Washington DC falls in the USDA Zone Chart of 7b. Use the chart below to help guide you when planning and planting your garden. Some crops can be planted in both the Spring and the Fall for an extended harvest.
This chart is provided by:
http://www.veggieharvest.com/Vegetable-Planting-Calendar/zone-7-vegetable-planting-calendar/
Vegetable Planting Guide from UMD