The woolly warriors fighting Scotland’s most dangerous plant

On a quiet riverbank just south of Macduff lies a new secret weapon in the fight against poisonous giant hogweed: a flock of sheep.

In the shade of the trees this colony of sheep have developed a taste for the invasive plant, munching down stems until they are little more than stumps in the ground.

But the sheep didn’t get there by accident.

The Scottish Invasive Species Initiative (SISI) are at the root of this ingenious project, and over the last three years have been testing the feasibility of using sheep to control this insidious plant.

Known for its toxic sap that can cause burns, blisters and even blindness, giant hogweed is a non-native species which has been spreading uncontrollably across Scotland for decades.

For years landowners have waged chemical warfare on this giant weed but with little long-term success.

Now, however, it appears the solution to tackling one of the country’s most loathed plants may have been quietly grazing in the local farmer’s field.

Giant hogweed is one of the worst offending invasive species in Scotland.

It can easily grow two metres tall, forms dense, impenetrable stands and will outcompete all nearby plants, destroying natural flora and fauna.

Originating from the lower reaches of Russia and Georgia, this plant arrived in the UK back in the 1800s.

The Victorians loved its impressive clusters of white flowers, massive leaves and thick, towering stalks, while its Caucasus Mountains origins offered an exotic twist to their ornamental gardens.

Giant hogweed may have been all the rage in the 19th century, but few Victorian gardeners could have suspected they were cultivating a monster that would go on to invade riverbanks, pastures and wasteland the length and breadth of the country.

Its phototoxic sap burns the skin of anyone who touches it and so is a serious hazard for walkers, anglers and anyone trying to enjoy the countryside

But while humans and other animals such as dogs must steer well clear, giant hogweed isn’t poisonous to all.

Sheep are immune to its aggressive sap, and the SISI are currently in the final year of a practical experiment investigating how best land managers could use sheep to control giant hogweed.

With the River Deveron rushing by in the background, Karen Muller, a project officer at SISI, has been working at this site since the project began and says the team have learned a lot in that time.

“This is the third year of the project. In 2019, we started off with a larger number of sheep – 25 – grazing for a longer time.

“That was the first time they (the sheep) had encountered giant hogweed so it took them a little bit of getting used to and they had to be kept in a park for a little while until they really developed that taste for giant hogweed.

“But once we let them loose they did a really, really effective job of grazing the giant hogweed on the site.”

She explains how the sheep did too good a job, clearing the area of hogweed but also munching on everything else in their paths.

This type of overgrazing is detrimental to the biodiversity of the area, so the team made changes.

“In 2020 we reduced the number of sheep and reduced the number of sheep grazing days by about half, to see what would happen then,” Karen said.

But a mild winter and long periods of heavy rain turbocharged the spread of giant hogweed on Scotland’s river banks and the Covid-19 lockdown made tackling the problem even harder.

With lockdown restrictions in place, Karen and her colleagues were unable to monitor the site as they usually would.

Luckily, however, the sheep weren’t subject to stay-at-home orders and the farmer herded them onto the test site over the summer.

“What we found is that the sheep still really effectively grazed the giant hogweed and they overgrazed the rest of the vegetation to a lesser extent,” said Karen, “but there was still some overgrazing going on.

“So this year we are keeping 12 sheep on-site and we are cutting the sheep grazing further – so actually compared to 2019, that’s actually about a 70% decrease and we are looking at if the sheep will still graze the hogweed effectively and what that does to the rest of the vegetation.”

Throughout the summer the team will be back to monitor the sheep’s progress.

In autumn, a final count will be done to measure exactly how effective the sheep have been at grazing the hogweed.

It is expected that the number of giant hogweed plants will reduce, as was recorded in the previous two years, but this doesn’t mean the hogweed is under control.

“Sheep grazing the plant once or even twice won’t kill the hogweed,” said Karen.

“It needs to be a continuous effort over several years until the taproot’s energy has been entirely depleted.”

Like carrots and dandelions, giant hogweed grow from a taproot.

These long, thick roots are deeply anchored in the soil and contain enough energy to regrow the plant several times until it finally dies.

Every time a plant is munched away by sheep, it must use more and more energy to return until it eventually dies off.

The experiment has proven that sheep definitely have a taste for giant hogweed and that grazing can reduce the spread of the plant and eventually kill it off entirely.

Once this summer is over, the SISI will be putting together some practical advice to help farmers and landowners recreate their success on their own estates.