Zone 6b - Mid-Atlantic Gardening

The USDA divided the country into Plant Hardiness Zones based on average annual minimum temperatures, divided into 5°F (2.8°C) differences.

In the Washington Metro area, the USDA assigned the parts of Montgomery County with lower elevations, Prince Georges County, and most of Maryland north of there as part of its Zone 6b. In Virginia, Arlington County, the northern parts of Fairfax County, and the parts of Loudoun County with lower elevations as in Zone 6b.

More southern parts of both Virginia and Maryland are in Zone 7a, which runs slightly colder. Although your local conditions might vary (hilltop versus valley, for example), I think that recent winters suggest that we should err on the side of Zone 6 plants in the entire area.