The USA’s National Abor Day Foundation has updated it tree hardiness maps (which are used to suggest what species of trees will grow in a particular region) based upon data from 5,000 National Climatic Data Center cooperative stations across the continental United States. The site includes an animation of changes between the 1990 and 2006 maps, which shows how the tree hardiness zones have moved north over the past 15 years.
The new map reflects that many areas have become warmer since 1990 when the last USDA hardiness zone map was published. Significant portions of many states have shifted at least one full hardiness zone. Much of Illinois, Indiana, and Ohio, for example, have shifted from Zone 5 to a warmer Zone 6. Some areas around the country have even warmed two full zones.
… Hardiness zones are based on average annual low temperatures using 10 degree increments. For example, the average low temperature in zone 3 is -40 to -30 degrees Fahrenheit, while the average low temperature in zone 10 is +30 to +40 degrees Fahrenheit.