The autumn bloom on the High Line, Manhattan's new and most radically engineered park, rages gold and green in the sun. The park, half of which opened to the public in June this year, lies, absurdly, 30ft above the street on an iron-bound, 22-block stretch of elevated 1930s freight railbed that slices through the Lower West Side. Originally built because the short-haul freight trains were killing too many insufficiently agile New Yorkers – Lower Tenth Avenue was called Death Avenue in 1930 – the rail runs generally north/south from the huge 30th Street freight terminus to the cluster of 19th-century warehouses in the Meatpacking District, around Gansevoort Street. The last train made the short commercial hop down the rails in 1980, delivering to one of the meatyard warehouses that have sidings running into their second floors. It must have been close to Thanksgiving Day: the payload was three boxcars of frozen turkeys.

Twenty-nine years on, the railbed has been reincarnated as New York City's flaneur mile, drawing up to 25,000 visitors a day to its spectacular steel-framed planting beds. Its current northern entry at 20th Street and Tenth Avenue opens on to a three-block-long section called the Chelsea Grasslands, after the New York neighbourhood that was itself named after the garden-rich London borough.

Dutch master horticulturalist Piet Oudolf, called in to design the High Line's plantings four years ago, has painted the Grasslands in his trademark wild-prairie palette – swathes of meadow sage, loosestrife, three sorts of burnet, stonecrop, prairie dropseed, switchgrass, ironweed, wild quinine, prairie dock, purple love grass and 16 other species. Glorious at this time of year are the black-eyed susans, the deep-purple "Gipsy Queen" clematis, the "Sundown" echinacea and the spiky, almost extraterrestrial eryngium, or rattlesnake master, whose bloom heads have turned black.

"Piet's particularly good at the sequencing, or what we might call the rhythm of the plantings," says Patrick Cullina, the vice president for horticulture at the Friends of the High Line, the public/private consortium that built and manages the park with the City of New York's Parks Department. "He's also captured micro-seasons, as with the asters, which are blooming now, or with other patterns of blooms and hibernation that he's orchestrated to occur within days of each other. The switchgrass turns gold, the autumn moor grass will turn silver and its top foliage gold, and your eye can catch that silver and the other patterns as they repeat."

This is Oudolf's signature, a meticulous but layered-as-God-would-strew-it planting algorithm that he's brought to the High Line. His beds are both radical and conservative: radical in that one does not expect them where they are, but conservative in that they look natural where they are, because they've been tailored to the hostile urban conditions under which they thrive. Forty per cent of the species Oudolf put on the High Line were already there, dropped by birds and blown by the winds on to the railbed during its derelict years. Sustainable may be an overused adjective, but Oudolf embraces the notion, in that what was put there by nature was, by definition, sustained.

"Perhaps we can call what I work in layers of patterns," says Oudolf, at his home in Hummelo, the Netherlands, in the week before departing for a horticultural conference in Berlin. "It's about finding species and understanding the conditions where they might be native – for instance, a few years ago I took a long drive around the American prairies. And some of these conditions might also be similar to the conditions atop a railroad such as the High Line. Of course with the High Line, as with all of nature, there's also a narrative: you move from grasslands, to trees and shrubs, back to grass, and then to woodlands at the southern end."

Because the massive black trestleworks and riveted balustrades are vintage 1930s, at street level the High Line exudes an air of heavy post-Industrial Age permanence. It looks like an airborne railhead in Leeds. But on the trestle the park unfolds itself to the visitor delicately, cinematically, as if the High Line is enveloping you in its own film about the future of New York City. One exits the forever-mad stream of midtown-bound truck and taxi traffic on Tenth Avenue, mounts three flights of steel stairs to the former railbed, then, framed by warehouses and by the Hudson River to the west, Oudolf's long grassy meadow unfolds south of 20th Street, washed by the light and the wind off the water. The Hudson is a mile wide in this stretch, so the experience approximates that of walking along the crest of a great dune above the ocean.

"There are just 14in to 20in of soil possible in most of the beds in the Grasslands, and most of these species grow in what gardeners would call junk," explains Cullina, the man charged with the overall maintenance of the park. "In other words, not high-fertility soil. On the High Line, they're in loam. When we were opening this section last spring, we'd push the plants with a little water to get them bedded in. We hand-water some, but in the Grasslands the species are mostly very tough and they're going to get what they get. The whole experiment is a testament to the power of drainage. Up here, the wind plays an enormous drying role. That's part of the point of Piet's choices. They have to be able to take it. It's a very 'prairie' feeling."

Bold as the High Line is, it's important to remember that New York – now happily receiving the many millions in taxes brought on by the real estate boom under, around and over the park – is the city that, for 23 years, let the High Line lay derelict. It was a rust-caked dragon's tail of black-iron urban blight, blotting out the sky and raking through the centre of blocks west of lower Tenth Avenue. The homeless lived under it; untenable buildings around it were razed and made into parking lots; weeds and trees later to be lovingly catalogued and replanted by Oudolf sprouted in the stone ballast under its railroad ties.

The point is that the High Line had no easy birth in any of its phases – in inception, design, or execution. First, the city fathers had to be persuaded by enough private economic momentum in the derelict neighbourhoods through which the High Line ran – in other words, enough risk-loving artists, gallerists, restaurateurs and boutique owners had to create a frisson of hipness and commerce – to even entertain the notion that building a park on the railbed might matter. In fruition, the process is best seen in the former warehouse and light industrial districts of Soho and Tribeca, now home to some of New York's priciest loft residences, shops and restaurants. Exactly this happened in the late 1990s in West Chelsea and the Meatpacking District. Then, in 1999, Chelsea residents Robert Hammond and Joshua David founded the Friends of the High Line, the advocacy organisation for the park's reclamation, and now its governing body.

Significantly, the Whitney Museum of American Art is building its downtown venue directly under the High Line at 17th Street. A further 10 blocks of High Line, from 20th up to its great westward bend over the railyards at 30th Street, have just undergone their engineering renovation and will open in late 2010. The city enabled this new construction with uncharacteristic alacrity, since the park had already proven its economic muscle with the real-estate frenzy engendered by the southern section. Surprisingly, given the park's success, the High Line's last quarter of a mile is under threat of being removed in favour of other development.

"We can call something like the High Line 'greening'," says James Corner, with just a whiff of disdain for the cliché. Corner was the leader of the 80-man High Line design team and the principal of the landscape architecture firm Field Operations. "But I think what we're seeing is a need in communities to take these industrial spaces – be they old ports or harbours, or transport infrastructure such as the High Line – and repurpose them, imagining new uses for how these places can be re-occupied."

Corner is British, a Mancunian whose New York-based firm has designed some 40 projects worldwide, including a 278-acre island off the Korean coast and a 4,500-acre, million-tree reforested park in Tennessee. Compared with those, the High Line has been a rather delicate job.

"There's a lot of engineering in the High Line that one doesn't see," he says. "The paving system – the aggregate planks – had to be able to expand and retract so that they can keep their shape after a hard winter or a really hot summer. We had to have open joists so that we could drain and collect rainwater. Then there was Piet. I think it's safe to say that Piet very much does not do boxwood hedges and manicured lawns. He has literally thousands of species in his head, and he knows how to put them together for good compositional effect. He sort of dismembered the original meadow that was on the High Line and then put it back."

A hint about the kind of effort it takes to refashion the High Line as a garden can be had on a short walk up the unopened section, from 20th to 30th Streets. Here, the rails have been taken up and the screed-layer has been poured, to serve as the foundation for the plant beds. On this day, a team of workmen is yanking the last few rusted train rails from the 30th Street spur, chalking them with number and location. The last "original" – meaning derelict – plantings of ironweed and Johnson grass can be seen wafting in the wind on the third east-west section they have yet to rip up.

"The structure was robust, built as it was to carry two freight trains," says Nahyun Hwang, a High Line team lead project designer and member of Corner's Field Operations. "But obviously we pulled up, catalogued and saved the rails. We tried to save the timber and the stone ballast, but that had been so contaminated with asbestos and fuel that we were going to have to send it off to be specially washed. We decided: OK, we can live with new stone and new timber."

There are European projects with the impact of the High Line. The direct antecedent is Paris's Coulee Verte, a 19th century elevated railbed converted into a park in 1992. Of more recent note are the Quai Branly Museum's green wall, also in Paris, and the Eco Boulevard in Madrid. The re-imagining of European cities is accelerating, but because they are mostly laid out on plans fixed hundreds of years before New York's, the greening is more piecemeal.

"London has installed 230,000 square metres of green roofs in the last four years – that's more than New York or Chicago," says Dusty Gedge, president of the European Federation's Green Roof Association. "But most of it's private, so nobody sees it. We're starting, but I'm afraid we just haven't seen the political will."

One UK public project that will outweigh the High Line in scale is the Olympic Park, currently under construction for London's 2012 Games. It's a truism that the "get" of the Games often spurs city planners to architectural and horticultural heights, some of it bombastic and dictatorial – Beijing 2008 springs to mind. The brains trust for London's Olympic Park – James Hitchmough, Sheffield University professor of the department of landscape, and his colleagues – are anything but bombastic. Hitchmough is a leading proponent of the "meadow-ist" school of planting, preferring in his designs an even more naturally chaotic chaos than Oudolf.

"Piet and I are on a spectrum," Hitchmough explains. "Natural as they look, Piet's designs will still be carefully composed, whereas ours are more like nature on steroids. For the Olympic Park we'll use native English plants; there will be wetlands and woodlands and meadow. But we've been studying how to get species from all over the world into the park. At the moment we're looking at some South African tall grassland species. It is the Olympics, after all."

Two of the High Line's most striking architectural features are its rail spurs – bridges that lead from the main track into what were once warehouses. The southern spur, at 15th Street, will soon be converted into a small children's playground, while the northern spur, which crosses 10th Avenue at 16th Street, is fully planted, glade-like and resplendent in October-blooming asters. One block south of the southern spur is the stage for the human fauna of the park, the sundeck, sponsored by film and shopping mogul Barry Diller and his designer wife, Diane von Furstenberg.

The sundeck lies directly over a busy petrol station at 10th Avenue and 14th Street, isolated from the streetside bustle. It's in this refuge, especially on sun-drenched autumn days, that New Yorkers retreat to read, reflect and – not least – flirt madly while in the garden's embrace. That the High Line's natural display is flat-out sexy has not been lost on anyone. When the Standard hotel opened earlier in the year, some of its guests, in flagrante in their rooms, ripped back the curtains, turned on the lights and put on a show for the assembled parkgoers down on the High Line.

Leading a tour of the flowerbeds on a fine, warm autumn day, High Line vice president Patrick Cullina stops briefly next to the sundeck, which in this leftover bit of summer resembles a beach resort, with dozens of people splayed out over the chaise longues. "I had a family stop me the other day," he says. "They'd noticed the sound of the crickets. They said: 'How did you broadcast that recording? We couldn't figure out where the sound system was.' I had to tell them that the crickets were real. The crickets arrived, as all nature would when provided with proper habitat. That's the thing about greening a city. You think that nature is so far away, so difficult to get. But actually, if you provide it with the right conditions, nature is right there next to you, just waiting to happen."★