Tony Kirkham can talk “for ever and ever” about trees, he says, and he is true to his word. “I have had a passion for 40-odd years, since I was 10 or 11 and I'd go climbing for conkers. When you're up there, it's like a different, magical world. Peaceful. You see trees in a totally different perspective. All the diversity! Stop me if I'm going on a bit.”

As you can see, Kirkham, Kew Gardens' head of arboretums and the unlikely star of BBC Two's The Trees That Made Britain, rather likes trees, and he wants you to like them too. And to help us, he's made, he says, “a bit of an icon”.

Kew's Xstrata Treetop Walkway, which opens on Saturday, is Kirkham's device “for getting up-close and personal”. And, boy, are you close. A lift, or a staircase for the steely thighed, raises you 18m (59ft) on to a walkway among the treetops. Children are welcome but this is not one for vertigo sufferers.

The floor and walls are steel mesh, veiling, but not entirely obscuring the hefty drop below, and the railing is at chest height, so while you don't engage with the immediate knee-trembling plunge, you are well aware it's there.

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Woodpeckers and squirrels, bats and insects, tawny owls and plain old sparrows live up here in the chestnut (sweet and horse, in full bloom), oak, pine and walnut trees, blissfully unaware of the chasm underneath and the eye-boggling views from their des res of the Gherkin and Wembley's arch.

This is Kirkham's magical world. Peaceful, as he says, the walkway is hugged by branches bursting with May leaves and swaying in the breeze - unlike the walkway, I should add, which is as solid as terra firma. Though the treetops moving all around you do make you feel a little at sea in a force 9.

The magic of Kirkham's hidden kingdom has already rubbed off on the builders. “I call it Hobbit land, like The Lord of the Rings,” says one, tightening a bolt. “You don't even think about all the crap going on down there,” says his mate, nodding to Planet Earth.

Rarely have I witnessed builders proffering opinions about deciduous versus coniferous, or the merits of sweet chestnut. “But building this has been a little bit different,” says Dave Marriot, from the steel contractor W.S. Britland. “We had - what were they? - Egyptian geese nesting in here a few weeks back. The chicks just hatched and fell out of the sky. That doesn't happen on a normal building.”

And who better to build a showstopping, sky-high visitor attraction on a sensitive site than David Marks and Julia Barfield, the creators of the London Eye? They are still overshadowed by that famous creation, which remains the most visited paid-for attraction in Britain.

While this husband-and-wife-led firm has since built schools and art galleries, and has, Barfield says, “an absolute fear of being pigeonholed”, it is Marks Barfield's unerring ability to create massive contraptions for viewing the world below that seems to keep getting them work. In Brighton, for instance, they have planning permission for a 183m observation tower at the foot of the ruined West Pier.

“I have no idea why we keep getting these jobs,” Barfield adds. “The Eye helps, of course. And, yes, we're interested in our projects offering real experiences of being in a place, but no more so than any other architect.”

What thrills them instead, Marks says, is “that mix of architecture, science, engineering, social and ecological responsibility we grew up with in the 1960s”, expressed in the work of, say, Buckminster Fuller. “We like creating man-made structures inspired by the geometries of nature.”

At Kew, the rhythm of the struts holding up the walkway echo the Fibonacci sequence, a 13th-century mathematical theory akin to the Golden Section - that half scientific, half New Age bunkum theory of innately satisfying proportion. Leonardo Fibonacci suggested that a sequence in which each figure is the sum of the preceding two (1, 1, 2, 3, 5, 8, 13...) lay behind much of the formal elegance of nature as it appears in shells, leaves, spirals or fractals.

All Kirkham wanted, though, was “something quiet in the woods. I'm a bit of a minimalist. I like man-made things that are a bit subtle. I don't really like all this modern stuff - this excepted, course.” The treewalk may be several hundred tonnes of steel, but it's only quietly iconic. Its massive triangular columns consciously echo the trees, cradling the walkway in their three branches, in prerusted weathered steel.

The new arrival is the last in Kew's recent building spree, provoked by its designation, in 2003, as a World Heritage Site. Since 2005, Kew Palace has reopened, restored, and been joined by new additions, such as Wilkinson Eyre's dinky admiral's hat of an alpine glasshouse, John Pawson's elegant Sackler Crossing across the ornamental lake, and Walters & Cohen's glass Shirley Sherwood Gallery of Botanic Art.

Some have accused Kew of aping Alton Towers. But these are gentle thrills compared with the structures in Kew's original landscape. The Royal Botanic Garden was stitched together from two 18th-century gardens, Kew and Richmond, worked on by the age's finest architects and landscape designers - William Kent, Capability Brown and William Chambers - to create an archetypical landscape in the English picturesque tradition. It is arguably Britain's greatest contribution to environmental design.

This was a theme park for the intellect, punctuated by follies - temples of Pan and the Sun, an Alhambra in blue and gold stars, a faux mosque with two minarets and an interior of papier-mâché green stuccoed palm trees, for example - to emulate the sublime, a rush of beauty to the head caused by the artful interplay of man and nature in an artificial landscape.

Which are exactly the emotions Kirkham is hoping to conjure up. “What was once a desert,” Chambers wrote, “is now an Eden.” With this final piece of its newly thrilling landscape, Kew is Eden once more.

The Xstrata Treetop Walkway opens at Kew Gardens, Surrey, on Saturday May 24. No booking required

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