Technology can only do so much. The best way to find out what's growing seasonally is not to browse an electronic map. The best way is to hike over to one of the District's numerous farmers' markets and browse the new and good in person.
If, however, you're planning ahead -- or, say, stuck in the office on the prime weekend days for visiting those markets -- then Epicurious has a sweet tool for you. This state-by-state seasonal ingredient map lists what's growing where and provides recipes for every ingredient.
Unfortunately, the ingredient descriptions do not break down to state-specific information. So clicking on "cantaloupe" gives you the generic profile about cantaloupe and not, say, what differentiates Virginia cantaloupe from the rest. The recipe recommendations draw from the large Epicurious library, offering such entries as sweet-and-savory cantaloupe soup.
It may not change lives for foodies well versed in the doctrine of Alice Waters -- but it could be something of a gateway to seasonal eating for new cooks whose first instinct is to turn to Google for recipe ideas.