Another Life: The struggle against the strangler in our midst

To travel the shores of Killary Harbour, Connacht's big fjord, just now is to be bathed from time to time in the rosy glow of Rhododendron ponticum. Wayside hedges of it extend through Connemara, and new mounds of it adorn rushy pastures and bog margins.

From Killarney's old oak woods to the rocky hill of Howth, rhododendron is popularly enjoyed as a most beautiful "wild" flowering shrub. Its new botanical tag of R 5 superponticum – the 5 denotes it as a hybrid – seems also, however, to acknowledge the plant's special resilience as the most troublesome terrestrial invader. In both Ireland and Britain the rearguard battle with its spread through native woodlands has already cost millions, rarely with total victory in sight.

Compared with the behaviour of Rhododendron ponticum in native Spanish and Portuguese habitats, the shrub that has plagued the oak woods of our national parks, shading out tree seedlings and poisoning their soil, is a supervigorous strangler, thriving in our moister air and pushing up stronger leaves to many metres high.

Imported from Iberia to England in 1763, it was originally kept indoors as a tender, decorative plant. Later, crossed with frost-hardy species from the Appalachian Mountains, in North America, it was tried outside.

A severe British winter in 1879 proved its new resilience as an evergreen windbreak and wildlife cover on big sporting estates, including those of Ireland. By the break-up of big-house estates, rhododendron was already running wild.

The ponticum in the name refers to Turkey, where there are plants up to 400 years old and where it has spread, modestly enough, through natural layering. But the rampaging Anglo-Irish hybrid now unique to these islands is spread mostly by windblown seeds, of which a well-grown clump may produce a million every year.

Seed production needs pollinators, and among scientific research into the secrets of the shrub's success is the work of Prof Jane Stout, the Trinity College Dublin botanist. In the first study of its kind, she spent hour after hour, over days, watching insects that visited the big rhododendron flowers in clumps on Howth Head and a hillside in the Wicklow Mountains.

The flowers exude abundant sugar-rich nectar, and Stout followed each insect as far as possible to see how many flowers it visited and if it brushed against the pollen-laden stigma. She tracked the manoeuvring of 555 insects, most of them bumblebees of six species. Even though only a third of them connected with the stigma, enacting the transfer of pollen so vital to seed production, any flower was likely to be pollinated at least once every three hours during the peak flowering season. This, Stout concluded, explains the high set of seed that has nourished the spread of R ponticum through the Atlantic oak woods of the west, so rich in rare mosses, liverworts and ferns.

Now she seeks to discover why the shrub puts out toxic nectar (toxic, too, in honey for humans). How poisonous is it for bumblebees, or is it just to to put off nonpollinating insects? And how will its growth respond to global warming?

Although the shrub’s spread began around the start of the 20th century, it went largely unheeded for some 50 years. Then its smothering impact on native plants and wildlife, and the regeneration of trees, began to concern the ecological guardians of Ireland’s new national parks. In the woods of Killarney, for example, by the 1980s more than 260 hectares were solidly and darkly entwined.

From 1981 to 2009 the NGO Groundwork, supported by the Irish Wildlife Trust, ran voluntary summer camps, subsidised by the National Parks and Wildlife Service, to clear rhododendron from key Killarney hillsides. About 5,000 young people, drawn from Ireland and abroad, used chainsaws and mattocks to clear 40 per cent of the woods. They also revisited each area every seven years, advancing in lines to root up any new seedlings or regrowth. It was hard manual labour but as thorough as it comes.

From 2005 the park management decided to take over the clearance through contractors, using, in place of hand tools, herbicides sprayed on leaves or painted into chainsawn stumps. In 2009 any further voluntary groundwork was declined.

Last year small Groundwork teams revisited sections of woods they once had kept clear for 20 years. R ponticum takes 10 years to flower, and none of the areas was now free of substantial flowering plants. Some had already scattered seed, and others looked certain to do so before the next maintenance visit scheduled in the park's clearance programme.

All this is detailed and mapped in Groundwork's report, accessible online (groundwork.ie/news.htm). And the ginger group Save Killarney Oakwoods is already lobbying for urgent action before the newly named Rhododendron 5 superponticum can gather its forces yet again.