On the back of seed packets you'll find a map of "hardiness zones," or zones where plants are best suited to grow.
The zone map is created by the United States Department of Agriculture and has been used for decades by amateur and professional gardeners as a guide to yearly planting.
So why all the changes?
The USDA has found through research from the past few decades, that a general warming trend has taken place across the U.S. from 1990 to 2005.
This means the new map should be more precise than the previous map published in 1990.
Each zone on the map is based on 10 degrees Fahrenheit variations in the coldest average temperature expected in the region. Zone 2 is the coldest region for the U.S., where the average minimum temperature is -40 to -50.
The Arbor Day foundations, suggests "the new 2006 Hardiness Zone Map is consistent with the consensus of climate scientists that global warming is underway."