Just as gardeners across the city are raking in new dirt and fertilizer and planting seedlings, City Hall is embarking on its own spring gardening project--albeit 12 floors up in the sky and with the assistance of a 14,000-pound crane and truckloads of specially formulated lightweight soil.
Work on Mayor Richard Daley's ambitious City Hall garden roof gets under way Tuesday, commencing a $1.4 million experiment to see whether the tops of Loop buildings can be transformed into lush mini-Edens and help cleanse the air and reduce energy bills in the process.
The project, expected to take 12 weeks to construct, involves transforming the roof of the 11-story City Hall into a rolling terrain of formal gardens designed with 20,000 plants from 150 different species of shrubs, vines, flowers and grasses. The vegetation will even include two full-size trees, a cockspur hawthorn and a prairie crabapple, all hauled up to the roof via freight elevators and cranes.
And that, in a sense, is the easy part.
Planning the project to avoid having the roof collapse under the weight of nearly 350 tons of plants, dirt and water was an even trickier feat.
A team of structural engineers, architects and landscape designers was hired to come up with ways to keep the weight down. The solution: replacing topsoil and manure with a special blend of compost, mulch and spongelike ingredients that not only weighs less but retains water better.
"It maximizes the amount of water it can hold and minimizes the amount of weight," said Environment Commissioner William Abolt.
More than 12,750 cubic feet of this soil will be hoisted to the roof once a layer of gravel that covers the roof is removed and a sheet of water-permeable membrane has been rolled into place. The plants are already being grown in the lightweight soil mixture at a greenhouse in west suburban St. Charles.
The experts also determined where to place plants to minimize stress on the roof. For example, the trees, which weigh the most, will stand atop 18 inches of soil over a structural column. Thick concentrations of shrubs and flowers will be laid in 8 inches of soil over raised portions of the roof that once were skylights but have long since been covered. The rest of the roof will be covered with 4 inches of soil that will be filled with smaller plants.
But first, crews will install a layer of egg-cartonlike plastic squares that can hold water under the soil to keep plant roots moist during dry spells.
Since Daley announced plans for the garden late last year, the concept has changed as experts took into account technological and aesthetic challenges of gardening in the sky. The overall design has changed from prairie-style to a formal garden. Boston ivy and other vines were added to the vegetation menu, and stately oaks included in the original concept were replaced by the hawthorn and crabapple.
"The basic plant selection chosen won't require a lot of water, will need limited maintenance and will survive in the winter," Abolt said.
The cost, to be paid with funds obtained through a settlement of a long-standing dispute with Commonwealth Edison, rose $400,000 from the original budget because of additional reinforcing layers needed to buttress the aging roof and help it support the weight.
The tree roots will be kept in place with vertical barriers, and the branches will be pruned like bonsai trees.
Planting will begin in July, when the smaller plants will be wheeled to the roof on trays via freight elevators. The trees will go up by crane.
The project will have at least one thing in common with other City Hall ventures that so often cater to the clout-heavy. The roof will be off-limits to the public. The only views will be available to people in offices in surrounding high-rises that tower over City Hall.
The city has also has set up a mini-weather station to measure rooftop temperatures and compare them with temperatures on top of the adjoining County Building, which will remain barren. Rooftop temperatures on Loop buildings can be up to 40 degrees higher than at ground level during the summer, forcing air conditioners to work harder. The goal is to see whether the vegetation brings down the thermometer on the City Hall side and cooling bills.
City officials expect the experiment to chip $4,000 annually off the City Hall air-conditioning tab. Officials also hope the project will inspire developers and residents alike to build rooftop gardens. Experts say a critical mass of such projects could reduce ozone pollution in the city, which in summer has the third worst air quality behind Houston and Los Angeles.
"This is unique to Chicago," said Richard Price, director of community design with the Virginia architectural firm William McDonough & Partners. "No one else has looked at rooftop gardens to mitigate urban heat island effect, which is a process whereby highly urbanized areas with hard surfaces tend to be degrees hotter than green areas."
Environmentalists say that urban areas, with their hard surfaces and black tar roofs, absorb heat, raising the city's temperature a couple of degrees--prime conditions for producing smog.
William McDonough is among a team of experts that have designed City Hall's rooftop garden. The company built a similar green space for the Gap's corporate headquarters in San Bruno, Calif., and at Oberlin College in Ohio.
Like last summer's Cows on Parade display and now ubiquitous rows of wrought-iron fencing, the rooftop garden idea sprang from Daley's European travels.
Even before the first seedling is planted atop City Hall, the Environment Department has begun plans for a second rooftop garden on a city-owned building in the West Loop. The city also has persuaded the developers of the Block 37 shopping, office and residential complex to put one there.
And city officials hope the garden-in-the-sky idea will branch out.
"We'd like to see it replicated by both the public and private sector," Abolt said.