The Windy City is blowing the competition away in the green roof movement, according to the AFP:

Some four million square feet (370,000 square meters) 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.

“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: 74 degrees F (22 degrees C). The black tar roof next door was a scalding 152 degrees F.

When I started growing tomatoes on my West Village roof fifteen years ago I didn’t even realize that covering your roof with vegetation was a great way to keep your building cooler. We just wanted to grow our own food and flowers and have an outdoor oasis in the middle of Manhattan.

Our roof garden grew to include everything from hops vines to hollyhocks, and it was heaven on earth–for a few years, anyway. It wasn’t until we dismantled it and moved the entire garden to the terrace of a free-form radio station in Jersey City (a long, strange trip I’ll spare you) that we noticed how much hotter our building got under bare asphalt.

The city of Chicago is well aware of the cooling benefits of green rooftops; after Chicago’s disastrous 1995 heatwave, which caused an estimated 600 heat-related deaths, the city began to incorporate green rooftops as a way to combat the “urban heat sink” effect of our concrete jungles and make its buildings more energy efficient (rooftop gardens keep buildings warmer in the winter, too.)

Green roofs also help reduce pollution by filtering the air, and reduce stress on the city’s sewers by absorbing rainwater. They provide precious habitat for wildlife, too. In fact, two of Chicago’s city-owned roof gardens even have beehives whose honey is sold to fund afterschool programs. Sweet!

Chicago started by greening the rooftop of City Hall. Now, the city’s businesses are following suit, with a green-topped McDonald’s and a roof garden above an Apple store, too (no word on whether it will include an orchard.)

The city is also encouraging other business owners to landscape their roofs:

Organic grocery store owner Paula Companio received a 5,000-dollar 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-square-foot garden covers half of the roof on top of True Nature Foods and produces a small crop of onions, potatoes, herbs and tomatoes.

Companio estimates that her building has been 15 to 20 percent warmer in the winter, and “noticeably” cooler in the summer since the garden was planted.

As an avid urban gardener, I have long dreamed of the day when all our unused asphalt acreage might be converted to lovely, productive gardens. Happily, my dream is finally coming true!

Learn more about the movement to green our city rooftops at greenroofs.org.