The tropic of Cornwall: How a nondescript field was turned into an unexpected sculpture park

Anna can't quite believe her eyes

A parcel of land came up for sale just outside Penzance in Cornwall and Neil Armstrong, who practices as a GP in the town, bought it. Armstrong doesn't live on the land, or ever wish to, and yet every weekend for the past 15 years he's come on his own to this secretive valley, delicately clearing and planting to make a retreat that is now quite staggering in its richness and complexity. Why did he do it?

"Well," he says, slipping away from an answer, "for the first five years you know it was mostly clearing. Not planting." But the land was south facing, sheltered from harsh winds, and provided a huge range of possibilities for introducing his favourite plants: bamboos, ferns and palms. "It's just been about planting things I like in places where they will do well," he says at another point. But that's no answer either, although it's true. He knows what his plants need and takes care that the needs are met.

The land he bought (now 20 acres) had, for 600 years, belonged to a long line of the Tremenheere family. The last of them, Seymour Tremenheere, planted the oaks, beeches, sweet chestnuts and hollies that still flourish in the steep-sided valley and provide a backdrop to Dr Armstrong's exotic introductions: fabulous tree ferns (forget the standard dicksonias – we're in cyathea territory here), underplanted with begonias and tropical-looking colocasias.

This complex layering of plants is one of the wonders of the place. Your eye is caught by something growing alongside the path, an arisaema, perhaps, or the red-bronze frond of a woodwardia fern. But then just above, you take in the massive leaves of a rhododendron reaching into the mid-storey. Beside and above that will be the tall thin trunk of a tree fern, it may be Cyathea medullaris, with fronds five metres long breaking from furry monkey fists at the crown. Finally, silhouetted against the sky, you get the arched dome of the Tremenheere beeches, carefully limbed up by Armstrong to allow all these wonders to happen beneath.

You have no idea what lies ahead when you first turn up at the place. A narrow lane leads into a field with a handsome little building (housing a café, loos and a shop), very new, with its shingled roof not yet greyed by the weather. The path to the garden leads quietly out of the field on the right-hand side and joins the track that used to be the carriage drive leading to Seymour Tremenheere's summer place (now gone) on the hill above. His initials and the date 1849 are marked on the stone bridge that crosses the stream coursing down the valley.

When you leave the line of the old drive and plunge off on a path to the left, the things you know soon close in behind you. The noise of the stream drowns out the traffic, the helicopters. Unfamiliar leaves loom in on all sides, impossibly bright and lush in their greenness. Stands of bamboo are carefully cleaned and thinned so you can admire the striped perfection of their stems. In their season, magnolias flower. Huge-leaved Rhododendron maddenii flower. But that's not why they have been introduced. It seems to be the stature, the structure of particular plants that wins them a place in the Armstrong Arcadia.

By the time you reach the head of the valley, you've already been immersed in four different plant-worlds: native Cornish woodland, the exotic woodland that is already well established in the valley, the boggy bottom which the introduced begonias and ferns love, and a string of pools, each reflecting a different amount of sky.

But then you come into a clearing and are confronted with a row of pine trunks, 11 of them, about a metre high, quietly beginning to rot round the edges. By now you've passed plenty of fallen trees, upturned rootplates and twisted hanks of ivy that seem in themselves to be sculptures, but you know this installation is definitely not a natural happening. It's the first manifestation of another of Dr Armstrong's passions: art – more specifically, sculpture.

This piece is one of two in the garden made by the Japanese artist Kishio Suga. I don't particularly need sculpture to mean things, and Tremenheere is blessedly free of panels 'interpreting' stuff for you, but if you want to, you can read the line of life incised into the tops of the trunks, shattering into cobwebs of lines at various points on the way. There's some quiet manipulation going on in a little dell nearby too, which I liked even more.

At this point, though, you can see that the trees and the cool, dark shadowy paths that you've been following, are coming to an end. You emerge, rather suddenly, into light, the kind of blinding, sparkling light that only Cornwall can do, with a long view out to St Michael's Mount and its castle, tenuously attached to the mainland by the causeway.

Here, with no preparation, you are transported to the fynbos of South Africa. Or the dry deserts of Arizona and Mexico. Or anywhere else where agaves and restios, palms and tall spiky eryngiums populate the land. Here they are planted on the sparse, gravelly slopes of the bare hill that sits above the valley. It's a dramatic, very sudden transition. The rich, lush greens of the ferns have gone. You are in a landscape of beige, bleached cream and grey, with bizarre grass trees (species of the Australian native Xanthorrhoea) erupting from the slope among Butia capitata and Phoenix theophrasti.

This moving from darkness into light (or the other way around) happens all the time in gardens, but I've never been so aware of it, as a recurring theme, as I was at Tremenheere. You pick it up, not only in the way the place is planted, but also in the superb installations. Crowning the stony hill is a James Turrell piece where a dark passage leads to a womb of light, with an oval eye gazing at the sky and the clouds chasing across it. Below, deep in the wood, is a second Turrell masterpiece, hidden in the underground tank that once supplied Penzance's water, which gave me the most astonishing interaction I've ever had with a work of art.

You need a day here, to do it justice, to see the charred wood group installed by sculptor David Nash and the intriguing camera obscura built by local artist Billy Wynter; to see the blechnum ferns and the beautiful agaves and the Yucca rostrata.

And now, with the café and loos newly installed, you can. For Dr Armstrong has decided to raise the curtain on his private paradise. Today sees the grand opening of the Tremenheere Sculpture Garden, near Gulval, Penzance TR20 8YL. Visit Monday-Saturday (10am-5pm) or Sunday (11am-5pm), admission £6.50. For further information, call 07974 996089 or go to the website at tremenheere.co.uk.