According to the U.S. Department of Agriculture, in 1990 Chicago was in plant hardiness zone 5 (with an average annual low -of 10 to -20 degrees F). Now it’s at the north end of zone 6 (average annual low of 0 to -10 F). The National Arbor Day Foundation has recalculated the zones as of 2006 and turned them into maps and animations. (Hat tip to Page 3.14.)

Kathy Purdy has a sophtisticated and readable discussion of hardiness zone maps. (Hat tip to Lake County.)