Alan Power has become, in many ways, the voice of National Trust gardens. He has worked for the Trust for 25 years, at two of its most iconic gardens, Stourhead and Mount Stewart, and for over a decade he spoke to BBC Radio 4 audiences every autumn with bulletins from Stourhead on the PM programme, describing the magical effect of the landscape changing day by day. This in turn led to presenting on Gardeners’ World and on the series Gardens In Time, treating viewers to a bird’s-eye view of Stowe (another 18th-century landscape garden) from the top of a colossal cedar; Alan trained as an arborist after studying horticulture, and first came to Stourhead in 1996 to survey and manage its collection of trees.

Present some head gardeners with a voice recorder, let alone a television camera, and they wince but not Alan. He stands up straighter, lowers his voice, and begins to perform. He’ll say it’s the love of language, of storytelling. It’s not a development he would have predicted – he was a painfully shy boy, who hated school and was always a bit on the edge. As a teenager, he’d take off to the mountains of County Kerry in southwest Ireland as often as he could. It was his mother who taught him gardening; he would see her face light up, he recalls, whenever he showed an interest. She was thrilled when he chose horticulture as a career.

Alan has felt his spirits soar, walking in the garden at Stourhead with his toddler son

“Gardening for me is about the story it tells,” he says. And the story that has inspired him for decades is that of Henry Hoare, the art-loving banker who created Stourhead in Wiltshire. “I feel I know him quite well, and his grandson Richard Colt Hoare. I’ve read all their letters and I’ve nearly been in tears reading when Henry lost his wife and his kids, and then his joy when his daughter was getting married into a good family. It’s heartbreaking. They called him ‘Henry the Magnificent’ and he was: he had all the weight of tragedy and loss on his shoulders, the burden of making a success of his business – but still he held his head up, maintained his passion for his creation. He had the confidence to do something contemporary and different – and then to upset his family by leaving the estate to his grandson, because he had an instinct that he would cherish the garden. The family is what the garden is all about: it was their successes, their sorrows, their happiness, their pain that made Stourhead.” Above all, he feels, the garden embodies Henry’s astonishing capacity for joy.

And Stourhead has become a repository of sorrow and of joy also for many of the 450,000 visitors who pass through the gate every year. Some are recorded in the garden’s memorial book. Others, crying quietly into the lake at the end of the day, have just told him their stories. Alan, too, has wept into the lake at bad times, and also felt his spirits soar, walking in the garden with his toddler son, earnestly naming buttercups and cow parsley and ‘stinky garlic’. It is hard to walk away from such memories.

“But it feels as if the stars are aligned,” he says. “It’s the right time to leave Stourhead.” His two older sons are settled. The months of closure in brilliant sunshine, have felt like a parting gift – a chance to barrel through his to-do list – but also an exquisitely prolonged farewell. His destination is Derreen on Kerry’s Beara Peninsula, a sub-tropical seaside garden warmed by the Gulf Stream: he can’t wait to get his hands on its astonishing plant collection.

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Above all, he feels Stourhead is in good order. Last year he had a visit from John Sales, former head of gardens at the Trust, and initiator in 1978 of a 100-year conservation plan for Stourhead – the first of its kind. The congratulatory letter of thanks he received from Sales and his wife brought yet more tears to his eyes. “That meant more to me than any report I’ve had in the past ten years – I couldn’t have been prouder.”

If John Sales believed he’d done a decent job, he could walk away with a light heart. And start planning how, over the next 20 years, he will turn romantically tangled, little-known Derreen into another world-class garden.

Useful information Stourhead, Stourton, Warminster, Wilstshire BA12 6QF. Tel 01747 841152,