Giant hogweed, (heracleum mantegazzianum) is more than just an invasive plant species; it is a genuine threat to people’s health, and it requires an extremely aggressive level of treatment to remove infestations. A growing number of UK parks, public spaces and areas of natural beauty have reported appearances of giant hogweed, causing a number of issues from severe skin blistering to the destruction of local plant species and weakening of flood banks.
David Thorpe is Biodiversity Specialist at Natural Resources Wales and has worked on the infestation on the River Soch in Wales. Located in rural North Wales, the Soch is popular with anglers and walkers, sitting a few miles away from the village of Abersoch to the east, and just over a mile and a half away from the beach at Porth Neigwl. The area has thousands of visitors each year, from holidaymakers to surfers, meaning the presence of giant hogweed causes serious issues for the local council, residents and visitors.
Protecting the Afon Soch
David, who is qualified through a recognised standards and auditing organisation for herbicides, fertilisers and related sectors called BASIS, has been advising on how best to tackle the giant hogweed infestation. Continued use of glyphosate in both spray and stem injections is expected to provide the River Soch with some much-needed respite.
“Glyphosate is, effectively, the only reasonable tool we can use against giant hogweed”
“We’re currently running a three-year treatment plan on the Soch,” says David, “but we’re not sure if that will be enough. Giant hogweed does produce quite large seeds that can survive longer than three years. We’re hopeful that after three years there will be enough of a reduction in plants that any further treatment won’t need to be as extensive. Hopefully, we’ll just have to walk up and down the river with a backpack, spraying individual plants every other year.”
David is passionate about his work, and is trying to preserve the local landscape for future generations. “I’ve been involved in control and management of invasive species since the year 2000, and I’m qualified to advise on the use of herbicides against species such as giant hogweed and Japanese knotweed. I also sit on the Wales Invasive Species Group board. Giant hogweed is actually quite restricted in North Wales, although it’s more common along the River Dee, and in Chester there’s been quite an infestation.
Because a number of flood banks are involved, in the infestation it could get quite serious as other plants are shaded out leaving weak areas in winter months. The Dee Invasive Non-Native Species Project has been targeting sites for control, doing so in partnership with local authorities, Natural Resources Wales and the Environment Agency in England. Because of the possible risk to human life, as well as the possible damage to the flood defences, giant hogweed has been a major cause of concern for us.”
Controlling giant hogweed
The control programme at the River Soch was initiated after concerns regarding the plant’s spread. People have suffered with blisters after accidentally touching giant hogweed that had been cut down. “There have certainly been cases of our flood defence staff on the Dee, while strimming the banks, being affected by the plant due to it appearing without us realising it. They’ve had bad blistering to the hands and arms, from the sap.”
Giant hogweed can grow up to three metres (over 10 feet) in height, and is a close relative of cow parsnip or hogweed (heracleum sphondylium). The difference, when both plants are young, can be difficult to identify. Giant hogweed is a non-native species to the UK, thought to have been brought over by Victorian gardeners from Eastern Europe due to its exotic nature and impressive height.*
Unfortunately, as it’s a non-native species, giant hogweed is more difficult to control. “Critically, it’s growing in an environment it hasn’t evolved in, so we don’t have fungi, pathogens, aphids and other insects to control it. This means it has the capacity to outgrow the native flora, which is being naturally controlled.”
Eradicate or reduce
David recognised the fact that glyphosate was one of the few options for combating the aggressive plant. “Glyphosate is, effectively, the only reasonable tool we can use against giant hogweed. We could cut the plant every year, but it will grow back from the roots, so we need something to target it at the roots.”
There are alternatives, but the cost and manpower would make the treatment far more difficult and, potentially, dangerous. “We investigated using a shovel in springtime to sever the root, but it does still leave a viable root in the ground which can re-sprout. This method weakens the plant, but doesn’t kill it. Glyphosate, being systemic, doesn’t just kill the leaves but the roots as well. Injecting into the stem means you use less chemical in a more targeted way, and have the potential to prevent viable seed from setting. From a manpower and safety point of view, glyphosate is far better. It can be applied far quicker and without needing to come into direct contact with the sap, which is what causes blistering.”
Untreated, giant hogweed could stretch to the nearby village of Abersoch. “Without intervention, it will spread to wider areas and become unmanageable. Although giant hogweed can spread up to 10-20 metres a year, water acts as a natural dispersion tool and can speed up the spread. The nearest village of Abersoch is at the bottom end of the River Soch, and would eventually be affected, along with local schools, rights of way and fields. The closer the giant hogweed gets to Abersoch, the more aspects of everyday life get affected.”
A positive outcome
In spite of this, David is hopeful that the programme of glyphosate treatment will produce solid results. “I would expect to see an 80% reduction in giant hogweed, as a ballpark figure. In three years’ time we would expect newly emerged seedlings to be visible, and for all of the older plants to be gone. I don’t think we’ll be able to completely eradicate the plants immediately, because the giant hogweed takes two or three years to flower, so full eradication is more likely to take 10 years.”
Plant growth is being monitored using an annual survey, so the desired targets can be accurately assessed after the three-year period. Through this careful monitoring, and with targeted use of glyphosate, the River Soch should see a significant reduction in the giant hogweed along its banks.