Ferns for all

This rain-forest summer has been great for the pteridophytes, as if a great glass bell had been set over Connacht to conjure a moist Victorian fern-garden from every wall and hedge-bank. Green plumes of fern and lady fern, male and female fern, soft shuttlecocks of spleenwort and polypody, polished ripples of hart's tongue - with such an elegant collection of plants, who needs aspidistras?

It is also, unfortunately, yet another good summer for the one kind of fern we have too much of: Pteridium aquilinum, or bracken. Any journey through the hills shows its smothering advance across the greener and better-drained slopes: the sunny mountainsides of Killary Harbour are now becoming a classic bracken landscape. The fern's sturdy fronds fill the clearings of the old western oakwoods - but also the margins of green fields and pastures right down to the sea.

The story is the same from the highlands of Scotland, through the northern uplands of England and Wales, to Wicklow and the west of Ireland. In the UK alone, bracken may cover up to 6,000 square kilometres of land; in Ireland, it waits at the margins of farming, to colonise any half-hearted hectare with astonishing speed and aggression.

Its modern expansion began once cattle and horses were taken off the hills and sheep numbers rose in their stead. Cattle used to trample the young and tender shoots, and hill farmers cut bracken every year for soft and absorbent bedding in the cow-byre. A dozen traditional uses helped to control one of the world's oldest and most invasive plants.

Bracken spreads by extension of its root system, a network of fleshy, black-skinned cables, or rhizomes, that can grow outwards at a metre a year. Each thicket of bracken in the sheep pasture outside my window is a separate plant and more are now creeping through our thorn hedges. You can go on tugging up the fronds summer after summer - as I do - without making much impression.

I have always admired the jaunty, clenched-fist salute of the emerging shoots in spring, and the filigree grace of their unfurling. I should miss the glowing, rusty reds of withered bracken in December, and, composted over the winter, the litter makes a splendid, peaty mulch for the garden. Yet the acrid whiff of bracken sap reminds me of the fern's darker reputation.

Bracken's toxic alkaloids, long known to cause a complicated series of diseases in farm livestock, were indicted a decade ago as a human carcinogen. The International Bracken Group at the University of Wales at Aberystwyth warned bracken could contaminate water supplies in hill areas, that there's risk in the milk from bracken-munching cows and that even to ramble through the bracken in late summer may be to incur cancer from inhaling the spores. Many ramblers' groups will no longer wade through the fern, and some even carry face-masks in case there is no way round it.

Modern chemical control of bracken has relied substantially on a herbicide called asulam, sprayed in July or August, sometimes by helicopter, at considerable cost and with its own ecological hazards. But five years ago, scientists at the International Institute of Biological Control in Berkshire, in the UK, felt able to offer a more natural solution.

For a native plant of such ancient lineage, bracken has remarkably few natural controls. Only 27 species of invertebrate will eat it in Britain (correspondingly fewer here), and there are few bacteria or viruses that do it any harm. In the mountain regions of South Africa, however, the caterpillars of two moth species, Conservula cinisigna and Panotima feed on bracken and nothing else.

The introduction of exotic species from one country to another has had its notable successes: an Argentinian moth cleared 25 million hectares of the prickly pear cactus when introduced into Australia, and similar introductions have been made against more than 80 weed species in a score of countries, apparently without adverse effects.

But there have also been appalling mistakes in the long history of introductions (especially on islands) when exotic species turned out to have unforeseen appetites. Before the IIBC proposed the use of Conservula and Panotima, they tested the bracken-or-nothing diet of thousands of the caterpillars, and they failed to survive on any other plant.

Permission was given by the Department of the Environment and the Ministry of Agriculture, Fisheries and Food for the release of Conservula into escape-proof hillside cages for a three-year-trial. But this experiment has not happened: there were problems over funding, and over which government department should take responsibility if anything went wrong.

This must be to the relief of the UK Nature Conservancy Council, which was unhappy with the scheme from the start. Bracken control is a major task at many nature reserves, especially those trying to bring back the original vegetation on heathland. But most conservationists would reject the idea of help from alien moths, even if these made only marginal inroads into the fern.

Some, indeed, are quick to point out that bracken, for all its aggression in the man-made landscape, provides shelter for adders, and breeding grounds for nightjars, whinchats and scarce fritillary butterflies. Counting out the snakes, Ireland no doubt has a comparable fern-friendly fauna.

Countrydwellers, however, might think only of the sheep-ticks, some bearing Lyme disease, for which bracken is a notorious habitat. How do you safely extract a tick without leaving its mouth-parts to fester under the skin? Ramblers may appreciate this technical advice from a wildlife warden: grasp the tick with tweezers, close to the skin, and "unscrew" it, twisting anti-clockwise without pulling or jerking. "It will come out cleanly after five or six half-turns."