A cold frame protects plants from cold weather, extending the planting season for gardeners. A simple cold frame can be made from almost any material you have on hand as long as it can hold in heat and hold up a piece of framed glass or clear plastic. Building a cold frame from recycled materials like cinder blocks and old windows keeps waste out of the landfill and saves you money.
Measure the piece of glass, Plexiglas or an old window to use for the top of the cold frame. Subtract 2 inches from both numbers from both the length and width to determine the rough size of your cold frame's interior space. This may not be the finished size, but it is the maximum size your cold frame can be.
Select an area of your yard or garden which is in full sun for your cold frame, and place stakes at each of the corners based on your measurement. Remove the sod and dig the soil in this area up to 6 inches deep. Remove the sod around your cold frame dimensions 6 to 8 inches wide around the dug area.
Align cinder blocks along the perimeter of the dug area, and adjust them as needed until the inside measurement is the size you determined from Step 1, or smaller. Nestle the blocks into the soil. Pack soil in toward the bottom edge of the cinder blocks all the way around on the outside and inside of the frame.
Stuff straw, torn pieces of egg carton packaging, and/or balled up newspaper into the holes of the cinder blocks until each hole is full. Layer a second set of cinder blocks over the first row, staggering the blocks if possible. Stuff the holes of the second layer just as you did with the first.
Measure the width of the cold frame from one side to the other as well as the width of the cinder blocks you used. Cut two boards from plywood the width of the frame by the width of the blocks. Set these boards on top of the cold frame at the top and bottom to cover the holes of the cinder blocks.
Measure the length between the top and bottom boards and cut two boards at this width by the width of the blocks. Place these two boards to rest on top of the frame on each side between the top and bottom boards.
Place your piece of glass, Plexiglas or old window on top of the boards to close the frame. Leave the frame on during cold days, move it over an inch or two on mild days for aeration, and remove the glass entirely on hot days.