Conservation ranger Peter O’Toole stands in a clearing at Killarney National Park and looks at the rhododendron seedlings poking above the ground. Ten years ago specialist contractors liberated this site from a dense entanglement of the invasive plant and it’s now coming back.

“In three or four or five years’ time the whole place will probably be covered again,” O’Toole says. “It’s just an ongoing battle. People think that you can cut it down and that’s it. I wish that was the case.”

A single rhododendron ponticum can produce a million dust-like seeds – it’s impossible to clear an area once and expect it to remain cleared for good. The shade from its canopy of evergreen leaves eradicates all life beneath it; the plant can resist frost and survive fire. Given the chance, it would happily swallow Ireland’s ancient woodlands.

When nurseryman Conrad Loddiges first brought rhododendron to England in 1763, he could scarcely have imagined that two-and-a-half centuries later dedicated teams in every part of these islands would be locked into a seemingly endless war against the plant.

Native to Portugal and Spain as well as the Black Sea region, rhododendron found favour in the 19th century with landed gentry who used it to brighten up their gardens and provide game cover on estates.

Dr Jane Stout from Trinity College’s botany department says it’s unclear how rhododendron came to Ireland. It could be a hybrid brought over from the UK or it could have come straight from the Iberian Peninsula – “we don’t know, to be honest”.

The reason it’s doing so well here, she continues, “is because it just really suits the climatic conditions that it finds itself in. It doesn’t get grazed [because its leaves are toxic], so these little seedlings germinate really well in the damp mossy conditions in the west of Ireland”.

Whatever its origins, dense thickets of the plant, distinguished by its glossy green leaves and appealing purple flowers, can be seen all along the western Irish seaboard. It affects almost every county but nowhere is it happier than in the acidic soil and moist air of Killarney National Park, where it threatens the area’s oak woods and all that dwell within them.

“From a biodiversity point of view it’s a disaster,” says O’Toole. “It can wipe out the natural layers of the woodland floor and all associated animals and birdlife.”

Frankenstein plant

The park first attempted to get a handle on rhododendron in the 1960s. Horses and chains were used to rip mature plants out of the ground. Later methods involved cutting the plants at the stem and dousing the trunk in herbicide.

O’Toole says he first wanted to reduce the use of chemicals. Today the main method used to kill mature rhododendron is “stem injection”, making an incision with an axe or chainsaw and administering herbicide. For smaller saplings, workers cut the stem very close to the ground and spray it with a weak solution of herbicide and water, the “snip and spray” method.

Specialist contractors and volunteers work to clear and control about 40 sites around the park. Contractors remove mature plants and tackle dangerous areas, such as cliff faces. Volunteers, who during the summer include foreign university students, remove new plants from previously cleared areas via snip and spray.

It’s effective, says O’Toole, but some areas are beyond hope. About 3,000 of Killarney National Park’s 10,000 hectares have a rhododendron problem.

It takes decades to bring a rhododendron infested area under control. In Killarney, priority is given to oak woodlands; mountainous areas with coniferous trees are less likely to receive attention.

It’s ironic then that rhododendron is actually a protected species in its native Spain where climate change threatens its habitat. “It’s producing seeds but those seeds aren’t doing particularly well because there isn’t the damp, warm condition that we’ve got in the west of Ireland,” says Dr Stout.

Back in Killarney, O’Toole stops his 4x4 on Muckross peninsula to point across the water at Torc Mountain, which is covered in the bright green of this year’s rhododendron growth. “The cost of doing a hectare of that would be astronomical,” he says. “You’d have to do it by contract, all stages of it. It would cost millions and millions of euro to clear and even then you wouldn’t be finished . . . it’s only the beginning of a 30-year-long struggle.”

The Department of Arts, Heritage and the Gaeltacht has spent almost €3 million on the rhododendron clearance programme in the past 10 years. Of the approximately €899,000 spent since 2011, about €820,000 has gone on contractors and the rest on volunteer camps.

In recent months the department has been criticised, with Sinn Féin in particular saying the outlay doesn’t represent value for money. Controversy erupted last year when Groundwork, an organisation which ran volunteer camps in Killarney from 1981 to 2009, published a report claiming that areas declared clear of flowering rhododendron by the National Parks and Wildlife Service were actually on the verge of “exponential reinfestation”.

Groundwork chairman Trevor Halpin says they managed remote and ecologically important areas but from 2005 onwards the National Parks and Wildlife Service (NPWS) discouraged volunteers from working in those priority spots because they refused to use the snip and spray method, arguing it was slow and unsuitable.

The relationship between Groundwork and the NPWS came to an end in 2009 when Groundwork declined to work with the service’s prescribed programme. Halpin says the methods used at Killarney should be dropped in favour of an approach pioneered by his organisation. “We have no preference over who carries out rhododendron clearance in Killarney National Park as long as it is done in a scientific manner with proven results,” he says.

O’Toole declines to comment on the dispute but stresses his confidence in the current set up. “Our methods are more efficient and more effective,” he says. “I think we have a very good programme going and it will continue.”

Later he says there isn’t really an option, the programme has to continue. “If the maintenance programme stops, then in a matter of five years a lot of the areas would revert back again quickly to rhododendron. And as soon as they start producing seed then you’ve totally lost the battle.”