CHICAGO -- It's a bit outrageous, even for a mayor whose penchant for petunias and other greenery has led to accusations that he is out to "Martha Stewartize" his rough-hewn city.
But Mayor Richard M. Daley is dead serious about planting a prairie, complete with grass, ivy and two oak trees, on the roof of City Hall. Workers could begin hauling tons of dirt and seedlings to the flat top of the 11-story building within weeks.
Unlike the flower beds Daley has added to the city's boulevards, or the trees he insists be planted every 15 feet when a sidewalk is poured, this botanical initiative has almost nothing to do with beautification and everything to do with pollution.
Traditional black roofs absorb sunlight and radiate heat to surrounding air.
The warmer air accelerates the creation of ozone, a caustic gas that makes eyes sting and lungs ache in the summer. A chief ingredient of smog, ozone is formed in a chemical reaction fueled by heat.
Experts say a sky-high garden will short-circuit that process, if only over a small area.
Sunlight will be converted into energy for growth by the plants rather than into heat by the roof. Trees, they say, will provide shade and cool the air through a natural, perspirationlike process, "evapotranspiration."
The rooftop turf will also insulate the building to keep it warmer in winter and cooler in summer, the experts predict.
Thus, power bills, and the pollution generated by power plants, will be cut.
"It's funny how people think only people out west, who live in the redwoods, can talk about the environment," Daley said. "This can help us tremendously."
The plan calls for more than 20,000 plants of many varieties, including wild strawberry, creeping baby's breath and purple prairie clover, as well as a burr oak and swamp white oak.
Researchers hope to learn which plants do best in an inhospitable environment populated mainly by traffic helicopters and window washers.
Other than pruning the trees to keep them from growing too big, maintenance of the garden is supposed to be minimal. It won't be mowed. Visits will be discouraged. Irrigation will be provided by rainwater, led by gravity to the thirstier plants more and drier species less.
The garden is expected to cost $750,000 and is part of a pilot project being conducted with the city and the federal Energy Department and the Environmental Protection Agency.
Four other regions of the city will also try to cool themselves by lightening pavement, planting ivy, replacing concrete with grass and making other changes.
Funding will come largely from a $25 million-a-year, four-year commitment conservation programs by Commonwealth Edison Co. The company, Chicago's major power company, agreed earlier this year to the contributions to settle an arbitration dispute over its service to the city.
"We don't see rooftop gardens on every building. But it's one solution," said William F. Abolt, Chicago's commissioner of the environment. "We're trying to demonstrate it. We think on some high profile buildings it can be viable."
Everything that can be done needs to be done, Abolt said. Chicago's smog is second only to Los Angeles', he said.
"We have a serious smog problem and need to fix it."
Such initiatives are applauded by the Heat Island Group at the Lawrence Berkley National Laboratory in Berkeley, Calif. Researchers there study how cities trap and intensify heat, becoming "heat islands."
They've discovered that urban areas are getting hotter every year and run 6 to 8 degrees warmer than lusher countrysides do.
Even that small margin adds up to billions of dollars of additional energy usage each year, according to the lab.
The chief culprits: inky black asphalt roofs and streets that can be 70 degrees hotter than the air on sunny days.
"If we could simply make the surfaces that are dark lighter, it could make a significant difference," said Mel Pomerantz, a physicist and staff scientist to the Heat Island Group.
Lightening the color of roofs to make them more reflective can reduce surface temperatures by up to 30 degrees. Using light-colored topping on pavement produces similar results and does not diminish a road's durability or add to its cost, he said.
A city that managed to do both on a wide scale could achieve ozone reductions akin to the elimination of automobiles, Pomerantz said.
"It's one of the simplist things we can do, one of the least invasive to our lifestyle," he said.
Chicago's City Hall can handle the extra weight of the garden, which may be several hundred tons, because it was engineered to accommodate another floor of offices that was never built.
A mirror-image county government building, which is connected to City Hall, will keep its blacktop roof so temperatures and ozone can be compared.
Workers will first lay a water-tight membrane on City Hall, then a root-proof protective liner. After that comes a water-retaining layer -- possibly resembling plastic egg cartons -- to be topped by dirt ranging in depth from 4 to 30 inches.
City engineers have consulted with their counterparts in Hamburg, Germany, where roof gardens are common and where Daley conceived of the project on a visit.
Thwarted early in his administration in some ambitious building projects, Daley has established a reputation for finding simple ways to improve the city's livability.
Since taking office in 1989, he has overseen the planting of 600,000 trees, returned flowers to miles of roadway medians that had been left barren and pushed through the city council tough landscaping rules for developers, said Terry Teele, first deputy chief of staff.
Sometimes he tears out photos from magazines of attractive streetscapes in other cities and sends them to staff, Teele said.
"He has a terrific passion for it. There isn't an engineer who works in the city of Chicago who would dare submit a plan for a street that doesn't first come with a design for the trees and flowers," Teele said.
This has pleased members of the environmental community, who give Daley generally good marks, despite complaints about the city's recycling program and other issues, said Jack Darin, head of the Illinois chapter of the Sierra Club.
"He is the strongest advocate for urban greening and open spaces in the city's history," said Darin, who called the heat island project "very innovative."
Chicago's City Hall project is an interesting one but is unlikely to unleash a wave of leafy roofs because of the expense, said Pomerantz, of the heat island study group.
"That doesn't strike me as a very widely applicable solution," he said.
Daley acknowledges that most Chicago roofs will remain tree-free. But the project will draw attention to urban heat, and residents may follow through with better choices when they roof their homes and offices, Daley said.
"It's very important for us as a city to make that strong statement," Daley said.