A simple cold frame can extend your growing season several weeks by protecting your plants from frost, snow and cold temperatures. You don't need an expensive greenhouse; just a little elbow grease, plastic sheeting, PVC pipe and inexpensive lumber. This cold frame is two feet wide and eight feet long, but you can easily scale it to fit your needs.
Assemble the base frame by joining the short 2 x 4's to the ends of long 2 x 4's. Use two screws at each joint, drilling pilot holes to avoid splitting the wood.
Place the frame over the bed to be protected. Measure and mark increments along the long 2 x 4's on both sides at two-foot intervals; at the end, at two feet, four feet, six feet and the opposite end. Make the marks along the centre-line of the 2 x 4 edges.
Drill through the frame from the top and out the bottom at both ends using the marks as a guide. At the six centre marks, drill holes three inches deep at each mark.
Place the one-foot-long PVC pieces in the four end holes. Use the hammer to pound them into the soil. When they are flush with the top of the frame, use the spare short piece to continue until the stakes are seated about two inches down.
Create the hoops by inserting the ends of the five-foot-long PVC pipe pieces into the holes, going across the frame. Get each piece to the bottom of each hole.
Lay the plastic sheeting over the hoops. For a temporary covering to last a few weeks, use bricks or scrap lumber to weight the sheeting down. For a more permanent structure, staple the sheeting to additional 2 x 4 lumber. Trim away excess with a utility knife, but leave enough to weight the plastic down.
Don't let the simplicity of this cold frame fool you. On a sunny day, temperatures inside will rise quickly. Be sure to open the frame ends on warm days and close it late in the afternoon to conserve heat. If frost is expected, place two plastic jugs of hot water in the frame to keep plants safe overnight. Don't let plants touch the plastic if frost is expected. It will be easier to drive the foot-long PVC into the ground if you cut one end at a sharp angle. A scrap piece of 2 x 4 held over the stakes will prevent them from chipping or breaking as you pound them in. If you wish, substitute 3/4 inch polythene pipe for the PVC to construct the hoops.
Wear safety glasses while pounding the stakes in.