Though there are other types of coverings that are simpler and more easily moved from one part of a garden to another, sturdy cold frames maintain higher temperatures as a rule and therefore can be essential for protecting plants from severe winter cold. They may be less portable, but some designs are easy to build, work in, and ventilate.
A four-by-four cold frame design can be the simplest to construct. It requires the following tools: a hammer, handsaw (or portable power saw), screwdriver, measuring tape, and hand push drill or power drill. The following directions will guide you step-by-step in constructing a frame that will cover a 4’x4′ space.
Plywood will comprise the panels for the back, front, and both sides. The back panel will be taller: cut a 4-foot length to a 15-inch height. The front panel, of equal length, should be 9 inches high. The side panels will join these two; therefore, they should be cut at an angle from 15 inches at the high end down to 9 inches at the lower. Use 2x2s to center the side panels at the top: nail these in place, leaving a 2-inch space at each end. Nail two 15” 2x2s to the ends of the back panel (running vertically) and a 45” 2×2 along its top edge. Repeat this for the front panel, but use 9” 2x2s for its ends.
2x2s are also used to construct the lid frame. This requires cutting lap joints. Mark a line 1½ inches in from each end and then, from this line, mark down ¾” and draw a line out to the end. Make a crosscut with the saw to the ¾” depth mark, saw across to this cut from the outer edge, and then remove the wedge. When you’ve done this to both ends of 4 4-foot sections, they can be joined together and nailed to form a square. Apply carpenter’s glue along the joints before you nail the pieces. Insulate this lid with a 4′ length of fiberglass topped with crown molding; nail them in with 4′ lengths of wigglemold lain both beneath and on top.
The side panels can then be nailed to the front and back panel. Use a drill to make pilot holes for the hinges of the lid, and then secure the hinges with wood screws. All the wood that will be within 2-3 inches of the ground should be painted with asphalt emulsion to prevent rot. This is non-toxic, and will work as well as other wood preservatives that are more expensive. The lid frame and cold frame panels should also be painted, preferably white.
In order to keep the lid propped open while you’re working, you’ll want to construct a hook latch. Use a 1”x1” pole about 6 feet high and fit it with 4 or 5 screw hooks spaced 2-3 inches apart. This will allow you to adjust the height that you keep the lid propped.