Flower power!

The Pharoahs revered them. Pliny believed they could save your life. Now it's Chelsea's turn to fall in love with the Iris. Here, the best-selling author of 'The Tulip' explains her fascination with the flower that no gardener can live without this summer

The Chelsea Flower Show opens today, floating on designer water features and robed in imperial purple. The smart gardens there this year have mostly forgotten about lawns. Suddenly the patch between the flowers where the grass used to be has melted away. Instead there are dark reflecting pools, rills and canals. The show gardens, which used to be such solid affairs - plenty of concrete paving slabs surrounded by banks of knicker-pink rhododendrons - have become mysteriously insubstantial, flickering, glittering with water.

The Chelsea Flower Show opens today, floating on designer water features and robed in imperial purple. The smart gardens there this year have mostly forgotten about lawns. Suddenly the patch between the flowers where the grass used to be has melted away. Instead there are dark reflecting pools, rills and canals. The show gardens, which used to be such solid affairs - plenty of concrete paving slabs surrounded by banks of knicker-pink rhododendrons - have become mysteriously insubstantial, flickering, glittering with water.

Heaven knows what it's doing to bathrooms in the gilded homes of Kensington and Chelsea. Is anybody getting a morning shower? Are the dolphin taps running dry? Has the entire mains water system in the borough been diverted to the showground to keep the fountains bubbling?

The pools with their dark linings appear bottomless. The plantings around them are husky mixtures of purple and deep blue. Glass and Perspex glitter in the snatches of sunshine. I didn't see a windscreen wiper on any of these new space-age garden buildings, but that moment cannot be far away. If you can train a computer to shoot bolts of water from fountain to fountain, you can surely teach it to clear the view so that you can see your installation in all its expensive glory.

The Chelsea show has always been good at flowers: flawless sweet peas fanned out in vases set against black velvet, delphiniums as massive as Corinthian pillars. But it has been slow to catch up with good design. This year it's different. Garden design at Chelsea has taken a giant step forward, leaving behind it not only the lawn, but the traditional herbaceous border. This year, Chelsea has style by the bucketful.

You might be on your way there this week to see it for yourself. If not, close your eyes. Think first of sheets of water. Introduce some iris - deep purple or rich blue. Give them a few thistly things for company. Abandon concrete and think cool, cut slate. This is what should surround your pool. Surrender that antediluvian teak bench and replace it with an enigmatic hunk of burnt oak. Then plant some more iris. If you are thinking straight, you should now have an image of the new super-cool, super-sophisticated Chelsea garden.

The apotheosis of style is contained within the billowing box hedges of the Gardens Illustrated garden at Chelsea, made by English designer Arne Maynard and by the celebrated Dutch nurseryman and garden-maker Piet Oudolf. Neither have been at Chelsea before. Both felt strongly that the garden should work as a garden, not just as a showpiece. Both have strong ideas about colour.

The backdrop to the garden is a wall, rendered and painted a deep, soft red. Along the front runs a suggestion of a wall, broken into simple oblongs, rendered and painted with the same extraordinary colour. Three long slabs of Ancaster stone, marbled in buff and cream and grey, hang like paintings against the red backdrop. It is actually a waterfall, but the water runs down the stone so subtly, you have to blink to make sure it isn't just the grain of the stone blurring in front of your eyes.

In the front of the garden is a circular pool edged with the same stone. Buried underground, a hammer silently shakes the enclosing basin, so the surface of the water ripples hypnotically. The hedges either side are unclipped. They roll like cumulus clouds behind two sternly rectangular blocks of dark, saturated planting. "Two years ago, when we were first thinking about this garden, we walked round Piet's garden in Holland," explained Arne Maynard. "We picked posies of flowers, tried different mixtures in the bunch and settled then what we were going to use. All flowers of the season. Unforced."

So, for the now look, mix deep-pink valerian with dark purplish-blue salvia Mainacht. Add the wine dark astrantia called Claret, purplish spurge, and stitch the whole thing together with the blackish purple leaves of cimicifuga "atropurpurea". There are iris too, grouped round the circular pool, erect, aloof, among the fluffy seedheads of a fancy grass called deschampsia. Not a rose in sight. Roses? Was there really ever a time when people planted gardens with nothing but hybrid teas? Instead, here are these tapestry plantings, deep, rich and strange, separated by four low cubes of box, tightly clipped to cushion the four circular steel dishes set into them. They are fountains too. From time to time, they throw jets of water to each other in an enigmatic, surreal game of aqua-tennis.

That's another thing. It's not enough just to have water, lying about in the garden. It's got to be doing something, and the things it does get more subtle and Zen all the time. In Christopher Bradley Hole's garden, which is more water than anything else, jets are submerged at metre intervals in an underwater grid. Gently, water bubbles out of the top of the jets to make a series of ripple images. Each set of concentric circles bumps into its neighbour, merges and is followed by the next wave.

It's a mesmerising performance and the designer (who's also an architect) has put in a seat that you can push out on runners over the water. Bradley Hole has got iris too. They are planted in a dry-stone wall, built under the water, so the surface of the pool just breaks over the top of the wall and laps round the roots of the iris. Oh, it's subtle. So subtle that practically nobody, I fear, will be able to afford to copy it. That's the funny thing about style. The more of it you've got, the less there is to show for it.

For the full iris effect, you have to go into the new, twinned marquees. The Royal Horticultural Society has played a master stroke in bringing the soul of Birmingham's NEC into the centre of Chelsea. But nevertheless, here among the most fabulous displays put on by any nurserymen anywhere in the world, you will find the Kelways stand, featuring nothing but bearded iris. They are insultingly superb: louche and yet distant, all at the same time. I'd be prepared to throw myself overboard for them, if only they would show a few signs of being interested in me. Love needs a little encouragement now and again and I've been let down by iris too often to want to go through the whole humiliating process of rejection again.

But then, each year as they emerge, caution, prudence and all the other things your mother reminds you about from time to time, are thrown to the wind, and I am hooked again.

It's been happening to people for centuries. Millennia even. Bearded irises were brought from Syria into Egypt by Thutmosis III, who was making gardens 1,500 years before Christ was born. The flower still shines out in bas relief on Thutmosis's temple at Karnak.

Pliny later gave elaborate instructions on preparing iris roots to make medicine. Subsequent physicians endorsed its powerful qualities. It could fasten loose teeth. It could "provoke sleepe and bringeth out teares". It was good for "gnawinges in the belly and for them that have taken a thorowe cold". Beauty counsellors of the second and third centuries promised it would scour out freckles, spots and other life-threatening blemishes.

In the sixth century, the iris was adopted as an emblem by Clovis, King of the Franks. Trapped between an army of raging Goths and a dangerous bend in the Rhine near Cologne, he noticed a patch of yellow iris growing far out in the middle of the river. They were a signal that the river, at that point, would be shallow enough to ford. He led his army past the shimmering iris, to safety on the far bank. The flower remained the badge of his descendants.

So, a powerful flower. And it knows it. It looks at you with a wonderful, daring insolence. You can only take this kind of treatment for so long. Battered, I swayed away from the Kelways stand, past the turquoise and pink bromeliads in the Newington Nurseries display. I restored my equilibrium with the help of the cool green foliage of Martin Rickard's fabulous ferns and stepped blinking out of the marquee into the fresh air. In front of me, beckoning suggestively from the front of Arabella Lennox-Boyd's Italianate garden were bearded iris, toffee-coloured tongues lolling suggestively. I give in. They win.

The Chelsea Flower Show starts today, and is open to the public on Thursday and Friday. Admission, by advance booking only, costs between £12 and £27; a credit-card hotline is open 24 hours a day, on 0870 906 3781.