Yesterday we introduced you to cold-frame gardening—an easy and fun way to extend your growing season through the winter. Today we’ll dig in to building the actual cold-frame box. Tomorrow, we’ll show you how to top it off with the cold-frame light.
The following is an excerpt from Four-Season Harvest: Organic Vegetables From Your Garden All Year Long  by Eliot Coleman . It has been adapted for the Web.
The Cold Frame Box
Any cold frame design that protects plants will serve you well. To be enjoyable to use, however, the design must be simple, attractive, pleasant to work with, and dependable. Having tried them all, we settled on the traditional design. The simplest cold frame is a rectangular wooden box, 8 feet long and 4 feet front to back, with a slight slope to the south. We build them out of 2-inch lumber to make them strong, but 1-inch stock would be adequate. Three 8-foot boards are necessary: two boards 12 inches wide and one board 8 inches wide. One of the 12-inch-wide boards is used for the back wall. The 8-inch-wide board is used for the front wall. The second 12-inch board is cut into two 4-foot pieces, which are each cut diagonally lengthwise so that they are 8 inches wide at one end and 12 inches wide at the other.
It is easiest to put the frame together with the boards sitting on a flat surface and the diagonal cut edge of the side walls facing up. When you do this, you will notice that the bottom edge of the frame is flat, whereas the upper edge has a slight discontinuity where the diagonal cut meets the front and back walls. In order for the lights to sit on the flattest surface, you should turn the frame over before using it. Any discontinuity of the other edge is then hidden by contact with the soil. The frame will slant slightly to the south, allowing more light to enter.
Attach a 4-foot-long 2×2 to what is now the top. This piece extends across the middle of the frame, running front to back. You will want to cut notches in the top of the front and back walls so this cross piece sits flush with the top. (See drawing.) This helps keep the sides spaced and also provides a handle that one person can use to lift the empty frame and carry it to a new location. If you use 1-inch wood, you might want to place more of these stiffeners across the frame.
We use standard pine or spruce for our frames. We purposely do not use treated wood, nor do we treat the frames with a preservative. Even the supposedly safe products should not be used in close proximity to food crops. Wood rots where it is in contact with the earth, however, so we attach a strip of scrap wood about 1 inch thick to the bottom edge of the frame where it touches the soil. In a few years, when this strip begins to rot, we replace it with another. The rest of the untreated wood frame will last for many years.
We also do not paint the frame. Yes, if the interior were white, it might reflect a little more light than the gray weathered wood, but paint is just one more complication. Rather than having to scrape and paint every few years, it’s best to keep things simple.
Tune in tomorrow for the third and final installment of “Build Your Own Cold Frame”: The Cold Frame Light.
Photo courtesy of Homegrowers Exchange.