Garden in the Sky

Gardeners and environmentalists in Chicago are moving into uncharted territory in their effort to moderate summertime heat: up on the roof. Rooftop gardens are sprouting on buildings throughout the city, redefining urban green space in the process. Chicago is only one of many places trying to capitalize on the fact that plants are natural air conditioners. Meanwhile, city dwellers are discovering that flowers, grasses, and trees also make great upstairs neighbors.

Discuss rooftop gardening at the Nature conference in Café Utne’s:

‘Rooftop gardening is going to become more popular as land becomes more precious,’ says Liz Serritella, co-owner of Chicago’s Old Town Bed & Breakfast and a new rooftop gardener. ‘I love to dig in the dirt. We had gardens before, and when we moved I couldn’t garden anymore,’ says Serritella. Then she put in two rooftop gardens, to the delight of her guests.

Environmental designers would like to see the same thing happen on a much wider scale.

‘Flying into O’Hare airport, you see acres and acres of bare roofs. They all could be green, and that could make an enormous difference,’ says David Yocca, principal of Conservation Design Forum, a design group of landscape architects, ecologists, and botanists. The firm has been greening up a large area on the 38,800-square-foot roof at Chicago’s City Hall, building a garden that as soon as this spring will harbor as many as 150 different plants. Their efforts are part of the U.S. Environmental Protection Agency’s Urban Heat Island Reduction Initiative, which aims to reduce smog by lowering the temperature in five congested cities.

Like dark clothes, dark rooftops absorb the sun’s rays, heating city buildings and streets by as much as eight degrees. Hotter buildings need more air-conditioning, which leads to higher energy use and more fossil-fuel pollution. The heat also cooks the pollution, creating ozone-heavy smog. Other cities in the urban heat island project–Baton Rouge, Houston, Sacramento, and Salt Lake City–plan to use reflective roof surfaces to control the heat-island effect. Chicago is the only city testing whether green rooftops can accomplish the same thing.

Plants cool the air by releasing water vapors through their leaves, a process called evapotranspiration. According to computer models conducted by the Lawrence Berkeley National Lab, which is participating in the heat island project, widespread use of reflective surfaces and green roofs could reduce summer temperatures in cities by several degrees.

Rooftop gardens aren’t new; historical accounts date back 2,500 years to the Hanging Gardens of Babylon. During the Renaissance, Pope Pius II had a roof garden built at his summer residence in Pienza, Italy. More recently, settlers on the American plains built sod houses to insulate themselves from extreme cold as well as heat.

Green roofs are starting to catch on across the United States, says Matt Carr, Garden Roof product manager for American Hydrotech, a company that manufactures roofing membranes, including the one used on Chicago’s City Hall. Three years ago, says Carr, ‘we were working with two or three green roof jobs. Today, we’re overseeing about a hundred green roofs throughout the States.’

The potential benefits of green roofs are nearly as varied as the plants that can grow on them. In Portland, Oregon, where the challenge is not heat but rain, builders are encouraged to use green roofs as a possible way to win the city’s ongoing battle with stormwater runoff. The celebrated eco-friendly Gap headquarters in San Bruno, California, also features green-roof architecture.

In Europe, green roofs are catching on even faster. According to Theodore Osmundson, a landscape architect and author of Roof Gardens: History, Design and Construction, 43 percent of German cities offer financial incentives for building green roofs. But that’s not surprising: Many of the drainage and barrier technologies that have made green roofs a realistic solution for today’s structures were developed in Germany. European rooftop gardens function not only as runoff control systems, but as small parks, complete with ponds and, in one case, even with a miniature golf course.

A green roof consists of several layers. At Chicago’s City Hall, the roof was covered with a seamless membrane made of hot, rubberized asphalt designed to last as long as the building. Above that sits a synthetic mat that creates a grid-like canal system between the barrier membrane and the overlying gravel and soil. Though more than half of a typical rainfall will be caught in the gravel and soil, a layer that ranges from 3 to 30 inches thick, the remaining water can thus drain off without it drowning the garden–or dripping into the offices below.

When the City Hall garden is complete, it will contain 20,000 plants, from shallow-rooted sedums and ivies to shrubs, hawthorn, and crabapple trees. Many of the plant species will be of native origin because they are hardier and can withstand drought and high winds. Most native prairie plants have root systems that are too deep for roof gardens, but species that are found in hilltop prairies have shallower root systems, green roof designer David Yocca says. Ground-cover plants, including sedums, mosses, and grasses, can tolerate both too much and too little water. These plants will go dormant and turn brown during a prolonged drought, then green up again as soon as it rains, Yocca adds.

The City Hall’s roof is not open to the public, but it can be seen from a number of nearby buildings. Local scientists will monitor a green garden’s effect on climate, with the help of infrared satellite photos that register heat levels. The roof’s insulating capacity is expected to save the city $4,000 a year in cooling and heating bills. In addition, the soil and plant cover protect the roof from the elements, which should extend its life and make it easier to maintain. ‘Green roofs can last 50 to 100 years as opposed to a 15-year roof,’ says Yocca.

A green roof on top of City Hall is only part of the solution, of course. The city intends to install green roofs (and solar panels) on other public and private buildings. But their impact on summer swelter will be limited until private building owners follow the city’s example. Simpler versions requiring only a few inches of sod planted with low-growing plants are relatively inexpensive, and city officials hope their efforts will start a trend.

From Conscious Choice (July 2000). Subscriptions: $24/yr. (12 issues) from 920 N. Franklin, Suite 202, Chicago, IL 60610.

Discuss rooftop gardening at the Nature conference in Cafe; Utne:

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