Hardiness Zones

What are the zones that you refer to in your articles. I have not been able to find them anywhere. I was told they are on the back of seed packages, but cannot locate them there either.
  I would appreciate any help you can give me.

Zones typically refer to cold hardiness. These zones are based on the average minimum temperatures for an area. Heat hardiness zones are used less frequently. It refers to the average number of days with temperatures over 86 degrees. Check out the American Horticultural Society's Heat Zone Map. Perennials, trees and shrubs are rated for their cold and sometimes heat hardiness. Selecting plants that thrive in your zone will increase your gardening success.

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