Virginia's "humid sub-tropical" climate creates growing seasons from 180 to 200 days in length in most parts of the state. The temperature falls below freezing during the winter, but a sturdy cold frame extends the growing season for vegetables through most of those months too. Those extra weeks provide time for additional crops of cauliflower or bok choy as winter settles in---or an early start on lettuce and spinach in spring.
Cold frames capture solar energy. One side stands higher than the other so the transparent top tilts toward the sun; the shorter side of the box on the south side tilts the box toward the winter sun. On warm sunny days, the top opens to let excess heat escape. This avoids overheating and scorching plants. Box sides may be lumber, polystyrene foam or insulated vinyl or aluminum. Glass, acrylic or plastic sheets make lids transparent.
Extend the Season
Cool-season crops like cabbage, cauliflower, broccoli and spinach can survive light frosts. Start lettuce, parsnips, onions, carrots and salsify in the cold frame to grow throughout the winter. Biennials like bok choy and parsley started in the fall and grown in the cold frame through the winter are ready for harvest the following summer. Garlic, onion, turnips and additional underground crops also make good cold-frame crops. Asparagus and rhubarb are perennials in Virginia; try planting them in the cold frame in the fall for winter harvest. Potatoes, a Virginia garden favorite, are another cold season crop that can start late for winter harvest; however, they require more room than may be available in a single cold frame.
Get a Jump on Spring
Pea, radish, turnip, celery, cauliflower, carrot, cabbage and beet seeds all germinate in soil where the temperature is below 45 degrees F, making them good candidates for winter cold frames. Spinach, lettuce, parsnips and onions germinate in soil a few degrees above freezing. Be sure to mulch around the base of the cold frame or mound with garden soil to keep cold winds from nipping seedlings. Plant evergreens or shrubs along the north side of the cold frame to shelter it from cold north winds.