Planting a fall garden in Washington, D.C. can be a rewarding experience, as the cooler temperatures are conducive to the growth of a variety of vegetables and flowers. Here are some options for what you can plant in your fall garden:

Leafy Greens:
Fall is an excellent time to grow leafy greens like lettuce, spinach, kale, and Swiss chard. These cool-season crops thrive in the milder temperatures of autumn.
Brassicas:
Vegetables like broccoli, cauliflower, and Brussels sprouts can be planted in late summer for a fall harvest. They can tolerate cooler temperatures and may even taste sweeter after a light frost.
Root Vegetables:
Carrots, beets, turnips, and radishes are great choices for fall planting. These vegetables develop well in the cooler weather and can be harvested before the ground freezes.
Onions and Garlic:
You can plant onion sets and garlic cloves in the fall for a spring harvest. They establish strong root systems over the winter.
Herbs:
Herbs like parsley, cilantro, and chives can thrive in the fall garden. They can be used fresh or dried for future use.
Perennial Flowers:
Fall is also an excellent time to plant perennial flowers, such as asters, mums, and goldenrod, as the cooler temperatures encourage root development before winter.
Bulbs:
While not for immediate fall enjoyment, planting spring-flowering bulbs like tulips, daffodils, and crocuses in the fall will reward you with beautiful blooms in the following spring.
Cover Crops:
Consider planting cover crops like winter rye or crimson clover to improve soil health and prevent erosion over the winter months.

Make sure to prepare your garden bed by adding compost and mulch to enrich the soil and retain moisture. Also, keep an eye on the weather, as the first frost date in Washington, D.C. is typically in late October to early November. You may want to use row covers or other protective measures to extend your growing season into late fall or early winter.

Remember to consult local gardening resources or your nearest garden center for more specific information about the best planting times and varieties for your particular area within Washington, D.C.