Gardens: Sharp movers

Well – it wasn't what I was expecting, as I finally crawled up through the suburbs of south London on a hot, dusty afternoon. But Wendy Witherick and Mick Wilkinson's garden at 85 Croham Road in south Croydon is well worth a long diversion off the A303 followed by an hour's hold-up on the M25, followed by pretty much every aggravation that can be thrown at a driver in 21st-century Britain. OK, so I shouldn't be driving, but if you live in our under-endowed neck of the woods, you don't have much choice.

Their house is one of a pair of flint-knapped quarrymen's cottages, built in the 1850s. When they first came there, they'd never gardened before, but made the kind of garden they thought might suit the place, a typical cottage garden full of roses and peonies and delphiniums. First Garden, they call it, like archaeologists mapping a historic site. Then in the millennium year, they suddenly stopped gardening. "It had all got too much," explains Wendy. "We never went away."

They dug up the best plants, gave them away and went on an extended holiday. But two years later the bug bit again. And how! This garden, though, is totally different: olives, phlomis, figs, grasses, lots of box, clipped and unclipped. It's sharp (in more senses than one – there are more than 50 agaves), modern, bleached out in its planting, the ghostly greys and buffs of various foliage plants making a terrific backdrop for brilliant orange geums, yellow pokers and fountains of Bowles's golden grass. I don't often feel covetous of other people's gardens, but this one gave me very itchy fingers. It was so well put together. And it made such clever use of a potentially difficult site, a steep, wedge-shaped plot, 180ft long, but at best, only 25ft wide. When you get to the end, there's only 18ft between one side boundary and the other.

But the end is a long journey away, and well-disguised. There is more than enough to take in from your first stopping point in the small concrete yard by the back door. The garden runs up away from you on a south-facing slope, open, sunny, fast draining (the soil is chalk). In the yard is a handsome evergreen, Michelia figo, splayed out against the wall, with waxy cream flowers smelling of pear drops. In front, on the first terrace, is a staggering array of agaves in terracotta pots – all sizes, all shapes – sunk into a matrix of dark stones and pebbles.

Behind the billowing hedge of white-flowered lavender that surrounds the pool on the left are two more agaves, perhaps the most beautiful of all, huge sculptured creatures of white and grey (Agave americana 'Mediopicta Alba'), partnered by four standard olive trees, planted in a square, their heads loosely clipped into mops. A vast round barrel cactus in superb condition oozes out over the edges of its pot. "Homebase," says Mick. "Twenty-nine pounds," says Wendy. And I'm reeling, thinking of the extraordinary gear-change these two have made, and made so brilliantly: away with the cottage stuff, in with a completely different cast of strongly architectural plants that don't need watering, don't need staking and with which they have created Second Garden in a completely different style from the first.

But I don't see a greenhouse and many of this new brigade can't take our cold, wet winters. So where do they go? "The spare bedroom," says Wendy. "Sixty pots," says Mick. "At least," says Wendy. Mick makes a face. I guess he's the one who heaves them upstairs. But what intrigues me is how they set off on this completely new journey. They aren't great readers of gardening books or mags. They don't go garden visiting. They'd never gardened before they came to number 85. They didn't make a plan. Wendy shrugs. There aren't any explanations. "I just do what I like," she says.

And they evidently make a great team. It was Mick who found the 2,500-year-old bricks to make the path that takes you past the olives, underplanted with steely blue festuca and wheatgrass (Elymus hispidus), between two tall standard figs in handsome Whichford terracotta pots, and past a similarly matched pair of tall, shaggy-barked vines and standard lavenders. "Ikea," says Mick. "Six pounds each."

By this time you are at the delphinium-coloured shed that you suppose to be at the end of the garden. You've already noticed the delphinium itself, a last remnant of First Garden. But a big bulk of clipped yew prevents you from seeing on into the top half of the garden, which opens up only when you are on its threshold. Blocks of yew, box both clipped and unclipped, are splendidly used to anchor the various levels of planting. The central route changes too, a long, thin runner of grass merging into a coral-covered carpet of stuff that looks like crushed waste terracotta.

The lesson they learnt from First Garden was that they needed form and shape to hold the place together while the flowers came and went. Hence the fabulous pair of pinkish-bronze Bismarckia nobilis rearing out of the lower half of the garden (a B&Q bargain), hence the spiky grey Eryngium agavifolium, the cardoons, bizarre New Zealand lancewoods, and the low mounds of a small-leaved box (Buxus sinica 'Tide Hill').

But these two are also great gardeners, in the sense that they've learnt exactly how to treat their standard vines and their figs in pots, hard pruning them so they stay well-balanced. They bother to cut the decaying leaves away from their alliums so they don't detract from the dried heads on their long, straw-coloured stalks. They feed a little but not too much (chicken manure pellets and Phostrogen), they train and tie in the plants that need it. The low box hedges that curve round the miniature pond at the top of the garden – simply a half barrel sunk in the ground – are crisply tailored. Yes, south Croydon is a surprising place. I ought to go there more often.

The garden at Elm Cottage, 85 Croham Road, Croydon CR2 7HJ is open tomorrow and Sunday 10 August (1-5pm); admission £2.50