Interestingly, the first edition of the plant hardiness zone map was created by Arnold Arboretum at Harvard in 1927! And believe it or not, the most recent version of the USDA plant hardiness zone ...

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However, bear in mind that hardiness zones don't account for micro-climates. Many areas have small pockets that may be hotter or colder (often because of hills or valleys) than indicated on the map.

Check out the map pictured here that shows the USDA hardiness zones. For 6a and 6b, average low winter temperatures are in the range of (-10) to 0 degrees F. If you want a plant that will come ...