A cold frame is a gardening structure with a transparent roof that allows gardeners to grow plants and vegetables during the cold winter months. It is usually a wooden box with a glass lid that traps and retains the sun's heat, raising inside temperature up by 20 degrees, thus providing less-hardy plants or those susceptible to frost damage a chance to grow. It works like a mini greenhouse, enabling gardeners a chance to grow plants long before they arrive in the market in spring, or extend their growing season in fall.
Measure and mark the plywood sheet to divide it into four pieces, each 2 feet square. Cut the plywood with a handsaw, and place the pieces aside. Measure and cut an 8-foot-long 2-by-2-inch wooden board into four, 2-foot lengths. Cut two 8-foot-long 2-by-2s in half so you have four 4-foot lengths to use for the top of the cold frame.
Arrange the four pieces of plywood over the ground to form a 4-foot-wide and equally deep box. These four pieces will form the front, sides and back of the cold frame. Hold the front and side piece together, ensuring the joint is flush, and drill wood screws to join them together. Repeat the process for attaching the other side, and then the back.
Place the assembled box over your desired spot, with its front facing southwards for maximum sun exposure. Dig a 4- to 5-inch trench along the front of the box, tapering it at the sides. Place the cold frame at an angle in the ground, with its back higher than its front.
Assemble the top of the cold frame. Place the 4-foot-long 2-by-2s in a rectangle on the ground. Butt the joints at each corner and secure together by drilling wood screws.
Cut both 8-foot lengths of fiberglass molding in half and place the pieces over the top four edges of the cold frame box. Use a power saw to cut the 8-foot-long fiberglass sheet in half.
Hold the fiberglass sheets side by side over the molding and determine where to drill holes to attach them to the frame. Mark the positions for holes on each fiberglass sheet and drill over them carefully. Replace both the pieces over the molding and join it in place by drilling a nail through each hole.