While Chicago's metropolitan area has more parkland -- more than 300 square miles (777 sq km) -- than practically any other US city, much of it is a dense tangle of shrubs and trees like garlic mustard and buckthorn that were either introduced or invaded from elsewhere.

"That's why fire is so essential. Native plants are built to survive fire, while invasives aren't and won't grow back," said Stephen Packard, a naturalist with the Audubon Society for whom such restoration projects have become a life's work.

"We've probably burned this field 20 times," said Packard. Surveying the restored bounty, he rattles off a dozen species within a step of where he's standing.

"Violet bush clover, wild bergamot, purple prairie clover, dropseed grass and heather aster -- it's easy to pick out 50 species within a metre of my foot. They're mostly rare. Each has its own associated insects."

Among the rarest is the prairie fringed orchid, but Packard won't reveal where it's growing due to poaching.

Joined by 1,000 dedicated volunteers, Packard has laboured off and on since 1977 to transform a few isolated patches such as the 90-acre (36 hectares) Somme Prairie Grove near the upscale suburb of Northbrook.

"A lot of them work behind a desk as lawyers, or computer programmers, so this physical labour appeals to them," Packard said of the volunteers. He watched as volunteers fed two blazes with dense stands of cut buckthorn, a prolific and extremely hardy woody species introduced by early settlers.

PUT OFF BY PRAIRIE

Apparently put off by tall grasses higher than their heads that winds blew into waves stretching to the horizon, immigrants transplanted familiar trees and shrubs to see which would thrive and interrupt the monotony of the prairie.

But buckthorn, which in the English countryside is tended into hedgerows, and other invaders grew wildly in the rich prairie soil.

Now, Midwestern lands not ploughed under or paved over have mostly been transformed into suffocating thicket. Once-common blazes ignited by lightning are put out.

Bringing back the prairie requires fire and the careful reintroduction of native plants, said Packard, who has published several essays describing his methods.

Echoing the pre-colonial Potawatomi and Illiniwek Indians who set fires to give them a clearer shot at game and their enemies, Packard has selectively cut, spread herbicide and burned -- with the permission of park overseers, of course.

He and his volunteers painstakingly collected seeds from remaining stands of native plants and coaxed them to dominance by setting controlled fires that drove out invasive maple, ash and dogwood trees. Completing the savannah is the occasional oak tree that grows cork-like bark which resists fire.

Linda, a volunteer, points to one grass species that, when rubbed for its seed, smells unmistakably like buttered popcorn.

Packard believes the thousands of prairie plants harbour potential medicinal and agricultural discoveries.

While restoring parks to prairie may be a noble cause, landscape designers and developers creating environmentally sensitive living spaces see it as a business proposition in what some analysts say could be a trillion-dollar industry.

A year ago, Clemson University launched an interdisciplinary program called the Restoration Institute where biologists, materials scientists, architects and others aim to teach development done in tandem with nature.

For instance, multimillion-dollar homes developed on Spring Island, South Carolina, blend into the landscape on which only natural fertilizers are used, said Barry Nocks, the institute's associate dean.

Meanwhile, there are endangered grasslands in Africa, Australia and Siberia where restoration is mostly unknown.

Often, small-scale efforts to create prairie landscapes along highways or in backyards fail because of a lack of variety, or because fire is not used to eliminate invaders, Packard said. Backyard gardeners, for instance, tend to select only the tallest, more spectacular species.

Carrying handsaws and a camera to record the work of volunteers, the bespectacled Packard stops to gaze at his handiwork from beneath his straw hat.