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?
This map, put out by the U.S. Department of Agriculture, defines climate zones for growing based on temperature. Each defined area of the interactive map (yes, you can zoom right into your background with the interactive map) indicates the minimum temperatures of a zone. The ability to withstand minimum temperatures defines how a plant can "winter" without dying.
Why does this aid the gardener? First, it's an important tool in plant choice when making any addition to the landscape. For example, an ornamental grass in Florida acts like a perennial with year round growth. Put the same plant in the ground in Maryland and the plant dies off with the first hard frost in fall. Second, each plant at the garden center should have an information tag attached to the pot. One would assume that vendors in Maryland would only sell plants that would grow well in Maryland. Third, try not to purchase plants outside of your hardiness zone. A plant grown in Savannah, Georgia, won't be too happy in a cooler zone.
Interactive USDA Hardiness Map - Maryland
Another aid for the gardener is the Heat Zone Map.
The American Horticultural Society's explanation is so much better than anything I could come up with...
The 12 zones of the map indicate the average number of days each year that a given region experiences "heat days"-temperatures over 86 degrees (30 degrees Celsius). That is the point at which plants begin suffering physiological damage from heat. The zones range from Zone 1 (less than one heat day) to Zone 12 (more than 210 heat days).Use these guides as part of the planning and learning process as it warms up outside (SLOWLY). By the way, Capital Weather Gang declared winter is over! The proof is here :)