Imagine a cross between summer solstice at Stonehenge and the Chelsea Flower Show. That’s what the latest exhibition at the Barbican Art Gallery is like. Radical Nature offers a survey of the work of some 25 artists and architects who, over the course of 50 years, have been trying to forge closer, more sympathetic bonds between Man and the planet on which he lives.

It’s obviously very timely. An anxiety that began in the early Sixties with Rachel Carson’s Silent Spring has developed into an extremely urgent debate. But what can art do? To be frank, this show feels like a pretty cranky convention. You will meet all sorts of freethinkers, from an architect who tries to explain his blueprints to a dolphin, to a girl who lures foxes with frankfurters to take self-portrait photographs.

The Barbican provides a fertile forum for debate as it transforms itself into a series of mini-gardens. The first thing you encounter is a little chunk of wilderness packed on to a car trailer: a lonely pine wolf, ready for transport like some circus animal, perches on a patch of natural environment. This is one of Mark Dion’s Mobile Wilderness Units. It would be funny if it wasn’t so sad. Dion speaks of our ludicrous urge to package up nature like any other commodity. This is the urge that the rest of the pieces in this show set out to challenge.

You drift off into a jungle of ideas asking you to look at the natural world from another slant: sometimes quite literally, as in the case of Henrik Hakansson’s Fallen Forest, a tropical jungle flipped on to its side. There’s a “farm in a grow-bag” type installation, a gazebo with a mirrored infinity of stately-home garden inside it, even an entire birch tree that has been felled (it was going to be cut down anyway apparently), sliced up and reassembled Frankenstein-like with great bolts through its trunk.

There is no chronology. The exhibition pays homage to pioneers. Here is a photograph of Hans Haacke’s 1969 Grass Grows, a lump of living turf that brought nature direct into the gallery. Here is a film of Robert Smithson’s 1970 Spiral Jetty, which turned art into landscape. And the relics of Joseph Beuys’s 1977 Honeypump in the Workplace — a mechanical ecosystem that shunted tonnes of honey round a closed circuit using engines powered by margarine — lie about like piles of dismantled plumbing. But ideas are not developed in logical sequence. This show grows organically, sprouting and ramifying and spreading off site. Agnes Denes’s 1982 Wheatfield — a Confrontation, planted among Manhattan’s skyscrapers, is recreated in Dalston. It’s on a much smaller scale , but still, coming complete with windmill to grind the harvest, it gets the point across: that urban and rural should not be divided, that Man needs to merge with, not dominate, his natural environment. This is the show’s fundamental message.

It’s all very worthy and often delightful. But people have been exploring the possibilities for decades: you can see that from the Star-Trek style designs of the architectural collective who dream of creating a diplomatic embassy in the realm of the dolphins, a floating outpost to promote communication between the species.

But do artists contribute anything practical? Or is this exhibition just a big carbon footprint planted in the middle of the Barbican, ugliest of our concrete urban confections? Where the contributors to this show move beyond self-congratulatory displays of their green credentials, they don’t get much farther than fantasy. Forget houses built on foundations of sand: here is a firm of architects who make a building from water. It’s a wonderful exuberant dream, but its purpose is all a bit hazy.

There is a bright side, however. Richard Buckminster Fuller’s dome might not have fulfilled the hobbit-house potential that hippies first dreamt of (a wooden example of his geodesic structure, made up of interlocking triangles, has been constructed on the Barbican’s terrace), but it has provided a much-used template. Wolf Hilbertz’s sci-fi plans for shell-shaped undersea cities are (by happy coincidence) helping to save our endangered coral reefs. R&Sie’s plans to create entire buildings that will eventually be subsumed by plants may one day, in our era of disposable architecture, prove a better answer than the landfill site. As one glances around the Barbican’s grim maze, one can only dream.

Radical Nature – Art and Architecture for a Changing Planet 1969–2009 is at the Barbican Art Gallery from Fri to Oct 19

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