The Arlington area is classified as Zone 7a. This zone extends down to the southern side of Richmond where it turns into Zone 7b. Here is a short link to the Virginia hardiness zone map, and for more USDA maps on hardiness zones (sometimes called planting zone or hardy zone), check out: http://planthardiness.ars.usda.gov/PHZMWeb/.
The hardiness zone is an indication, generally, of how cold it may get during the winter in an area. The actual low temperature, and its effect on your plants, is dependent upon the micro-climate around the plants.
For more on gardening in the mid-Atlantic, visit the Master Gardeners of Northern Virginia’s quick guide for gardeners.
Tried and True Plant Fact Sheets: Our Master Gardeners have put their heads together to identify and describe a broad variety of native plants that thrive in our local conditions and benefit our local wildlife. We’ve created more than 100 fact sheets to highlight the beauty, ease of care, size, color, and seasonal interest of these wonderful plant varieties, grouped in seven broad categories, like ferns, grasses, perennials, and more.