are growing horizontally along the ground before reaching its flowers towards the sky and roots start to form on higher leaf nodes, digging into the soil where the stem touches the ground. The trouble with this is the increasing likelihood of snapping the stalk during pulling, followed by the balsam stubbornly re-growing from the left over stem. A weed whacker can be really handy here to slash the stalk below the first node closest to the ground and curbing the balsam’s ambitions once and for all.

Casting our eyes on the future, there are always things to improve. For one, we still need to understand the distribution of balsam throughout our catchments better, so that we can control it strategically from the most upper extent moving down the river. We really appreciate reports of sightings to help us with that. The species is widespread and we know that long-term, sustainable solutions require landowners who actively take responsibility and more volunteers to support the effective removal of the plants annually. While many landowners, local communities, environmental groups and countless more are already getting involved we need to increase these efforts catchment wide moving forward.

Last but certainly not least, both this year and last year it is a must to acknowledge that every single one of our enthusiastic volunteers is absolutely wonderful and invaluable! You make it a joy to get to grips with balsam and you make the improbable possible. Not all heroes wear capes – some wear waders and gloves!

If this has inspired you to get involved with the Scottish Invasive Species Initiative or you’d like to find out more, visit our website www.invasivespecies.scot or email sisi@nature.scot