If the cold temperatures a few weeks ago damaged your garden plants, perhaps it’s time for you to build a cold frame. Because the runners of a nearby blackberry bush scurried under the base of the cold frame and sprouted a new plant inside the open box, and I procrastinated about removing it, I managed to plant my cold frame just before the recent cold snap. However, the protective environment of the cold frame will cause the seeds to germinate, and I’ll soon have happy lettuces and spinach for the rest of the winter.
Unless snow falls or the temperature remains below freezing all day, which rarely happens here in SC, I open the cover of the cold frame every morning. Winter vegetables do not enjoy temperatures much above 70°F; think about how easily your car heats to that temperature and beyond on an otherwise chilly day if it’s parked in the sun.
My husband and I made my cold frame, which is a box covered with glass, with a discarded shower door. Any glass or Plexiglas door or window would work; the glass allows sunshine and heat to reach the plants inside the cold frame. If you were going to open the lid daily to allow sunlight to reach the plants, even an opaque lid would work. The plants will be fine in the shade for a day or two if very cold weather threatens. After you raise the lid, make sure to attach it to the ground in some way so that strong gusts of wind do not suddenly close it and shatter the glass.
We made the sides of my cold frame out of treated lumber. We caulked the joints and put some weather-stripping along the top of the frame to prevent drafts. The back of the cold frame is about 18 inches high, and it slopes down to the front at about a 40-degree angle toward the southern sky; the front is about 8 inches high. This slope is supposed to maximize the amount of captured sunlight.
If carpentry is not your forte, use stacked hay bales, concrete blocks, or landscape timbers. My mother surrounded some of her vulnerable plants with black plastic bags full of leaves and found they provided sufficient insulation to protect them from much damage, especially if she draped a sheet of plastic over the top of the circle of bags. Plug as many cracks as you can.
Place your cold frame directly on the ground, fill your cold frame with compost-enriched soil, and plant the seeds or transplants. Because of the greenhouse-like moist environment, seeds sprout quickly and are the most economical choice. Water the soil when it begins to dry out and fertilize the plants as you would in your garden.
The best plants for a cold frame are lettuces, spinach, collards, and other cool-season greens. Carrots, beets, and parsnips also like the protected environment. Depending on your cold frame’s interior height, you might also be able to grow broccoli, cauliflower, and cabbage. This winter, I am growing broccoli inside a cold frame without a lid, because it shattered in a wind gust, and I am protecting them with a sheet of plastic.
Gardening supply companies carry prefabricated cold frames; harvesting your own salad greens instead of buying them will offset the purchase price quickly.