A fairytale Black Mountains garden that responds to the wild landscape

It takes courage and a certain independence of spirit to make a home in the deepest, darkest valley in the Black Mountains. It is a 15-mile round trip to buy a pint of milk, there are few neighbours and no broadband. Nearly 365 metres above sea level, winters are harder and longer, rainfall is higher and nature is a more powerful, uncompromising force.

Sue and Ian Mabberley's six-and-a-half-acre garden straddles the road up the valley. This was originally built as a railway for the construction of the dam and high-altitude reservoir that holds back the waters of the Grwyne Fawr, a tributary of the Usk. The former forester's cottage takes its name from the stream beside it. Nant y Bedd means 'stream of the grave', possibly an allusion to a prehistoric burial place on the highest ridge in the Black Mountains. A small hydroelectric plant means the stream provides them with electricity and a modest surplus to sell to the National Grid.

Last November, heavy rain turned the stream into a raging torrent that scoured the heart out of the cottage garden surrounding the house, leaving a mudslide of branches, sticks and silt. Their response was philosophical. 'We had to take it as an opportunity to do something different,' recalls Sue. They cleared the silt and then rebuilt the stone walls, and the garden has adapted. Irrepressible meadowsweet, along with common valerian and sweet cicely, are re-emerging, taking advantage of the bare ground. The best gardens are those that work with nature, as is evident in the larger, wilder part of the garden that leads down to the sparkling waters of the Grwyne Fawr.

We walk past a line of utilitarian ex-forestry sheds, homely Norwegian-style wood stacks and a rusty cast-iron boiler containing a miniature still life of viola, moss and golden saxifrage. Through an unassuming wooden gate, we step into a sunlit clearing - and as if into another world. Wild wayside flowers - purple spires of toadflax, orange poppies and acid yellow parsnip flowers - wrestle with broad beans, hops and climbing peas.

A rope bridge crosses over ferns, while flowering ground elder and coppiced willow lead through a small meadow to a natural swimming pond. Bordered on one side by colossal Douglas fir trees and open pasture on the other, the still water reflects the drama of the sky. A submerged timber frame separates the pool from a shallow fringe of native marginals, such as flag iris, loosestrife and marsh marigold. Planted in soil-less gravel, they draw nutrients from the water, keeping it clear. Newts, dragonflies and damselflies animate the meadow verges.

Though bluebells, ferns and grasses may prosper at the edges, conifer plantations are dark, dusty and lifeless places. Sue and Ian have brought new life to their forest by removing lower branches and carving out sections of the forest to create footpaths and rills lined with gold saxifrage. They honour the beauty of broadleaf trees by clearing space around them. Four years ago, the emerald carpet of wood sorrel didn't exist: a lesson in how quickly plants can colonise.

The commitment, ingenuity and sheer hardiness to garden in a place like Nant y Bedd is matched by artistry and sensitivity in observing and learning from the landscape. They have the courage and drive to make changes, but also the sense of when to stop. With Sue's designs and fluid style, Ian's practical know-how and their shared love of nature and intimate sense of place, they have got the balance right.