Showtime!

The old way of celebrating May's rebirth was Beltane, a Celtic feast of fire, sacrifice and general mayhem. Now we have the Chelsea Flower Show: genteel sandwiches with the crusts cut off; pinstriped men in panama hats; the low, happy buzz of gardeners on a spree. At Sloane Square, the Tube station closest to the show (held since 1913 in the gardens of the Royal Hospital, Chelsea), regular commuters reel with shock as people up from the country for a day at the show say "No, after you, please" at the ticket barriers and smile pleasantly at the motorists who have unwillingly stopped for them on the zebra crossings of Lower Sloane Street.

I love Chelsea. I love the smell of it. It creeps up on you slowly as you step inside the magic enclosure. First, there is the smell of bruised grass, as intoxicating as the smell of malted hops. Then, when you enter the inner sanctum, the Great Marquee, you are positively assaulted by scent - roses, sweet peas, honeysuckle, exotic daturas. It hovers hypnotically over the exhibits of some of the world's best nurserymen.

Part of Chelsea's magic is the excitement that surrounds all ephemeral tented events: the sudden transformation of the setting, the atavistic lure of an itinerant life for anyone whose future is normally shackled to the 8.05 from Woking, the particular quality of the light as it filters through canvas.

You can get that at any agricultural show, of course, but Chelsea's strangeness lies in the fact that all this happens in the middle of the most densely populated city in Britain. All these pulsating, growing, flowering things suddenly arrive in the middle of a place generally marked out by concrete and barrenness.

There is a delicate balance to be kept at the show between exhibits that are there because they bring in money, those that push forward into new horticultural or design territory, others that are there for spectacle, and those that relate directly to most people's experience of gardening.

Since it was set up, the Royal Horticultural Society, which puts on the show, has considered it an important part of its role to educate the public. This is now an unfashionable thing to do. Fortunately, the society, magnificently impervious to fashion, has continued to give Chelsea a firm underpinning of educational displays.

You can find these along the western side of the Great Marquee. Here the Welsh College of Horticulture, from Mold in Flintshire, has put up a display of the flowers and vegetables brought to Britain by Caribbean immigrants in the Fifties and shows how they have been assimilated into the ever-growing pool of introduced plants. The Institute of Horticulture may persuade you to "come out" as a gardener. The University of Reading fuels gardening nightmares with its exhibit on aphids. At the end of one year, a single aphis can have exploded into 250 million tons of progeny.

There are certainly factions who consider these stands a waste of space and who would gladly trade the Institute of Groundsmanship's turf clinic or Capel Manor's history of composting for another lush conservatory, kitted out, Southfork-style, with every accessory a fevered interior designer could dream of. Except plants. Those are generally ousted in favour of cane furniture and cushions.

The conservatories hover round the central marquee at Chelsea, hissing "lifestyle, glamour" at the prim educational exhibits inside. Were the Chelsea Flower Show put on for entirely commercial reasons, conservatories would have ousted compost long ago. Fortunately it is not.

In the larger equation between plants and furniture, plants win hands down. The Chelsea Flower Show is primarily about green, growing things, not hardware - and this is surely an important reason for its success. It is a celebration, not a marketing opportunity, and it is pre-eminent.

With its usual greedy eye, News International (which owns The Times and The Sunday Times) decided six years ago that it wanted some of this action for itself. Its marketing gurus decreed that Easter was the best time to separate the gardening public from its cash. With much fanfare, a gardening show duly opened over Easter at Wembley Stadium. It failed spectacularly.

Though Wembley is hallowed ground for my football-mad sons-in-law, as a venue for gardeners it has all the allure of an unmanned railway station on a November night. I thought it was about as glum a site as could be imagined. Plants, in their magic way, might have been able to transform even that barren landscape. Unfortunately, in the sea of white plastic and what are still quaintly termed "sundries", green and growing things were scattered as thinly as on a slug-ridden seedbed.

Chelsea is not everyone's cup of Thea sinensis. Few disagree on the magnificence of the plant displays in the Great Marquee. Having slugs as big as tortoises in the garden, I have never been able to grow a decent delphinium. So, at Chelsea, I stand in front of Blackmore & Langdon's fabulous display of the things like a pilgrim in front of a shrine. It is my fix for the year.

The show gardens, though, arouse deeply divided emotions. They are judged on the Monday before the show officially opens. The results are published early on Tuesday morning. Then the accusations and recriminations start ricocheting around the ground like bullets, bouncing off yew hedges, skipping over elegantly installed fountains, burying themselves finally in mounds of elaborately tweaked rhododendrons.

The sensible designers keep their mouths shut. The rest tell anyone who will listen that "the judges don't understand what I am trying to say". "Nor do I," one is tempted to reply, gazing at a crude mishmash of orange and pink flowers, crammed into a chequerboard of purple-and-buff hard landscaping. Few of the show gardens have much relevance to the way that people really live, but I don't think that matters. These are peepshows, theatre sets. They are there to inspire rather than be copied.

The most important thing that marks Chelsea out from other horticultural shows is the standard of its plant and garden exhibits. A stand of lawnmowers looks much the same anywhere, but you will rarely see such a beautiful stand of lupins as the one the Woodfield brothers are bringing to Chelsea this year, or such ravishing columbines as those staged by the Three Counties Nursery from Dorset.

"Oh, lupins!" said a style-crazed acquaintance to me, as she waved dismissively at their still, elegant spikes, displayed in Chelsea's Great Marquee last year. "Such Fifties flowers." But that's another of Chelsea's great strengths. It is old enough, strong enough and sensible enough not to bow too low to the transitory whims of fashion. There are hiccups - such as the Year of Ecological Correctness, when the top award for gardens went to one featuring windswept dunes (very little vegetation), brackish pools and rabbit-grazed turf - but they are few and far between.

These days, it is hard for gardeners to celebrate the arrival of spring, the leafing of trees, the surging growth of roses, without some gloom- and-doom merchant looming up to pour metaphorical weedkiller on our optimistic dreams. Chelsea is our nirvana. There, we are safe.

Anna Pavord's guide to the Chelsea Flower Show is on page 14