Weatherman Bill Giles believes that global warming will give us a Mediterranean climate within 20 years. Whether he's right or wrong, the gardens of Britain are already metamorphosing, Anna Pavord discovers
Olive groves in Oxfordshire? Bill Giles, the BBC's weatherman, thinks so. Despite some particularly vicious nights of frost last winter, he is a firm believer in the idea of global warming. So much so that he is planning to plant olive trees in his own south Oxfordshire garden. In 20 years or so, he reckons, with the typical summer climate moving north at a rate of 10 kilometres a year, Dundee will be as balmy as Berkshire. France, he says, will be "a desert". Hard luck on the Dordogners.

His sums seem a little awry - at the rate of 10 kilometres a year, surely we will be waiting at least 100 years for a significant change - but no matter. "Why olives?" I ask, since there are so many more interesting trees that he could plant, if his bit of country really is going to get much warmer than it is now. "It was only an example," he says rather wearily. "It could have been vines. Or I could have talked about the impact of the changing climate on the leisure industry and the need to build good roads in Cornwall."

Poor Bill Giles. He's going to be haunted by those olives, the way that his colleague Michael Fish has been by the hurricane that he said wouldn't happen. He won't be able to pick up a bottle of salad dressing without someone saying, "Your own oil is it, Bill?"

I am not holding my breath for the day that Mr Giles opens his olive grove for the National Gardens Scheme. It's not that olive trees can't grow in this country; they can and do. In the Chelsea Physic Garden, London, there is one that is already 30ft across, planted, thinks the curator, Sue Minter, sometime in the 1880s. But the temperatures in London, a heated island wrapped in escaped central heating and car fumes, and insulated by heat-absorbing tarmac and concrete, bear little relation to what's going on in the rest of the country. Angus White, of Architectural Plants, remembers a night in February when temperatures at his nursery in sunny Horsham were 13C lower than in Notting Hill.

Then there is the question of wet winters. Mediterranean plants die here, not because they are cold (so is France, away from the coast) but because they are wet at the roots. They rot, rather than freeze. Bill Giles's definition of global warming includes warmer, wetter winters. Only on very well-drained soil, such as Sue Minter has at the Chelsea Physic Garden, will olives have a chance of seeing more than their first birthday.

Despite this, Reads Nursery in Loddon, Norfolk, sells between 150 and 200 olive trees a year, most of them, says Judy Read, destined for cool greenhouses where they can stay dry over winter. That's where most of their lemon trees end up, too. But growing lemons in England is old hat. Lord Burghley, Queen Elizabeth I's Chancellor, was at it way back in 1561. "If the prise be not too much," he wrote to Thomas Windebank in Paris, "I pray you procure for me a Lemon, a Pomegranate and a myrtle tree." Windebank did his stuff and sent the trees. The lemon cost Lord Burghley 15 crowns.

A hundred years later, the diarist Samuel Pepys went off with Mrs Penn to see oranges growing in Hackney. "Some green, some half, and some a quarter and some full ripe on the same tree," he reported. He pulled one off and ate it when the gardener wasn't looking. That was in 1666, two years before the coldest period ever recorded in England. Then, for 21 miserable years, frosts came in August, and winters lasted for all of eight months. In the winter of 1683-84 the Thames was frozen solid from November all the way through until April. That year, John Evelyn noted in his diary that there had been widespread damage to "sipris" (cypress) trees. It couldn't have done the Hackney oranges much good either, even though they would have been packed away in a glass-fronted orangery for the winter.

Evelyn was the first person to use the word "conservatory" (or so says the OED). He built one of the first in this country and experimented with growing masses of tender plants: pomegranates, passion flowers, mimosa, cannas, daturas. They are all things we might hope to grow outdoors rather than inside if global warming is a reality.

But a long-term trend can be upset by some very nasty short-term shocks. On two nights at the beginning of this year, temperatures of -22C were recorded in the garden at Crathes Castle, Scotland, causing the worst damage in living memory to the trees and shrubs growing in the sheltered walled garden there. And the head gardener, Mike Nelhams, will never forget the devastation caused by the January frosts of 1987 that killed 80 per cent of the plants in the garden at Tresco in the Scilly Isles.

"Even 80ft Norfolk Island pines were killed," he said. Tresco, until then a frost-free paradise of tropical, Mediterranean and Australasian plants, suffered temperatures of -25C. Just one blip like that, even if it runs counter to a well-established trend, can mean death for marginal plants.

What will happen to the wider landscape, if the climate warms up? Remember August last year? Already, beech trees all over the country were shedding their leaves in a last-ditch attempt to cut down on the amount of water they needed to stay alive. It was a desperate sight. Having lost our elms, we can't afford to lose beeches, too. I would trade in global warming any day (and olive trees, too) for the sake of a beech wood in spring, carpeted underneath with bluebells. Siberia, here I come.

Cabbage palm (Cordyline australis) Nothing like a cabbage. Or a palm. Big clumps of strap-shaped leaves more than a foot long. Tie up the leaves to protect the heart in winter.

Red hot poker (Kniphofia caulescens) Fleshy blue leaf rosettes on short trunks topped by burning poker flowers in late summer.

Angel's trumpet (Datura arborea) Fabulously scented trumpet flowers, which can be white, yellow or apricot-coloured. Big paddle-shaped leaves. Poisonous.

Castor oil plant (Ricinus communis) Deeply lobed leaves a foot across, rich bronze in the variety "Impala". Perennial in its native country, Africa, but usually grown here as an annual.

Canna (Canna hybrida) Huge purple, bronze or green-leaved plants from tropical and sub-tropical America. Flowers in splendidly shocking colours: bright pink, orange, vivid scarlet.

Banana (Musa basjoo) A hardyish banana. Cut down by cold, it will regrow from the root, up to 10ft in a summer. If global warming fails, protect it through the winter with a chimneypot.

Plants available from: Architectural Plants, Cooks Farm, Nuthurst, Horsham, West Sussex (01403 891772); Great Dixter Nurseries, Northiam, Rye, East Sussex TN31 6PH (01797 253107); Beth Chatto Gardens, Elmstead Market, Colchester, Essex CO7 7DB (01206 822007); Reads Nurseries, Hales Hall, Loddon, Norfolk NR14 6QW (01508 548395).

Never mind a lawn, we've got gravel

Beth Chatto had few qualms about ploughing in three-quarters of an acre of mown grass at her famous gardens and nursery in Elmstead Market, Essex.

Where once she looked out in summer over sad, struggling, "biscuit-brown" grass, now there is a burgeoning new gravel garden, richly planted with daisy-flowered anthemis, French lavender, spiky New Zealand cordylines and South African poker plants. Using drought-tolerant species, she first set out a framework of bold shrubby plants, both deciduous and evergreen, filling in with soft mounds of sage and santolina. Lapping round between them she put in pools of bergenia, lime-green euphorbias, branching heads of crambe and creeping thymes. When all the plants were in place, the whole area was covered with a layer of gravel, two inches thick.

Living in one of the dryest parts of the country (average rainfall 20 inches a year), Mrs Chatto is no stranger to drought, but from the beginning, she decided her new gravel garden would be planted in such a way that it would never need watering. "I wanted to test plants for their durability in these conditions," she said, "to help gardeners who may, in years to come, be obliged to accept the effects of global warming."

There are other advantages, too. You are relieved from the tyranny of the lawn mower. Throw the noisy thing away and buy a sun lounger instead. You can also congratulate yourself on your ecological correctness. Biological diversity is the buzz-phrase now, but a well-kept lawn is just the opposite - a monoculture of several million blades of fescue all living together in a botanical ghetto. Forget lawns. Go gravel instead.

Out go the roses, in come the cannas

"Roses have dominated gardens and gardening for far too long," said the famous gardener Christopher Lloyd, as he ripped his own roses out of the Lutyens-designed rose garden at his home, Great Dixter in Sussex.

He replaced them with plants more commonly found in the jungle than the rolling acres of the home counties.

Out went the thorns and the black spot. In came cannas, castor oil plants, banana palms and dangerously seductive daturas.

The recent spate of long, warm autumns has favoured his new exotic garden, which gets into its stride in August and then charges full pelt towards the first frost in a mass of orange, red, dark purple and yellow flowers backed by masses of luscious foliage.

The leaves of a paulownia, stooled down to the ground each season, grow as big as tablecloths while banana fronds rattle in the breeze as convincingly as they ever do in the tropics.

Global warming, however, isn't the reason for the dramatic changes at Great Dixter.

Christopher Lloyd is simply a gardener who likes to experiment, and the old roses, he argues, just weren't doing enough to earn their keep.

Does he believe in Bill Giles's predictions of a Mediterranean climate for Britain?

"Not a bit," he replies robustly. "Every summer produces some summery weather in Britain. That is as much as one can depend on."

His cannas are still lifted and stored for the winter in a frost free cellar.