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