Highland show: An inspired walled garden deep in the Scottish countryside rivals its surrounding scenery

Hardworking: Gardener Will Soos at Dundonnell (Ashley Coombes)

They say it's the midges and the rain that keep summer visitors away from the Highlands. Bring them on, say I. Well, perhaps not the midges, though they must be one of the reasons why the place swarms with swifts and swallows, curling and screaming across the pasture. But the rain is superb. From steep slopes of birch and bracken, gulley and rock, it conjures up an extraordinary number of waterfalls. Clefts that you usually read as no more than dark creases in the terrain foam with white water, in such a hurry to get down from the hill that it drowns all other noises in its thunderous fall.

It was like that the day that I drove over to Dundonnell House at Little Loch Broom, Wester Ross. No, it was better than that, because the rain alternated with bursts of sun that brought bits of rainbow to hang between the massed flanks of Meall an t-Sithe and Coill a Bhun. When you're high among the bogs and it's raining, all you think about is the hostility of the place. But then you drop off the hill, cross the narrow stone bridge over the river and arrive in a different world.

Down in the valley, the first thing you notice are the trees – huge limes, beech and sweet chestnuts, as well as sycamore and ash. Then rounding a corner, you come upon the house, a big, superbly strong, gabled building, white-harled, standing in a meadow of long grass. I've always admired the place and thought how fortunate it was to have owners who hadn't smothered the front with trellis and creepers, urns and climbing roses. But though I've often lingered in front of it in a way that might cause a suspicious owner to call the police, I've never been into the garden. Until this year.

I first knew of Dundonnell when it was home to Bunny Roger, his two brothers and his wardrobe. Fourteen years ago it was bought by Tim Rice. Since then, with great subtlety and vision, his wife Jane has transformed the three acres of walled garden that stretches out behind the house.

She managed to lure over from the well-known garden at Inverewe a brilliant young gardener, Will Soos. As shrubs and trees died (this last winter finished off several of the camellias from the Roger era) he's replaced them with herbaceous plants, so the garden is now a glowing mass of crocosmia and monkshoods, geums and scrambling geranium.

The front of the house gives you no clue what lies at the back, where tall stone walls enclose a space divided by long paths into a chequerboard of squares. You are scarcely aware of the divisions, though, because of the huge trees in the garden, which fudge the lines so you never quite know what lies beyond.

As you come through the gate at the side of the house, the first tree you see is a monumental yew, broader than it is high, and thought to be at least two thousand years old. It's a powerful, still anchor at the centre of the garden, standing in a long wide band of mown grass with low yew hedges leading to and curving round it on either side.

The house makes the fourth side of the walled garden, with a terrace pushing out alongside a conservatory. It was Jane's idea to put in the intricate stone mosaic, made with long, thin pebbles brought up from the beach, an interlaced Celtic knot outlined in slate. Beyond is a wide lawn and, against the west-facing wall, the first of the borders, a pair of them either side of a long gravel path which leads all the way to the bottom of the garden.

This is when you begin to wonder whether Will Soos ever sleeps. He gardens single-handed at Dundonnell and herbaceous borders are far more time-consuming to manage than trees and shrubs. But he keeps making more and more of them. And he's laid out a model kitchen garden which at first you only glimpse between a screen of apples trained over hoops. And he's planting up an arboretum on the far side of the river. And he propagates a whole greenhouse full of plants to fill the new borders. And he's got a croft, a wife and two young children. The man's a miracle.

Old pear trees drip out of the border by the lawn, with agapanthus and phlox, asters and sanguisorba holding each other up on the other side with the boundary wall behind. "I try not to do much staking, despite the wind and the rain," says Soos. "I like things to look natural. It's all about finding the right position for a plant, the right microclimate. That's what I most enjoy." So his phlox don't flop. Eupatorium rugosum (now Ageratina altissima) 'Chocolate' stands in a handsome, upright clump.

The wall provides a splendid backdrop all the way down on the right, but on the left the view changes: first the lawn, then a delicious water garden, then a first glimpse of outrageous cabbages in the vegetable patch. And wherever you look there are massive trees: Magnolia campbelli, a mossy-looking cryptomeria, Pinus densiflora, superb acers, Quercus pontica, huge eucryphias, a tulip tree.

My favourite border runs for 100 metres along the south wall from the cast-iron Edwardian greenhouse beautifully restored by Jane to the beech hedge which stops you falling into the river beyond. Vast Hydrangea villosa with lacy flowerheads in deep mauve flop out over the path with climbing dicentra scrambling over the wall behind. Halfway along the wall, a water spout framed in golden hop trickles into a cool pool. On the left you are given long glimpses back to the house at the other end of the garden and a chance to gasp (I did) at the vast holly tree (actually two trees – a male and a female – planted in the same hole) which must date back to the time of the Mackenzies. It was they who in 1769 gave the house its classical exterior.

The rain had stopped by now and behind the spires of Ligularia 'Zepter', I could see the peaks of An Teallach through the cloud. On the other side of the valley, Creag Chorcurach cleared to give an equally heart-stopping view. It's astonishing any work at all gets done in this place. I'd spend half the day just locked in to those hills. But the spreads of meconopsis and primulas, the careful pruning of trees, the inspired placing of ferns, the intricate training required in the long tunnel of laburnum, the cobbled paths, the snaky new borders presently being made on the west side of the garden, well-fed box hedges – none of these things suggest that Soos wastes much time looking at the view. This inspired garden is open on Thursday (2-5pm), admission £3.50. If you're in the Highlands, don't miss it.