Feat of clay: A top ad executive reveals his ambitious country garden in Herefordshire

Brockhampton Cottage, Herefordshire. It sounded such a cosy address. I imagined black and white timbering, a box-edged path to the door. Hollyhocks even. And the garden, opening today for the first time, a retirement project perhaps for its owners, Peter and Ravida Clay. Instead, I found a thruster, once deputy managing director of the aggressively successful ad agency BMP, stitching together a landscape so big you can scarcely see to the end of it. "Cottage" is an ironic misnomer.

But this isn't a case of city money buying up country acres. The house belonged to Clay's grandfather, who left it to him 13 years ago. He remembers it imprisoned by high thuja hedges, which filtered the wind, but blocked out the view to the sweeping, undisturbed views beyond. When he first took over the place with his wife, an interior designer, and their two children, now aged 13 and 10, what he most wanted to do was to connect the house with its landscape again.

Getting rid of the hedges was easy. But thinking what to do next, wasn't. He had a vague memory of his grandmother's garden, made elsewhere in the hollow of a chalk down. "It was a billowing sort of place. Romantic. I wanted it to be like that." But what happened then was unexpected, irrational even. He puts it down to the male menopause and a curry date with his school friend, Mark Fane.

Fane had started his working life as a venture capitalist, but then switched to landscaping. By the time he and Clay met for their curry, he had already put together half a dozen gold medal gardens at the Chelsea Flower Show. Both, in different ways, had been seduced by the garden dream. By the end of the meal, Clay and Fane had sketched out on the back of a paper napkin the ideas that in 1999 came together in Crocus, a new sort of nursery and landscaping business, aimed at people like them. Crocus is like a fashion outlet. The plants you can buy there are glamorous, smart and snappily presented via a design-conscious website. Clever marketing (and a clear understanding of who their customers are) has made it a huge success.

The birth of Crocus brought Clay in touch with Tom Stuart-Smith, just then at the beginning of his glossy career as a garden designer. Tom suggested a vision and made long lists of desirable plants. For Clay, the green "Go" light suddenly shone bright and for the past 10 years he's been obsessively engaged with the garden and its Herefordshire setting.

All this is a long preamble to a garden visit, but I was intrigued by Clay's vision (as well as envious of his access to a first-rate designer). It can't be bad, either, to part-own a nursery that you can dip into for the odd hundred oaks, or a herbaceous border's worth of veronicastrums and spurges, geraniums and thalictrums. But there were problems, too. The site is not an easy one, with the house perched high and the land sloping steeply away in two directions. Once the corset of thuja was taken away, the house seemed to balance slightly uneasily on its summit, like a hat not fitting a head.

Stuart-Smith's first suggestion was to anchor the place into its setting by means of a series of level spaces. The sloping ground was pushed up to make a big stone-paved terrace, with generous herbaceous borders below, backed by a stone retaining wall. The Clays put few restraints on the scheme – "We only said we didn't want orange or yellow" – and the borders are planted in big drifts of grey and purple, blue and deep pink. Delphiniums the colour of melting icebergs pierce through mounds of deep-blue salvias. The leathery grey-green foliage of Helleborus corsicus jags into feathery masses of Selinum wallichianum, one of my favourite plants. There's white willowherb, lots of dark-flowered astrantia, great anchoring masses of spurge, towering verbascums, oregano – in short, all the must-have plants you'll find on the crocus.co.uk website.

Behind, the house is tied to the stone terrace with great webs of climbing plants: hydrangea, honeysuckle, roses such as the coppery-pink rambler 'Francois Juranville' plastered with purple Viticella clematis (the clematis are particularly spectacular). Parthenocissus clutches at the windows, fumbles into the gutters. To the side, an equally exuberant vine covers the pergola that shades an outdoor dining table, scented by big pots of lilies. Clay wanted billow. He's got it. Beautifully.

Nevertheless, I'd guess that his heart lies the other side of the railings that mark out the garden proper from the land beyond. To the south, the land drops away steeply and Clay has planted the whole of this huge pasture with perry pears, well-spaced in the old way, standing on impressive six-foot trunks.

And wandering east from the front of the house, with its pots of tobacco flower and neat box parterre brimming with lavender, you are led into wildflower meadows laid out on an astonishing scale. There's another irony. The earth in Herefordshire is generally rather good, but Clay had all the topsoil stripped away to create this landscape where purple knapweed and white yarrow bloom among the remains of moon daisies and yellow rattle.

Below the meadows and their waving grasses, wide mown paths lead through newly planted woodland. Here is Clay the Collector, amassing special sorts of oak: Quercus pyrenaica, Q. rubra, Q. palustris, Q. pontica. Like the perry orchard, it's a magnificent gift to the future. At the foot of the valley, the path winds round a lake, where once again, the planting is on an astonishing scale: swamp cypress, alder, hickory, wild cherries with swathes of miscanthus, hemp agrimony and a beautiful flowering rush (Butomus umbellatus) set along the banks. Clay falls silent and we watch a family of swans coast over the water, the cygnets all in line. "This is where my strength is restored," he says eventually. Mine too.

The garden at Brockhampton Cottage, Brockhampton, Herefordshire HR1 4TQ is open today (10am-4pm), admission £5, children free