If your windowsill farming is crowding you out or your plants are beginning to get leggy from warm temperatures and lack of light, a cold frame is the answer. Even if you do not have one, you can improvise. Four 12-inch boards connected at the corners will do. Even old bricks or cement blocks will do for the sides. An old window is fine for the top. If it doesn't have glass any more, fasten on a piece of plastic.
Last year I made a very satisfactory cold frame out of four bales of old hay and a window placed on top. It provided about 16 square feet of sunny, wind-protected, extra-early garden area. When the cold frame had served its purpose, the hay was used for mulch.
The ideal cold frame should slant to the south to catch the sun. Twelve to 14 inches at the back and 6 to 9 inches at the front are the usual measurements. If it is to be a permanent structure, the cold frame should be placed on well-drained ground.
The frame should be built out of long-lasting wood or concrete with a foundation several inches below ground level to keep out grubs. And it should be filled with a good all-purpose soil mixture, such as one-third garden loam, one-third compost, and one-third sand.
A portable cold frame can be made to rest on top of the soil. Unless the top is connected by hinges, there should be space behind it for easy removal and storage of the glass. There will be days when you won't want the glass on at all. A few pieces of wood should be kept on hand for raising the sash to various heights for ventilation, depending on the sun and wind.
Never leave the frame closed tightly on a hot day because it will become a steam cooker.
Depending on the intended use, you may also want wood or mat coverings for extra warmth on bitter cold nights or lath covering for summer shade.
Except in midwinter, a cold frame offers many of the advantages of a greenhouse. With the addition of an underlayer of newly rotting manure or a heating tape, it becomes a hotbed.
You will find your cold frame invaluable for hardening off plants in the spring, for starting seeds and rooting cuttings throughout the entire growing season, and for protection of less hardy plants and storage of bulbs and plants for forcing during the winter.
You also can use it in the summer as an intensive-care unit for pots or plants that need a little pampering.
The nearness of a deciduous tree will offer needed shade during the heat of summer while letting in the light when it is needed in early spring. The reach of a hose can make a big difference between this and the general garden. Just the defined extent of the sides keeps you from letting your hopes run away with your time and energy supply.
So buy, build, or improvise some kind of cold frame.