Tackling the spread of infernal ferns

Another Life:The decline and fall of bracken seduces my eye every autumn: the dry fronds curling through delicate ochres and golds, the wet ones glowing perversely as they crumble into embers of hot russets and burnt siennas.

Connacht at the year's back-end is well worth a week-end's gamble on the weather. Try Leenane, now they've fixed the bridge, and take the hill road through the bracken on the north side of the fjord.

Pteridium aquilinum, was one of the pioneer plants of the world, missing now only from the Arctic and Central America, and is seemingly still determined to fill every gap that is left to it on the drier hillsides of these islands. From the Scottish Highlands, through the northern uplands of England and Wales, to the Wicklow uplands - and, of course, the west - it remains "a problem weed".

Its fronds crowd the clearings of the old western oakwoods, but also the margins of green fields and pastures right down to the sea. In a landscape of part-time farming, it leaps into any half-hearted hectare where cattle would once have trampled its young fiddleheads to death. It spreads by wind-blown spores - those mysterious, dust-fine, embryo-less entities that germinate even in the dark. Once, gathered on the right night, they made you invisible; now, enough of them, inhaled, might give you cancer - how times change.

But bracken spreads even more readily by mere extension of its its rhizomes, a network of fleshy, black-skinned cables that grow outwards at a metre a year. Each thicket of bracken in the pasture beyond my window is a separate plant with a 20-metre spread and more are now creeping through our hedges. I go on tugging up the stems on my side every summer, without making much impression.

What to do about bracken is an old problem. The reserves of carbohydrate in its massive underground rhizomes are its secret weapon, letting it survive, not merely the winter, but any burning of vegetation, and then push up new fronds to fill the vacant space.

The one bracken-specific herbicide, the costly asulam, is sprayed in mid-summer and translocates just like it says on the tin, killing off most of the buds on the rhizomes for a whole year. But then the network regathers itself and draws on an undiminished larder to spring forth all over again.

In the mid-1990s, the UK seemed to be shaping up for its first large-scale venture in biological control: an assault on bracken with two species of moth caterpillar that eat nothing else when at home in South African mountains. Classical biological control imports a natural enemy of a pest species - often, a pest that has itself been introduced. The most spectacular success was the importation of an Argentinian caterpillar to clear much of rural Australia of the invasive prickly pear cactus, one of many catastrophic introductions to that continent.

Since then, America, Canada and New Zealand have seen other successful controls. In New Zealand in the 1920s, heather seed imported from Ireland, Britain and France was liberally sprinkled on the red tussock lands of central North Island as food and cover for introduced grouse. The grouse failed but the heather flourished and now blankets some 10,000sq km, including the world heritage Tongariro national park, smothering native vegetation and indigenous species. In 1992, heather beetles (Lochmaea suturalis) were introduced from northwest Europe (where they can, indeed, be a serious pest) and are now chewing away to slow but visible effect.

In Europe itself, however, there has not been a single such biological release. The UK bracken-moth project got as far as government permission for trials in "escape-proof" hillside cages, but fears that the caterpillars might, after all, choose to change their diet, and persuasive arguments that even bracken has its own ecological benefits (as shelter for adders, for example) helped finally to scupper the scheme.

"Biological control of weeds remains in its infancy in Europe" - this was the refrain of the 12th international symposium on the subject, held in Montpellier in France last spring. "Biocontrolling" scientists see themselves as a scattered community, fighting an uphill battle against both real ecological concerns and fear in government departments of taking potentially blameworthy decisions.

Meanwhile, invasive alien "weeds" continue to change native landscapes and ecosystems. Across Killary Harbour from the northern bracken slopes, the "giant rhubarb" of South America's Gunnera tinctoria continues in wholesale eruption. Japanese knotweed creeps along our roadsides and invasive Japanese wireweed spreads along our coasts. An African waterweed, escaped from ornamental ponds, is choking parts of Lough Corrib. Besides that lot, our native "fern" seems positively neighbourly.