In order to combat the urban heat island effect and to improve urban air quality, the City of Chicago began construction of a rooftop garden in April 2000.

The garden consists of 20,000 plants of more than 150 species, including shrubs, vines and two trees. The plants were selected for their ability to thrive in the conditions on the roof, which is exposed to the sun and can be windy and arid. Most are prairie plants native to the Chicago region. A supplemental irrigation system will aid in establishing the plants as well as provide supplemental water during extreme periods of drought.

The City Hall rooftop garden is a circular initiative, because it improves air quality, conserves energy, reduces stormwater runoff and helps lessen the urban heat island effect. Plants have the ability to cool the air, because of evapotranspiration, which occurs when plants secrete water through pores in their leaves. The water then draws heat as it evaporates. Plants can also filter the air by using excess carbon dioxide to produce oxygen.

Chicago City Hall’s greenroof saves $5,000 a year on utility bills and can retain 75% of a 1 inch rainfall before there is stormwater runoff into the sewers.

Photo by Celia Haven on Flickr