Among the images of Vivienne Westwood's menswear show for autumn/winter 2009/10 one lanky, elegant, bearded model stands out from the skinny young men. There's a clue to his identity: the collection, full of tweeds and leather, was tramped up and down a catwalk covered in gravel. Knee pads, thick gloves and even the odd trowel complete the look, for the model is Westwood's own gardener Andy Hulme, muse and inspiration for the whole collection.

At first, he thought the idea of a collection based on his look was a joke. "People were mentioning it to me and I kept thinking they were making fun of me. I even thought, This is a bit evil, they are all joining in on it!" And then he went through the strange process of watching the collection evolve. "The first time I turned up and saw a couple of photos of myself on the wall of the studio, it was a bit weird, but then I thought, What an interesting thing to happen."

Some fashion-industry commentators were rather dismayed at the idea of a mere horticulturalist kitted out in Milan's finest productions. The website opined that a gardener would hardly be likely to go out pruning in Lanvin-inspired high-tops. Yet all the evidence would suggest they are wrong. Today, Andy is wearing the most extraordinarily composed three-piece suit, topped off by a doubled pair of jaunty hats. In one hand is a roll-up; with the other he's digging out peonies.

He looks rather fine. Yet I wonder, isn't it a bit, well, uncomfortable, gardening in a Westwood jacket and tie? He gives an engaging grin. "Well, in 20 years, I've investigated every type of military, industrial, country clothing. And you can't beat Westwood," he laughs. He's happy to look heavily traditional for garden work: "I mean, the idea we have now of work clothes, leisure clothes, sports clothes... it's quite a new idea. If you look back, people wore suits to do everything."

Sartorial interest came to him young. "I got interested in clothes around the same time I got interested in girls," he recalls. "And my mother's father was a bit of a dandy, a country-gentleman type, and I ended up with some of his clothes." The gardening came later. "I was in trouble with the police,and I had to get a job."

He first worked with Westwood on the "Erotic Zones" show in 1994, where he was commissioned by her catwalk set designer to do the floral decoration. "We met outside a flower shop in Paris," he remembers, smiling. She commissioned him to create a vertical garden at her studio in Battersea, where wisteria and climbing roses span the summer, filling the air with scent and colour.

Just before I leave, Andy pulls out a black-and-white photo. It is a stark pyramid of brick in a wild open space, backed by railway bridges and a faraway glimpse of the Swiss Re building. Straggly buddleias flourish in the Tarmac cracks. The pyramid is weighty and silent, giving shape to the neglected urban space. "That's my garden," he says.

Back at Westwood HQ – a gorgeous Queen Anne house with gauzy floral curtains – I take a moment to peruse the labels and imagine how it must all look in full bloom. There's a remarkable selection of different orange cactus dahlias, Royal Wedding, Kismet and Arc de Triomphe, dotted in among other flowers in gaudy shades of marmalade: bright tiger lilies, and Crocosmia "Norwich Canary". The garden in the fullness of summer, with roses falling across the path, must be bewitching. But I'm still not convinced about gardening in a designer suit. "Well," Andy laughs, "you just gotta watch where you put the secateurs."