New machinery is tipping the balance in the decades-long battle to clear rhododendron in Killarney National Park, according to the National Parks and Wildlife Service.
The park, which is home to rare plants and animals, is one of the most important areas for biodiversity in Ireland, but the invasive species has been a major threat to that biodiversity.
In recent months, there has been significant progress made at the Co Kerry park to eradicate rhododendron.
The flowering shrub introduced from Asia in the 1800s thrives in the mild, wet climate and acidic soils of the park.
NPWS Divisional Manager Éamonn Meskell said: "We have 27,000 acres of Killarney National Park. 20% of that is lakes, so the rest of it has rhododendron in the woodlands and the boglands and the heathlands.
"We've been working consistently over the last 50 to 60 years with volunteers and ourselves and contractors," he added.
"Nothing - man, woman or beast couldn't go near it or go through it."
They have cleared thousands of acres at a cost of millions of euros and thousands of hours of work, both paid and volunteer, but they have not cleared all of the park, he explained.
Those parts that are cleared must be maintained, which means regular checks for fresh growth of rhododendron. One plant can produce a million seeds to be spread by the wind.
NPWS District Conservation Officer Mary Sheehan explained why so much effort is put in.
"Rhododendron is so dense it covers the woodland floor. It covers the ground floor, shrubs and trees. Therefore nothing really grows underneath it, or nothing can survive."
The native woodlands of Killarney National Park are some of the most biologically diverse areas in Ireland.
"We have a large number of species, right from fish species to bat species, we have all nine bat species in Killarney National Park," she said.
"Also, we have some of the rarest habitats in Ireland, including the yew woodland, for example, which is a rare habitat on the European stage. In the European Union, we have the largest tract of that.
"So it is vital, not only for Ireland, but on a European scale, to protect and enhance and expand these habitats and look after these species," she added.
Killarney has been leading the way on developing new ways to clear rhododendron.
The latest weapons are machines equipped with mulching blades to cut back the areas of densest growth.
"All of the wildlife associated with all natural layers has been wiped out... so from the biodiversity point of view it is a complete and utter disaster."
When RTÉ News visited Tomies Mountain, above Lough Leane, NPWS Craft Chargehand Conor Donoghue was operating one of the two machines in use.
"If we were tackling this by hand. You just couldn't do it, you couldn't, it was just so dense. Nothing, man, woman or beast couldn't go near it or go through it," he said.
Using the machines, up to an acre a day can be cleared. A layer of rhododendron mulch up to 10cm thick carpets the cleared areas.
Trees uncovered as the rhododendron is cleared have no branches or leaves for the first three or four metres of their trunk. The dense coverage of rhododendron smothered all growth below that height.
In Abbey Wood, between Killarney town and Muckross Abbey, a team of volunteers from Killarney Mountain Meitheal is using chainsaws to clear the margins of the wood of rhododendron.
The most densely infested areas have already been tackled with the tracked machines.
Peter Murtagh joined the group before he retired after 40 years of service with the NPWS in Killarney.
"If you look through the site, the devastation caused by the rhododendron infestation... all the natural layers of the woodland have been wiped out," he said.
"All of the wildlife associated with all natural layers has been wiped out as well, so from the biodiversity point of view it is a complete and utter disaster."
Mr Murtagh said the machines, which he calls "the flail mór", are great for clearing some areas but added that for him the big leap forward was developing the technique of injecting herbicide directly into the rhododendron plant.
"Once you injected a small amount of herbicide into the plant, the plant takes it up and it's locked into the plant's transport system and kills it from root to tip.
"It's a very environmentally friendly way of tackling rhododendron, because there's no collateral damage, nothing is affected surrounding the plant. And for me, that was a breakthrough," he said.
Mr Meskell accepts that some may have concerns about the use of glyphosate in the national park, but says it is used as sparingly as possible.
"We, over the course of 40 years, have done research on the amount of glyphosate which can kill rhododendron, and we have it down to a 14% solution now.
"And we also, when we're using the glyphosates, we do a spot spray on a particular area of the plant which we've treated. So it's a direct kill, and nothing around the edge of the plant or the floor of the woodland is damaged," he said.
Once areas are cleared the next stage is to allow the native woodland to regenerate.
In Reen Wood, Conservation Ranger Tim Cahalane showed a small fenced-off area.
"It would have been a dense thicket of rhododendron right up until the mid 2000s; it would have been cleared then of heavy rhododendron. It would have been maintained and follow-up phases would have been carried out by staff over the years.
"And then, when it was completely cleared in 2021, it would have been fenced off, and you can see fantastic natural regeneration inside."
The fence is necessary to keep out deer whose grazing would prevent plants establishing and the rangers also have to cull deer every year to protect the woodlands.
After just a few years, Mr Cahalane is happy with the results.
"It's mostly birch, as you see there, but there's also some mountain ash, there's some willow, there's some oak, and there's other species like holly, and you know, it's very successful in there."
While some of the trees are self-seeded, many are planted by NPWS staff.
"The new machinery.... will all be used in other areas of the country where there is rhododendron. And it's going to be the template."
NPWS Horticulture Supervisor at the park Gerry Murphy leads the team which collects tens of thousands of acorns every year.
They are cultivated on special rodent proof benches at Muckross.
"It's very important that you have Irish providence, that you're not importing plants from other countries that you could be bringing in disease, something like the ash disease, and also, when you have your own providence, they can climatise easier in their own areas," he said.
The wood for the fences that protect those young trees also comes from within the park, a circular economy solution.
Across the road from Muckross, NPWS worker PJ Murphy was cutting planks from a fallen tree.
"Last winter Storm Éowyn left a lot of dead fall across the roads and into the park, and besides leave it there to rot, we got the sawmill, and it seems to be producing timber pretty good for us."
The diversity of native trees found in the park means there is an appropriate timber for every job.
"In wetlands, you would use more larch or rot resistance timber for projects rather than Douglas or fir... conifers would be more for fencing. And there's other timber like oak and ash... harder wearing timber for different types of projects, like bridges or handrails."
Mr Meskell is confident the methods developed in Killarney can be applied in other areas with rhododendron infestations.
"The research we've done, the trials we've done on the most effective way of treating rhododendron, to kill it, and now the new machinery in the heavily infested areas of the first initial clearance will all be used in other areas of the country where there is rhododendron. And it's going to be the template."
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