Polytunnel promises a paradise in plastic

Another Life: Our little lean-to greenhouse, tucked half into the ground 20 years ago as a way of ducking the ocean wind, has now been quite upstaged by the big new polytunnel nested among the acre's young trees.

But it continues to raise young plants and to shelter its share of wildlife. Wrens, robins and frogs wander in and out; bumble bees need help to find the door again; centipedes and woodlice pursue their mortal feud beneath the flowerpots.

None of this has interfered with the annual miracle of conjuring tender seeds into growth.

A recent morning, however, found me aghast at little excavations in the compost of some 30 yoghurt pots collectively labelled "Sweet Corn". The entire sowing had been rifled by field mice, whose exquisite sense of smell told them exactly where to dig. Many gardeners whose peas or beans have failed to appear may also need to consider defensive measures.

My father used to roll peas in highly toxic red-lead paint before sowing them - a measure not, perhaps, fitting for today, when paraffin is the more usual choice. A layer of prickly holly leaves is a plausibly organic device, but one that seems to offer no certainty. Mouse-traps, as used against the house mouse under the sink, were suddenly unthinkable for the field mouse in the greenhouse: a blatant case of speciesism. On Ethna's inspiration, I sowed the pots again, set them into freezer baskets and slung these on strings from the crop bars in the polytunnel.

This new plastic domain is amazing, not least for its survival, so far, in strong gales from every quarter. On any modestly sunny day, heat flows out around the doors as from a sauna and even shirtsleeves can seem too much to wear within. Adjusting the doors as the weather changes, one seeks a balmy balance to match the soothing peace of the interior (this somewhat marred, for much of the time, by the patter of condensation on one's head).

And yes, every plant leaps up as if finding itself in the promised land: no more sulking in the seed-bed, no wincing from the wind. One moves through a verdure of vegetables doing their thing just like it shows on the packet: a sleekness of spinach, a fernery of florence fennel, broad beans in flower, cabbages curling into heart. In the central nave, a troop of tomato vines prepare to celebrate the space by climbing as high as they like.

Of wildlife, however, there has been little sign. A queen bumble bee, buzzing in to find the bean blossom, was given a personal address of welcome, and hoverflies, quite rare in this chilly spring, were drawn to the same early nectar. I can see myself inspecting such arrivals in a quite new way, as if presiding over a polythene pavilion of entomology.

There's a welcome notion in my Gardening Under Plastic book by Bernard Salt (Batsford) that carrot fly avoids a tunnel, but the corollary is that beneficial insects may stay outside as well. The warm, calm, humid conditions are perfect for the winter build-up of pests such as mealy bugs, aphids and red spider mite - insects that have rarely given any problem in a garden of unruly predators and rampant wind and weather. In autumn the tunnel will be prone to sinister mildews, moulds and fungal rots that thrive on poor ventilation.

There is an ecological price for "intensive gardening", just as there is for intensive farming. In southern Spain, the plastic polytunnels which keep northern Europe's supermarkets full of winter salads must be drenched in insecticides and fungicides. Here in Thallabawn, I must keep the doors open to as rich a diversity of wildlife as can be tempted inside (robins and wrens will find their way; frogs may be kidnapped if necessary). Diversity of crops will help, too.

There are now, of course, living biological controls - natural predators and parasites of pest species - that one orders by telephone or e-mail and that arrive by Swiftpost for immediate release (for one source, look at www.fruithillfarm.com, in west Cork). The polytunnel is an ideal environment in which the organisms can breed, exerting gradual control over whichever pest population is getting out of hand.

In the open garden, all this happens for nothing. The crops I plant outside - mainly brassicas and roots - will continue their imperfect, vulnerable but totally adequate cycles. The tunnel is a gesture to advancing years and increasing preciousness of time: it puts a little more order into what has been a cheerful (sometimes fretful) anarchy of effort, largely dictated by the weather. It's got a chair, a table and a radio for Lyric FM.

I'll let you know when it all blows away.