Japanese knotweed, ash dieback, Indian balsam: Understanding Ireland’s invasive species

Some non-native plants or creatures do serious damage to Ireland’s environment. But new attitudes mean we could yet contain the threats

As the story goes, the invasive grey squirrel was introduced to Ireland as a wedding gift. A pair was set free in Cork in the early 1900s as a ceremonial gesture to the bride and groom.

“I don’t really 110 per cent buy that,” says Leif Barry with a hint of diplomatic understatement. “Do people love grey squirrels? Yes. Were they all over Britain for a very long time? Yes, they were. Was someone most likely bringing them over and letting a few off in a park because they just loved nature?” Probably.

Barry is an environmental scientist who works as an Office of Public Works (OPW) guide on invasive species and runs biodiversity walks through Dublin’s Phoenix Park.

Unlike non-native species that coexist with native ones, invasive alien species are alarming because they establish themselves before taking over everything around them.


They cost the world at least €392 billion a year and are a leading threat to biological diversity, a UN report said this week. In Ireland, the National Biodiversity Data Centre lists dozens of invasive species that have been recorded here at some point, some of which may not still exist.

It is a specialist area but some names roll off the tongue – Japanese knotweed, rhododendron ponticum, giant hogweed. Among animal species, the sika deer, zebra mussels, noble false widow spider, even wild boar.

“They’ve done lots of things to try and eradicate it,” says Barry, returning to the familiar grey squirrel example. “It turns out if we eradicated all the grey squirrels out of the park here, the population would be straight back within a month. Because it would just come in from the other parks. It’s gone too far.”

Toured ahead of the biodiversity festival taking place this weekend, the park offers just a peek at what is a national issue. Strolling around a small patch of its woodland, Barry stops every few metres to identify an invasive plant or creature introduced to Ireland at some point over the centuries, some more threatening than others.

Many of them are colonial legacies, evidence of a time when the British establishment proudly imported plant life from throughout its empire. The well-to-do displayed them as status symbols, emblems of culture, sophistication and knowledge. Many of them thrive, long after their impacts have been understood.

There are other invasive elements. Ash dieback, the disease that threatens 95 per cent of Ireland’s ash trees, is caused by the invasive fungal pathogen Hymenoscyphus fraxineus.

Barry stops beside an ash tree near the park’s Knockmaroon Gate Lodge. Dieback is to the fore of the public’s imagination, he believes, because of the ash tree’s centuries-old place in Irish life. It is a reliable supplier of wood, and the first native spears were carved from its trunks, as was the first plane to land in the Phoenix Park in 1912. Barry points to signs of the disease. But there is hope.

“There’s a way out; 5 per cent [of trees are] genetically resistant,” he says, explaining that cutting down every ash tree would remove a possible solution. “It could still get contaminated but be able to fight off the disease ... there is a bacterium on the ash tree that will fight off the fungus.” Seeds from these trees could be inoculated and planted.

Japanese knotweed established itself as far back as the 1900s but has only come to prominence in recent times following a long lull, says Barry. Today, it is a serious threat to building integrity. Roadside warning signs not to cut the plant are common because, Barry explains, just the tiniest amount scattered is all it takes to spread.

The bright purple Indian balsam was once promoted and planted as a good foraging plant for honeybees. “Now it’s an incredibly invasive species that has taken over whole riverbanks,” he says. “It establishes itself, out-competes all of the native plants and then in the wintertime dies back and leaves the bare earthen bank exposed.”

In the late 20th century, the Harlequin ladybird was brought in from Asia to eat nutrient-consuming aphids on plant crops. Then it began to eat native Irish ladybirds as well.

Like ash dieback, much of the invasive species come down to importation. Rounding a large pond where north African emperor dragonflies whizz past, Barry says the toxic oak processionary moth, which has been much talked about recently and poses a threat to humans and animals, comes in on cheaper, imported trees.

A vital step is to encourage people who spot invasive species to report them. The oak processionary moth may yet be stopped here but the challenge is wider. Barry believes a new modern era of affluence and nature appreciation makes meeting that challenge possible. Times have changed.

“We have our comforts now. People start to look back; well I have enough to eat, I have a roof over my head, I have an income,” he says. “So what more do you need? Well, you need a stable environment to live.”

The Phoenix Park Biodiversity Festival & Honey Show takes place from Friday 8th to Sunday 10th September.