Here is a great way for a gardener to spend a cold winter morning: check out the new interactive USDA Plant Hardiness Zone Map. For the last 22 years, Chicago has been a zone 5. However, earlier this year, the USDA (U.S. Department of Agriculture) updated its Hardiness Zone Map, adjusting the zone upward by about a half-zone for much of Chicagoland. For Chicagoans, this means much of Cook County is now a zone 6a.
For those not familiar with this, USDA plant hardiness zones set the standard for gardeners to determine which plants are most likely to thrive at any given location.
The new interactive map is quite precise. The new zone roughly follows I-294 south of I-55 (including the city and suburbs of Burbank, Evergreen Park, Oak Lawn and south to Blue Island, Calumet Park and Riverdale). North of I-55, the new zone roughly follows IL Rt. 171 north to Chicago Avenue, including the near east suburbs of Berwyn, Riverside, Oak Park, and then veers eastward for the northern suburbs. Here, gardeners will see zone 6a appearing in pockets, very specific to where the grower lives. Using the interactive map and a zip code search, the user gets precise data with a click of the mouse.
According to the USA, these changes are largely the result of temperature change studies over a 30-year period as well as sophisticated mapping methods that consider elevation, nearness to bodies of water and topography.