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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The ins and sprouts of Giant hogweed control

Karen Muller, SISI Project Officer
June 2020

If you have ever come face to stem with a Giant hogweed plant or witnessed a dense infestation, you know it to be a truly impressive plant. Not purely because of its size – but because it makes the very most of itself in everything it does to ensure success. Huge leaves on a tall stem shading out other vegetation, an average of 20-50,000 seeds per plant which are viable for about 3 years (some studies say up to 7 years!), a taproot that stores energy and seeds that can float in water for up to 3 days on their voyage downstream to new realms. Easy come, easy grow.

It can be overwhelming to think about how best to tackle this towering hazard, but plant yourself here for a while and let’s talk about Giant hogweed control: methods, tried and tested by us, and the (mostly) weird and wonderful approaches that the internet spat out.

Before we dive in, safety first! Giant hogweed sap contains furanocoumarins, which enhance the sensitivity of skin to Ultra Violet (UV) light. Simply put, when the sap comes in contact with skin it reacts with direct sunlight causing photodermatitis (skin burns) and you’re often left with long-term sunlight sensitivity in that area. So, the first step is to get suited and booted and gloved to protect yourself. A face shield or safety glasses are also advised where there is risk of getting splattered with sap e.g. when digging or cutting the plant.

The second basic rule when tackling Giant hogweed is that you need to have a plan. Always start from the uppermost hogweed infestation along a water course, so that when you clear an area you don’t have to worry about any more seeds floating down and re-infesting your treated area.

Safety first; get suited, booted and gloved

Green and gone

At any point, while the plant is green and actively growing, a systemic herbicide with aquatic approval e.g. Glyphosate is perhaps the most straight forward answer for dealing with hogweed near water. Note that you require the relevant training and qualifications before you can use pesticides, particularly near water.

Using a knapsack sprayer to apply the herbicide is the most efficient option for dense infestations and extensive populations. For less dense areas or sites with occasional plants, you could consider wiping neat herbicide on the leaves with a weed wiper or directly injecting it into the hollow stem. However, these methods do involve getting closer to the plant – something you should generally look to avoid.

Hogweed can be resilient and so a second ‘mop up’ visit a couple of weeks later can be warranted in the worst infected areas to catch any plants missed and to make your control effective.

^{Herbicide application by knapsack spraying, weed wiping and stem injecting}

The taproot of the matter

Young Giant hogweed plants with their turnip-like taproot lend themselves well to digging up. This is more labour intensive but it reduces the use of herbicide, requires only basic training and can be done by anyone. Stick your spade through the root, cutting it approximately 15 cm below ground, then dig up the taproot and leave it above ground to dry out.

A sharpened spade is about as specialised as it gets with this method – but we have dug out plenty without sharpening, too. It just very much depends on the ground you are working on.

^{Small hogweed plants can be dug out}

Bud intentions

If you find that time has grown legs and the hogweed plants are forming flower buds or are already fully flowering, it is best to slash their ambitions with a long handled pruning saw. It should be an absolute priority to treat flowering plants – every time a plant sets seed you can add another three years to your treatment plan. Seed ‘em and weep!

Giant hogweed flowers only once after growing for 2-5 years and then only if the conditions and its energy reserves allow for it. So, if you are only seeing hogweed leaves for several years at your site, there will be flowers coming soon. Although the hogweed plant will die after flowering a lot of later treatment work can be avoided by removing the flowering head before it sets and disperses seed.

^{Giant hogweed bud and flower}

Long handled cutting equipment is advised – this allows you to keep a safe distance from the plant and any sap splatters as you cut it. If cutting is the only form of control you are doing be ready to revisit the site regularly during the growing season to cut any new emerging flower spikes on the lower stem or regrowth at the crown. Cut under the flower head first – removing it intact – then cut the stem in sections until you’re as close to the ground as possible. If you can combine this with chemical control – treating the cut stem with herbicide – you’ll get better results. Chemical application needs to occur relatively quickly after cutting to ensure the plant will take up the pesticide and transport it into the taproot before drying out.

Catch them early enough – while there is only a bud or white flower – and it appears safe to leave the flower head on site to degrade. However, if the flower head has turned green the seeds might still ripen on the cut head and be viable when they drop off. If the flower head is green, and if you can reach, carefully put a cotton bag or similar over the flower head and tie it off before cutting – then collect and burn.

^{Cutting flowering heads followed by herbicide application directly into cut stem}

All dressed up and nowhere to grow

Suppressing new Giant hogweed growth by covering with mulch or membrane is an interesting method that does come up frequently when looking for hogweed control solutions – but we haven’t used in in the project due to the often large areas we work in. It is most suitable for smaller areas, like a garden, and is likely to work best on areas where there are only seedlings coming through. The idea is to cover the area with sturdy black plastic and or a generous layer of mulch and suppress seedling growth by blocking out light – a reasonably standard approach to weed suppression in gardening. You do need to check back regularly to make sure nothing pokes through the covering. Let us know how you get on if you give this a try.

^{Giant hogweed seedlings and field ploughing (image – Stanze)}

In suitable locations, particularly in fields, ploughing can be a good solution. It is most effective if combined with chemical or mechanical control – cutting the plant low to the ground first, or treating it with herbicide and waiting for it to take effect, followed by deep ploughing the area. After that most seeds, or seedlings, will be far enough underground to not see the light of day again. If, after initial treatment, more mature plants are being ploughed the large taproots should be removed as much as possible to prevent regrowth. Like the covering, we haven’t tried this approach ourselves, but in the right location this could be well worth a whack.

Rooting for native species

Where possible, and particularly in areas of previous heavy infestation and a built-up seed bank, reseeding with native plants and trees can make control efforts more effective. The aim is for native vegetation to outcompete new hogweed seedlings which emerge from treated areas – this sort of planting can also help stabilise bare riverbanks following treatment or winter die back. Timing is everything here – long-established, dense hogweed infestations cannot be defeated by reseeding alone. Manual or chemical control is necessary until mostly just new hogweed seedlings are appearing and their density is reduced to a degree that they will not easily overpower the native species you have seeded.

Flocking to the rescue

Situation permitting, grazing can be an effective approach in Giant hogweed control. Sheep, goats and cattle will readily eat hogweed and pigs even go one step further by uprooting and feeding on the taproot as well. However, there are a few things to consider before sending the four-legged cavalry into the field.

Woolly warriors brought in to battle Giant hogweed

Hogweed infestations in fields, woodlands, or at the very least a few meters removed from the riverbank, can be suitable locations for grazing control. However, stock still need to be managed and so the costs of additional fencing, gates etc should be considered.

Hairy breeds of animals, with dark skin, are the best choice to prevent the sap having negative effects on them. Black face sheep, for example, have successfully and without injury been used to graze hogweed. Since 2014, such a trial has been taking place in the Deveron catchment, with a second grazing trial site set up in 2019 by the Scottish Invasive Species Initiative (read about it here).

Livestock prefer to graze younger or smaller plants, so grazing should start early in the growing year as hogweed emerges. The grazing of seedlings can be very effective due to the lack of energy stored in the taproot at that point in the plants multi-year life cycle. On the other hand, grazing of more mature plants might not be to every animal’s taste – although we have witnessed sheep taking down flowering plants during our trials – and while it prevents plants from growing larger, grazing doesn’t kill mature hogweed as the already stored energy in the tap root allows the plant to make a come-back that same year or the following year. In this instance, the grazers merely work from season to season to prevent regrowth and progressively deplete the energy store – preventing flowering and spread.

Like chemical or mechanical control grazing is required at a site for a number of years before you can call it a job well done and consider the plant eradicated. Ongoing review and assessment of livestock density and grazing days is also important to prevent overgrazing of other vegetation – otherwise you might just be replacing hogweed and grasses with native recolonising species such as nettles and dock.

^{The sheep in the trial developed a taste for Giant hogweed and made short work of these plants}

A word of caution

Household ammonia, salt, liquid nitrogen and heating oil, amongst other things, are at times suggested on the internet for use as alternative approaches to Giant hogweed control. We haven’t tried any of these methods ourselves and neither have we found any robust evidence to suggest that any of these would be effective. The potential negative impacts on soils, waterways and wildlife when other proven methods are available means we won’t be exploring these methods further.

Summing up

Tackling Giant hogweed isn’t for the faint-hearted and isn’t for everyone. Wearing protective gear is essential and specialised training is required to use pesticides safely. But, hopefully, there are control methods, such as cutting flower heads, digging of roots, mulching or use of barriers in your garden or grazing with animals in larger spaces that might work for you in your own hogweed situation and circumstances. So, don’t be afraid to try this at home – but do be careful and do some thinking beforehand. If you do decide to take hogweed on use a method that is suitable for you and your site, only proceed after proper planning and, if needed, training and absolutely always with the correct protective clothing!

Just remember, always check and clean yourself and your equipment before leaving a site to avoid further accidental spread, prioritise control strategically in a downstream direction and focus on plants about to flower.

If you are in the Scottish Invasive Species Initiative area and want to help our control work then get in touch (E: sisi@nature.scot) and we will see what opportunities are available in your patch. Read more on our website.