The plants tower above me. Three metres tall, they have frothy white flowers, spiky green leaves and prickly, dangerous stems. On a warm, airless summer day on the banks of the River Tay near Perth, I’m sweating in a protective suit as I wield a saw with a two-metre handle.
I’m also worried. Giant hogweed, with its angry purple-black blotched stalks, looks like something from a horror movie, and I know its reputation. I don’t want to be the next person making the news with horrific blisters.
But my guide, Mark Purrmann-Charles, has taught me well. I pull the saw sharply towards me and it rips through the hollow stem. The plant crashes down safely away from me, sap welling from the cut. Mark works for the Scottish Invasive Species Initiative (SISI) in the vast Tay catchment. Eradicating this hogweed is a big part of his job, and it’s hazardous. “It’s that sap that’s the problem,” says Mark. “I got some on my glove once and then scratched my neck. I won’t be doing that again…”
How did giant hogweed take over?
Giant hogweed, originally from the Caucasus, was brought here, like so many problem plants, by enthusiasts in the 19th century. The first reports of it in Britain are from Kew Gardens in 1817, with wild specimens seen in Cambridgeshire in 1828.
The sap causes a form of dermatitis called phytophotodermatitis. It stops the skin protecting itself from the sun’s burning rays, producing large, painful blisters, sometimes persisting for years. Mark’s neck burns – which fortunately cleared up quickly – are the only time anyone working with SISI on the Tay has been affected, but that’s only thanks to constant vigilance.
Is giant hogweed bad for wildlife?
The plant isn’t just dangerous to people, it’s an environmental nightmare: it shades out and kills off native plants, bursting out of the ground like a toxic zombie growth before other plants can get started.
Hogweed has now spread across the UK, lining paths and riverbanks in dense stands. Commonest in south-east England, in Scotland it’s mainly on east-coast rivers: it was often planted in ornamental gardens on Victorian Highland estates.
When rivers flood, seeds are washed downstream and settle on the lower banks. The plants are hardly noticeable in their first year, but in the second they reach perhaps 1.5m with big, broad leaves. Those leaves soak up energy to store in the plant’s roots, ready for the invader to show its true colours in the spring of the third year.
That’s when it races up, reaching up to 5m before it flowers, sets seed and the nightmare cycle starts again, with seeds blowing into the water and heading downstream to new sites. It also spreads into the surrounding countryside and creates impenetrable stands such as this one.
Localised efforts to remove it struggle as fresh seed arrives constantly from upstream, undoing all their efforts. But SISI’s approach is different: it aims to stamp out the alien invader with a careful, methodical top-to-bottom strategy. It starts at the top of the river catchment, finding the seed sources. Once those areas are controlled, with herbicide spraying and cutting, Mark and his volunteer helpers work downstream, knowing fresh seed can’t arrive.
How is giant hogweed being tackled?
Mark and I meet retired quantity surveyor and local resident Hamish Law at another site near Perth, on the banks of a Tay tributary, the Almond. He volunteered to clear giant hogweed here with SISI after spotting it from the nearby cycleway. “Some days it felt a bit like hand-to-hand fighting. There’s such a huge amount and you have to get through it,” he says.
As we walk across the site we crunch on dried, fibrous stalks, cut last year and rendered harmless by decay. Lush native vegetation has sprung up since the hogweed was cleared but, among it, Hamish spots first-year hogweed plants. The team will spray these next spring, before native plants come up, as part of the control process. “We look like something out of Ghostbusters,” laughs Hamish. “When they’re full the spray tanks are pretty heavy. We only work for half a day at a time: it’s hard work, your legs get tired and you don’t want to fall over in it. But seeing the results makes it worthwhile.”
While we’re on the Almond I spot a dense stand of Himalayan balsam. Mark explains that now toxic hogweed has been controlled, another invasive plant can move in. “It’s a bit like whack-a-mole,” he says. “But we keep at it.”
Giant hogweed work is over for this year, but volunteers will be needed again next year. They can also step up to help work on SISI’s other target species over the summer, and they provide thousands of hours of heavy lifting to the project. As well as giant hogweed, SISI tackles white butterbur, Japanese knotweed, Himalayan balsam, American skunk cabbage and the semi-aquatic American mink – invasives that spread on watercourses. It’s funded by government agency NatureScot but works through local rivers and fisheries trusts, which look after Scottish catchments.
There is no obligation on landowners to clear hogweed – although it’s illegal to spread it – but SISI finds that once it starts work, landowners realise the plant can be beaten so they put in their own time and resources to help with long-term control. The effect of people wanting to help, whether it’s landowners or local people such as Hamish, maximises the impact of SISI’s £650,000-a-year budget, according to SISI project manager Callum Sinclair. “The model is one that should be copied and shared,” he says.
Will giant hogweed ever be eradicated?
No sites in SISI’s area have been declared free of giant hogweed: seeds survive for up to 10 years and SISI is just seven years old. Sites have to be revisited regularly until the seeds are all gone, but Callum says, “There are some catchments where we have control from top to bottom, which is great.” The key is persistence. On the 193km Tay, SISI has come a long way, to the outskirts of Perth.
Back on the main river, Mark strides confidently in his protective gear into the deep giant hogweed thicket to demonstrate the complete control technique, slicing stems off a metre above the ground then spraying the remaining leaves and severed stalks.
The glyphosate liquid is dyed blue so workers can see what’s been sprayed, minimising the use of weedkiller, which is the only effective way SISI has found to deal with the plant. By the time he’s done, Mark is also sweating.
Later, scanning across the river from high on the valley side, he spots another huge giant hogweed stand. Days of work will be needed there. Within a kilometre of this site, Mark says, there are six or seven others. “When you look at sites like this, overwhelmed by hogweed, you can think, ‘What’s it all for?’” he says. “But when I look at the tens of kilometres of riverbank we’ve cleared and are being managed, that keeps me motivated. If you do it properly and have a strategy, you know you will deal with the problem.”
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