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.

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.

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.

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.