There are many places in Illinois where you expect to find a prairie. The roof of City Hall in Chicago is not among them. Yet there it is-20,000 square feet (almost half an acre) of shrubs, vines and small trees, 11 storeys above LaSalle Avenue. Planted in 2000, City Hall's "green roof" reduces the amount of energy needed to cool the building in the summer; captures water during rainstorms, thus reducing the amount of water flowing into Chicago's already overtaxed sewers; and combats the urban "heat island" effect, which makes cities warmer than nearby rural areas.
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