Another Life: Could ivy create the perfect storm for Ireland’s ash trees?

A reader who lives beside St Anne’s Park in Raheny, in north Dublin, is saddened by watching ivy “taking over the new trees as they grow”. Has ivy become a national pest, she asks, like rhododendron and gunnera, the “wild rhubarb” of the west? In this she echoes the long-standing regret that so many seem to feel at the dark dominance of ivy in Ireland’s wayside trees and hedgerows.

Hedera helix hibernica, the Atlantic ivy, is our most widespread climbing plant, shrouding not only inland trees but also the floors of woods, shaded cliffs, the walls of ancient buildings and the rocky crannies of our windiest offshore islands.

On Aran its leaves were collected from a holy well as a cure for abscesses, one of the plant’s myriad uses in traditional folk medicine. In an Ireland historically bare of wayside branches, and waiting on the planting of hedgerows, there was still enough ivy to harvest as winter fodder for cattle and sheep. (Its stems were also stripped by a multitude of dairy goats, poised on their hind legs.)

Beyond the reach of livestock and people, ivy could grow as old as the oldest surviving primal trees. In his classic History of the Countryside, from 1986, Oliver Rackham wrote: "On an island in a boggy lough in Co Offaly is an extraordinary wood of great ancient oaks hung with ancient ivies (one ivy trunk is thicker than a fat man)."

I hope it is all still there, but the abundant ivy of today has been left to climb the trees of a greatly changed farm landscape. Cattle feed on grass and silage, dairy goats are few, and manual husbandry is in decline.

Chief among ivy's hosts is the ash, Fraxinus excelsior, often self-sown. Its late budburst and airy foliage, offering the briefest shade of any native tree, all give ivy the light to climb up and grow a dense flowering bush – the wind-catching "tod" – in the canopy.

How far does this add to the menace of ash dieback disease, Chalara fraxinea, on its westward sweep across Europe? This is now confirmed in forestry plantations in 19 Irish counties and in trees in native hedgerows in 12 counties, from Donegal to Wicklow. With the prediction of more gusty winters, ivy must complete "the perfect storm" for Fraxinus excelsior in Ireland.

As still seems to need repeating, ivy is not a parasitic growth on trees. It clings by suckers, which do not penetrate bark or suck sap. Its competition is for light, space and a share of the water in surrounding soil.

In roadside hedgerows it can indeed be smothering, its dominance promoted by regular mechanical flailing that checks the growth of hawthorn and other hedgerow trees but leaves the vigour of ivy intact. On tall trees its offence is firstly aesthetic, in the damage to arboreal grace and shapeliness, and then in the risk, at peak winter-storm force, of the whole tree being wrenched out of rain-soaked ground.

Set against all this has been the modern awakening to ivy's value to the natural world, as cover for bird nests and spiders, late-season nectar for wasps and butterflies, and so on. There's even an ivy bee (Colletes hedera), a large, white-banded, solitary bee with an autumn flight period to match ivy's flowering. It is common in Britain and soon expected in Ireland. (Biodiversityireland.ie would like to know of any sightings.)

In light of such ecological service many tree owners decide to leave ivy alone rather than excise a length of stem, low down, to cut off the plant’s food and water.

Its pros and cons – and the need for more research – were well discussed in For Love of Trees, published 20 years ago by the remarkable Prof Risteárd Mulcahy, cardiologist and health campaigner, who was still cycling into his 90s.

He included a chapter on the effect of ivy on solid structures. It described the widespread damage and destruction of ruins and estate walls as ivy’s aerial roots penetrate old mortar and then swell to lever stones apart. A recent survey of Co Clare’s medieval churches pointed to just this role in “destabilising” old buildings, especially as the west’s increased rainfall washes out the original lime mortar.

Mulcahy's book also included opinions that ivy on sound walls can not only be harmless but also provide an economical insulation. This is reinforced in a study, commissioned by English Heritage, in which an Oxford University research team analysed the effects of ivy growing on buildings and old walls in five parts of the country (iti.ms/1VVMYT5).

It found that, far from destroying buildings, ivy acts as a thermal blanket, warming walls by an average 15 per cent in cold weather and cooling them in hot weather, all modifying extreme temperatures that could cause walls to crack. It also provides weatherproofing and a shield against pollution. But again, where there are cracks and holes, ivy’s aerial roots will eventually find a way in.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks