Cold frames extend the gardening year. In many places, they make it possible to grow flowers or vegetables year-round. Whether planting flowers early in spring, growing vegetables throughout the winter or just growing greens for a few more weeks in fall, use the plants that work in your cold frame to ensure a bountiful harvest.
Cold frames provide an early start to the growing season, making possible multiple plantings of short season crops such as lettuce and spinach. Organic Gardening lists the following "sure-bet" vegetables for home gardeners to start from seed: basil, broccoli, Brussels sprouts, cabbage, cauliflower, chives, leeks, lettuce, onions and tomatoes. Many vegetables, like other seeds, need a soil temperature of 60 degrees Fahrenheit (15.5 Celsius) or better; these vegetables might as well be sown directly, provided they are not long-season plants. The Alabama Cooperative Extension System (ACES) lists the following seeds as able to germinate in 50 degree (10 Celsius) soil--asparagus, corn and tomato. Pumpkins, parsnips, melons, tomatoes and other crops that need 100 or more days to mature are more likely to produce well in areas with shorter growing seasons if started early in a cold frame. Peppers also work well with cold frame starts.
Hardy perennials really don't work in a cold frame. It will throw their internal clocks off for their first year of life. Annuals, which live only one year, however, can get a fast start from seed or seedling in a cold frame. Organic Gardening lists alyssum, cosmos, marigolds and zinnias as reliable choice seeds. Easy biennials--flowers such as Shasta daisies, columbine and hollyhocks that grow only foliage during the first year--can live in a cold frame for their growing season until they are moved to their final place in the garden in the fall. Colorado State University Extension recommends African daisy, amaranthus, bachelor's button, calendula, California and Shirley poppy, calliopsis, china and native asters, dahlia, gaillardia, larkspur, lobelia, morning glory, nasturtium, petunia, annual phlox, salvia, snapdragon, statice, straw flower, verbena and vinca for cold frame starts.
Early, Late and Over Winter
The air and soil temperature in a cold frame depends on climate, days of sunlight and insulation. Mother Earth News suggests that gardeners north of USDA Plant Hardiness Zone 6 might want to supplement solar heating by adding black containers of water, making berms or stacking hay along cold frame sides and covering the top of the frame with blankets or straw at night to extend the growing season. Mother Earth News also recommends 12 winter crops for cold frames: arugula, broccoli, beets, cabbage, chard, bok choy, green onion, kale, lettuce, mustard, radish and spinach. ACES notes that parsnip, lettuce and spinach all germinate beginning at 35 degrees Fahrenheit (2 degrees Celsius) and beets, cabbage, carrots, cauliflowers, lettuces, celery, chards, parsley, radishes and turnips germinate beginning at 40 degrees (4.5 degrees Celsius), all within the range of many winter cold frame soil temperatures. You'll have to buy the tomatoes for the salad, though; they don't like chilly conditions.