Green Limerick: Giant hogweed ‘out of control’ on the banks of the Maigue

A RIVER in Limerick renowned for being a historical salmon fishery is fighting for its life against a “biodiversity crisis” brought on by an out-of-control invasive species.

The Maigue River, which flows from Cork into Bruree, Croom, Adare and later into the Shannon Estuary just west of Limerick city, stretches over 62km and drains an area of 1,000 km2.

A registered charity, The Maigue Rivers Trust, has been working with local communities to ensure that the rivers and lakes of the Maigue catchment achieve their full potential, both recreationally and environmentally.

Liz Gabbett, Project Officer with the trust, has warned that one of the most salient challenges facing the community and the county in protecting the biodiversity of the river is invasive species.

“Unfortunately we have to deal with invasive species. We have a major problem on the Maigue and that is giant hogweed.
“That runs the whole length of the main channel,” said Ms Gabbett.

She described giant hogweed as both “giant by name” and “giant by nature” with each plant spreading between 30,000 to 50,000 seeds.

Native to the Caucasus Mountains at the border of Europe and Asia, It was introduced to Ireland in the 19th Century as an ornamental garden plant. Since then, it has escaped into the wild.

With seeds that can remain viable up to 15 years after initial dispersal, giant hogweed is spread by wind, water, animal and human influence, and grows up to five or six metres in height.

“It loves Ireland and our native soil. In other parts of the world it doesn’t grow that big,” she said.

Giant hogweed poses a threat to human health due to production of a hazardous sap that can cause severe burns and scarring by sensitising the skin to light (UV radiation). Its large surface area means it shades out native species and its high volume of seed production means it easily propagates.

The species can also increase soil erosion along riverbanks, the National Biodiversity Data Centre says.

Ms Gabbett highlighted that it spreads like wildfire through strimming, which causes the sap of the tree to fly everywhere. She stressed the importance of farmers knowing what they are strimming.

In May and June 2022, the Maigue Rivers Trust secured funding from LAWPRO’s Community Development Water Fund and Limerick City and County Council’s Kilmallock and Cappamore Municipal Fund allocation to start giant hogweed control measures on the Morningstar River (a tributary of the Maigue) between Elton and Ballyvolane near Bruff.

Leading up to the eradication project, the trust identified a 5.5km stretch of the river before Bruff which was “overrun” and “out of control” with giant hogweed.

“There is an infestation there in Elton that has been there for years. It is going downstream. If it gets into the river, it is going to destroy the park in Bruff,” Ms Gabbett warned.

As a result, Knotweed Ireland Services removed 200 Giant hogweed flowerheads (estimated six million seeds) and injected the stems with glyphosate, along 6.5km of the river earlier this year.

However, Ms Gabbett stressed that the giant hogweed control work needs to be a multiannual project running until the plant’s seed bank is exhausted.

“To get the Morningstar River under control is a five-year project. To get the Maigue channel under control is a 20-year project,” said Ms Gabbett, who identifies as a citizen scientist.

Each year that nothing is being done, means that the problem is growing more and more, as the river carries seeds along the bank of the Maigue and its tributaries, she added.

While the outlook is bleak for the River Maigue and invasive species, the situation is not untenable.

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She stressed that the trust, which has 11 trust directors and only one official paid employee (Ms Gabbett) requires more funding and workers to continue eradicating invasive species from its banks.

She warned that if the trust does not get more funding, all the momentum gained, particularly work with farmers and landowners along the Maigue “will be lost.”

“We have a lot of lovely towns and village parts where the river goes through because the river connects everyone.

“It connects up these resources and is another reason to bring people to Limerick and show what we have,” Ms Gabbett concluded.