A Planting Helper - USDA Hardiness Zone Map for Maryland

We're very lucky in Maryland. Our growing season is fairly long - April through early November and boy, do we ever luck out at the abundant and varied flora available to grow in our landscapes.  Beautiful trees, shrubs, perennials and annuals thrive in our climate. A definite joy for any gardener or nature lover.

Rhododendron

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?