Cold frames are shallow structures which are used to shelter plants from the cold. Placed over vulnerable plants and set directly upon the soil, the clear covers allow sunlight in while the walls help to keep warm air in, offsetting the effects of cooler temperatures. One of the main advantages of cold frames is that they allow gardeners to get a jump start on the summer growing season, allowing them to start their plants early. This is particularly advantageous in northern regions where the short summer season can make growing some garden favorites, such as peppers, a real challenge.
Position the cold frame in a sheltered location where the emerging seedlings will receive full sun. Not only do the young plants need the sun to make energy, the radiant heat coming through the transparent cover will heat the air and soil surrounding the tender shoots and roots.
Prepare the soil at the planting site. Dig down approximately 12 to 18 inches and then mix compost, peat or well-rotted manure into the newly dug soil to create a light, well-drained, fertile planting medium.
Select your seeds. Greens such as lettuce, chard, endive and spinach grow well in a cold frame. You may also want to consider growing radishes, baby carrots and green onions, as they thrive in cold frames, though this is primarily due to the compact nature of their growth. A cold frame is also an ideal place to start vegetables such as hot peppers and tomatoes from seed, but the seedlings will have to remain in transferrable peat pots as they will need more room to grow once the weather warms. Containers which can be planted will make the transfer much simpler.
Plant your chosen seeds according to the directions listed on the back of the seed packet. Unless you are planning to transfer your seedlings to another location later in the year, pay close attention to the depth in which you are planting the seeds and the space you leave between them.
Water the plants once a week during the winter. As the weather warms, leave the top open on rainy days to allow the plants unrestricted access to rain water, fresh air and sunshine.
Keep an eye on the temperature. On a sunny day, the temperature beneath the glass can be significantly warmer than the temperature outside the cold frame. In the spring it leave the cover propped open any time the outside temperature rises above 45 degrees Fahrenheit.
Remove the cold frame once all danger of freezing has passed.