SKY GARDEN: Chicago City Hall's rooftop garden started seven years ago. The city now has 4 million sq ft of such gardens. -- PHOTO: AFP

Dozens more dot the rooftops of shops, restaurants, businesses and city-owned buildings in a patchwork of green aimed at cooling the concrete jungle.

Some four million sq ft of rooftop gardens have been planted on public and private buildings in the seven years since the first plants were placed atop city hall as part of a broader effort to reduce the Windy City's carbon footprint.

Inspired by similar programmes in Europe, Chicago now has one of the most extensive rooftop garden programmes in the world.

Corporate America is joining the trend and planting gardens atop a Chicago McDonald's restaurant as well as an Apple store, while other smaller businesses and landowners are converting to green roofs with the help of city grants.

'Chicago is at the head of the pack,' said Ms Amy Malik, regional director of the non-profit group International Council for Local Environmental Initiatives.

Concrete surfaces - especially those coated with dark tar - both absorb and radiate heat which significantly increases a building's heating and cooling costs and contributes to raising urban air temperatures.

The cooling impact of the gardens is dramatic.

Thermal images taken of the City Hall rooftop on a cloudy summer day found it was the same temperature as the air: 22 deg C. The black tar roof next door was a scalding 66.6 deg C.

'There are more than just aesthetic benefits,' said Chicago's Environment Commissioner Suzanne Malec-McKenna.

In addition to helping cool buildings, the plants also filter the air, reducing pollution and improving surrounding air quality.

The rooftops also 'stress sewers less by gathering rain water and using it', and a green roof can also extend the life of a roof by protecting it from the elements, Ms Malec-McKenna said in an interview.

The authorities do not generally open the 36 city-owned rooftop gardens to the public because of safety concerns.

But more than three dozen species of birds gather amid the 20,000 plants on the city hall garden alone.

It is a 20,000 sq ft oasis perched on top of an 11-storey building in the heart of the central business district, which hosts more than 150 species of plants, including purple comb flower, juniper and crabapple trees, bittersweet vine and sedum.

Sedum is a succulent, cactus-like plant ideal for green roofs because of its high tolerance for extreme temperatures and minimal need for water.

Honey from beehives kept in two of the city gardens is sold to raise money for after-school programmes.

'A market has been built around this,' Ms Malec-McKenna said. 'The economics of building green roofs have gotten much better. Now we have more than two dozen contractors across the Chicago region who know how to do this.'

Organic grocery store owner Paula Companio received a US$5,000 (S$7,000) grant from the city in 2006 to grow produce on her roof which she hopes to eventually sell in her store below.

The 1,500 sq ft garden covers half of the roof on top of True Nature Foods and produces a small crop of onions, potatoes, herbs and tomatoes.

AGENCE FRANCE-PRESSE