Mayor Richard M. Daley, who already has lined Chicago's downtown streets with more than 135,000 trees and countless flower beds in a much-heralded beautification program, is turning to flower power once again.

This time the greening is taking place on skyscraper rooftops, and the motive is not so much aesthetics as it is lowering sweltering temperatures and reducing choking air pollution during the city's notoriously hot summers.

It is a goal that may sound quixotic to some, but William F. Abolt, commissioner of the city's Department of the Environment, says "that's the plan." With Daley's go-ahead, he is putting $1 million into an experimental rooftop garden atop City Hall and other planting projects.

Work began Tuesday on reinforcing the 20,000-square-foot roof of Chicago's 11-story City Hall to prepare it for the planting of two oak trees and more than 21,000 plants and shrubs this spring as part of a five-city pilot project assisted by the Environmental Protection Agency and the Department of Energy. The project is aimed at controlling heat by restoring vegetation that once helped keep temperatures and air pollution at tolerable levels before overdevelopment.

With their black tar roofs constantly exposed to the sun, cities soak up heat, accelerating the creation of ozone and worsening smog conditions as each building is transformed into what thermal experts call an "urban heat island."

On a hot summer day, the temperature of a black roof can easily be 90 degrees hotter than the ambient air, rising to as much as 180 degrees and straining a building's air conditioning capacity.

However, the shading effect of a single tree with a 15-foot canopy of leaves can lower the temperature under it by three to four degrees and the cooling can be even greater as moisture is released by evaporation into the surrounding atmosphere. Additionally, because the leaves cool down at night, they absorb more heat from the air during the day, enhancing the cooling effect.

Studies using computer models have shown that one tree can have a cooling effect equivalent to an air-conditioning unit capable of comfortably cooling a 1,500-square-foot apartment. When combined with other trees, shrubs and thousands of plants on each of hundreds of rooftops throughout a city, such gardens could reduce overall temperatures in a city's core by as much as five degrees, according to computer models created by the Lawrence Berkeley National Laboratory in California.

Abolt said the city, using part of a $1.1 billion settlement that Chicago won in a municipal franchise dispute with the region's primary utility, Commonwealth Edison, will plant scores of varieties of plants on the City Hall roof, including wild strawberry, buffalo grass, prairie clover, western sunflowers, goldenrod and prickly pear cactus.

Researchers hope to learn which plants do best in the inhospitable urban environment. At the same time, they will make temperature comparisons using an adjacent black-tar roof over the Cook County offices to gauge the effectiveness of the experiment. In addition to the EPA monitoring, infrared satellite photos from the National Aeronautics and Space Administration will be used to measure the program's effectiveness.

Chicago officials also plan to plant gardens on top of parking garages and other structures in four of the city's 50 wards.

The other cities in the EPA's Urban Heat Island Project--Houston, Salt Lake City, Sacramento and Baton Rouge, La.--are concentrating mostly on the use of different types of roofing materials that reflect sunlight, as well as the use of more reflective paving materials, and the planting of trees and shrubs around parking lots and other open areas.

For several years, the Department of Energy, working with the nonprofit American Forests group, has directed a Cool Communities Program focusing primarily on planting trees at ground level in urban areas. There has been no organized effort to promote roof gardens as a way of reducing heat and pollution.

Abolt said he expects the City Hall roof garden will result in direct savings of $3,000 to $4,000 annually in heating and cooling costs, while at the same time reducing emissions from coal-fired electric power plants because power demand will fall. This could, in turn, ultimately reduce the utilities' environmental compliance costs, Abolt said.

"If you do enough of this, you can reduce everybody's air-conditioning costs because the overall temperature in the city is lowered," Abolt said. However, he said it will take a coordinated effort involving not only roof gardens but more surface green spaces, more trees around parking lots and more light-colored roofs.

Abolt acknowledged that "it's not for everybody because there are costs associated with it." But he said he already had heard from numerous developers and owners of office and apartment buildings who have expressed an interest in rooftop gardens because of the cost-savings potential. Comparison studies conducted during the pilot project and monitored by the EPA could convince more developers of the benefits, he said.

CAPTION: Urban 'Heat Islands' (This graphic was not available)