A cold frame is a mini-greenhouse that helps you extend the growing season of your garden. In the fall, a cold frame protects vegetables from frost, while in the spring the cold frame helps seedlings get a head start. With a cold frame you can enjoy fresh lettuce in December and tomatoes in June. You can purchase ready-made glass and plastic cold frames, but you can also put one together from scavenged objects.
Choose a sunny, sheltered location for your cold frame. A south-facing location up against the side of a house or garage is a good choice. The mass of the building will help block wind and retain extra heat.
Stack the hay bales or concrete blocks into a square or rectangle the same dimensions as the salvaged window. The hay bale or block wall should be about 2 feet high, or the height of one hay bale. Alternately, you can build a frame out of wood. The back wall of the frame may be made higher than the front, so the window is slanted, but this isn't absolutely necessary.
Place pots of plants inside this rectangle. In spring or fall, you can sew seeds such as lettuce or spinach directly into dirt in pots in the cold frame, or you can transfer seedlings such a broccoli, cabbage and carrots into your cold frame from the garden. Alternately, you can dig up the soil in the ground within the cold frame and plant in this.
Set the window on top of the cold frame. The sun shining through the window glass will heat up the interior of the frame and keep the plants warm enough to grow. Check the cold frame on sunny days. If it seems too warm, prop up one side of the window with a brick or two, but remember to remove the bricks before nightfall so your plants won't freeze.