The tall, burly man kneeling in a potato patch rubbing friable soil through his fingers as if he were making pastry is terribly good-looking in a Lawrentian sort of way — or Thomas Hardyish, possibly; anyway you wouldn't shoo him out of your kitchen garden if you came across him there. "You want to earth up your spuds," he says to the camera hovering over his head on the end of a long jib. "Keep them protected till frosts are over." He has curly hair, sleepy eyes and dirt under his fingernails; his dandyish costume ­ corduroy trousers, button braces, leather jerkin — has just a sufficient patina of use about it to escape parody. This, we think, is evolved nurturing man, anchored to the land, taking the long view while the rest of us are tossed around like jetsam.

Yet Monty Don, the nation's pin-up gardener and famously uxorious husband, the man who says that working the soil is a cure for his depression, has hardly been home since October. Filming for a new TV series on gardens of the world has so far taken him to Australia, New Zealand, Cuba, Mexico, Japan and China; his job as presenter of BBC Television's longest-running show, Gardeners' World, takes up two, sometimes three days a week, and he is working on three books on top of his regular journalism. Then there is the smallholding he set up with his own money, helping drug addicts turn their lives around through the redemptive properties of digging, composting and mucking out pigs (which, naturally, turned into both a book and a TV

series): his vision is that similar projects will roll out across the country. In three days, he is off to India to trace the story of the Mogul empire through the nation's gardens.

It might seem a rude question when we've only just met, but what is all this overachievement about, exactly? A small, weary, rueful smile. "I didn't plan it," says Don, "but like buses, jobs often come along together and you think if you don't do it, you'll always wonder what it might have been like. Anyway, when you're freelance, you don't want to say no to work." Don doesn't need to drive himself so hard, but he apparently can't help it, partly because he is ambitious and in demand, partly because of a Calvinist work ethic imbued in him by his parents, and partly perhaps — I don't think this is fanciful — to placate the demons that lurk beneath the surface of an apparently enviable existence. But more of this later; it is 10.30 in the morning, early for demons, and we are sitting in a small, cluttered room — an old scullery, perhaps — at the nether end of a rather graceful house in the Midlands, lent to the BBC, where the Gardeners' World team is halfway through a four-year garden makeover. People wander in and out — the usual cast of thousands that television seems to need — greeting Don with the friendly deference reserved for those set slightly apart by stardom. "Monty! How was China?" "Amazing." "Did you find your knife?" "Yes, in my shoe, funnily enough."

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Don once said that the physical work of making a garden, then writing and talking about it, was "exactly" what he wanted to do. Yet in his current frenetic existence there would seem to be little time for digging or tending the two acres of perennials, pleached limes and potagers at home in Herefordshire. "I miss it terribly," he admits. "And I miss Sarah, my wife; we don't want to live separate lives. Work is completely dominant at the moment and the way we handle it is to say that this would be a totally unacceptable state of affairs if it had no end. It's hardest on Sarah, who has to hold the fort." The whey-faced recovering junkies who turn up shivering to work on the smallholding must miss their inspiring friend and mentor too? He winces: "It's complicated. I'd love to be there but the way I salve my conscience is to acknowledge that rehabilitation is a long game — there are no quick fixes and I have to earn a living if the charity is to survive." He took the gardens of the world gig, he says, because it gave him an "authorial" voice rather than just presenting a script. "Vanity swung it, I suppose you might say."

The Monty Don story has been well documented in interviews, in his gardening columns, and in his book, The Jewel Garden, an account of the heady years when he and Sarah made fabulously bling costume jewellery — Elton John and Diana, Princess of Wales, were among their many A-list customers — over-expanded in classic Eighties style, then crashed and burnt under a mountain of debt. They lost all six properties acquired during the good times and retreated to Herefordshire.

The episode was the very antithesis of the young Montagu Don's upbringing — military, disciplinarian father, distant mother, no hugs — where God, duty, hard work and being seen to be busy were valued above all else. He rebelled, inevitably, got himself expelled from two schools, flunked his A levels and went to work on a building site. Three years later, he redeemed himself by getting into Cambridge where he met Sarah, married at the time to a man who rowed in the same boat as Don. "It was like recognising someone you've never met," he said. "You just know." They eloped in 1979: "Her husband was very cross, as well he might be," Don told an interviewer. "It was very messy, not at all noble and everyone was hurt." This self-confessed "breaking of a moral code" has been vindicated with a long, companionable and romantic marriage — Don wrote recently of coming home from a work trip and finding a trail of rose petals leading through the house to the bedroom where Sarah lay waiting for him — that has produced three now almost grown-up children and two very beautiful gardens.

But throughout this compelling back story, their lives have been dogged by Don's recurring bouts of depression, at its worst in autumn with darkening days — "The harvest is in and you are not saved" — sometimes reducing him to tears and hallucinations: he once "saw" a bull marauding through a restaurant where he was having lunch. At its nadir, Sarah told him if he didn't get help, she would take the children and leave: "Because I can't live with you like this." He had cognitive therapy and took Prozac, which he gave up after five years in favour of light boxes designed for people suffering from the now-recognised affliction of seasonal affective disorder.

How is the SAD these days, I wonder. Don crosses one long, corduroy-trousered leg over the other and grimaces. "God, I feel like an old trouper, trotting out the same songs — the only reason to keep talking about it is that it might help someone." He says that filming in sunnier climes this last winter mitigated the blues. "Coming home from New Zealand in December I thought, no wonder I feel like I do. England is an incredibly gloomy place at this time of year, in which it is not unreasonable to be miserable." Travelling with a TV crew was also oddly helpful. "There is someone to drive me, get me to places on time, put a mug of tea in my hand. If you want to, you can retreat into a cocoon and be mollycoddled."

In the early Nineties, after the failure of his jewellery business, Don had given himself up to depression and the dole. "I know what it's like to feel that you're at the bottom of the heap," he told me last year when we talked about his work with drug addicts. "I have a natural empathy with the underdog. It's all about trust and humility: here I stand naked before you." Gardening had rescued him from a deep melancholy and so, he reasoned, it might help these young men and women, caught in a spiral of drug dependency and crime. "At the very least we could offer them the chance to take some exercise out in the fresh air in a beautiful environment, to sit down and eat food they'd grown themselves."

Since childhood, Don had written obsessively — diaries, short stories, rejected novels ("most of them cringingly awful") — and his new enthusiasm for gardening, plus a pressing need to earn money, prompted him to try garden journalism. His first TV gig was a five-minute slot on This Morning with Richard and Judy, but, he says, when he was offered a weekly newspaper column it changed his life. Over the next 12 years, as he and Sarah toiled to create a garden out of a field, Don chronicled their progress: readers loved it, more TV followed and audiences warmed to his engaging directness. Today, his popularity has grown to the point where, despite being a self-confessed "rank amateur", he is described as the laureate of British gardeners. (There are those who dissent from this view, but we'll come to them shortly.)

These days he is routinely referred to as a celebrity gardener. "You never get sympathy from anyone," he complains. "They all think it's glamorous when really it's just sheer hard slog." He is, he admits, still in thrall to the work ethic, but it's his own, not his parents'. "After all that rebellion, I realised that the way out of this horrible middle-class conforming was self-motivation ­ getting up at 5am to write, training hard for sport so that even if I wasn't the fastest or highest, I was the fittest." He gets things done with a kind of remorseless rigour. "You take a punt, back yourself. You learn by taking risks and coming a cropper." The project with drug addicts failed initially when the 20-acre site he'd found in his local village was vetoed by neighbours. "He went about it the wrong way," says an observer. "He assumes others will see things his way."

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