Hardiness zone maps are helpful for choosing plants and for timing plantings to coincide with the weather. Have you ever seen those folks with annuals lining their walk in late April? It always fries my potatoes that tender annuals such as petunias and marigolds go up for sale so early in the growing season. My rule, and it's been a good one for me, is to never plant annuals until after my father's birthday of May 13th. I don't ever lose annuals to frost.
With all that said, what is the USDA Hardiness Map?