Giant hogweed: the 'royal beast'

The big invasive plant taking over our rivers

Warning: this post contains graphic images of burns/blisters caused by giant hogweed

Once upon a time in a distant land, on the southern slopes of the western Caucasus mountains, the seeds of a monstrous plant were captured and sent across the seas to the Royal Botanic Gardens, Kew. With sharply serrated leaves, mottled red stems and enormous flowerheads, it towered over flabbergasted Victorian spectators. Eager to share this staggering exotic plant, its seeds were planted in ornamental gardens around the country, where this alien plant unexpectedly escaped, setting seed and invading our watercourses...

If you hadn't already guessed from the great big hint in the title, we are of course referring to giant hogweed, Heracleum mantegazzianum!

Image: a photograph of a stand of numerous giant hogweed plants against a blue sky, standing at approximately 5m tall with huge umbrella shaped flower heads and sharply serrated leaves

What is an invasive non-native species?

We have many introduced non-native species in England that happily coexist alongside our native species without issue. However, some non-native species become problematic and are known as invasive non-native species. Invasive non-native species (INNS) are those that may negatively impact:

• native ecology

• the environment

• the economy

• human health

So why is this plant such a 'beast', and why is it problematic?

As Genesis correctly lyricised in their song 'The Return of the Giant Hogweed' from their 1971 album Nursery Cryme, we can see that giant hogweed has been a problem in England for decades:

They all need the sun to photosensitize their venom Still they're invincible Still they're immune to all our herbicidal battering

Fashionable country gentlemen Had some cultivated wild gardens In which they innocently planted The Giant Hogweed throughout the land

Botanical creature stirs! Seeking revenge... Royal beast did not forget Soon they escaped, spreading their seed Preparing for an onslaught Threatening the human race

Although giant hogweed is not quite a triffid level threat to the human race, this invasive non-native plant is unequivocally one of the biggest threats to biodiversity in our riparian landscape.

This monstrous plant out-competes native plants producing up to 50,000 viable seeds, growing up to 5m tall and forming dense monocultures native plants cannot compete with. Its early germination enables it to develop populations ahead of native species, growing large leaves which shade out native vegetation. This decreases biodiversity and reduces the likelihood of natural native plant regeneration, lowering the quality of available habitat for wildlife and affecting other species in the ecosystem.

Image: a photograph of a Tees Rivers Trust contractor being dwarfed by a huge stand of towering giant hogweed plants during a session of herbicide management

As if this wasn't bad enough, giant hogweed is hazardous to the human touch.

Giant hogweed plants produce a phototoxic sap that can burn and blister exposed skin in sunlight, and may cause blindness if contact is made with the eyes. A reaction between chemicals in the sap and UV rays increases the photosensitivity of skin to sunlight, causing pigmentation, burns, and blisters to form, potentially reoccurring for several years after initial contact was made.

Image: a photograph of large yellow blisters and burns on a hospital patient's hand caused by unknowingly touching giant hogweed

How do we beat this beastly plant?

It's not all doom and gloom, I promise! Thanks to the National Lottery Heritage Fund, we've been tackling giant hogweed with an amazing group of dedicated local volunteers, contractors and corporate teams.

Giant hogweed has spread gradually yet relentlessly through the River Tees catchment area since its introduction in the 1800s, but we're taking a whole catchment approach to manage this invasive species through a process of intelligence gathering, direct action, education and natural site restoration by:

• Mapping the distribution of giant hogweed

• Managing its spread with herbicide control

• Reversing the loss of biodiversity by re-introducing native flora

• Restoring access and re-establishing a safer amenity of great local importance

• Raising awareness and educating children and adults alike about giant hogweed

Image: a photograph of two volunteers along the River Tees using hand held knapsack sprayers to control the spread of giant hogweed with herbicide

Although managing this invasive plant will take many years, we are making great progress!

Stay tuned to read more about our challenges and work to rid the River Tees from this 'Royal beast' plant!

GIF: Before and after comparison of two photographs of the same location before and after giant hogweed was treated with herbicide. The before image shows a monoculture of giant hogweed plants. The after image shows a meadow native plants