   Building and Installing a Cold Frame - February 2, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Get an early start on your spring vegetable or flower garden by building a cold frame. Cold frames allow us to grow crops that may be slightly frost tender by using solar energy to heat the soil during the day. The lid is opened slightly during the day (the degree of opening varies with daytime temperatures) to provide ventilation and closed at night to trap the heat and prevent frost damage. Failing to open the lid can cook the plants inside. Cold frames are also excellent for hardening off seedlings to acclimate them to the outdoors. A simple cold frame can be built from second hand materials available from your local salvage yard. You might even have some parts in your own junk collection. I assume most readers have at least a small junk pile, if not, then start one. You can't go wrong. Materials: a couple of glazed windows, some 1x6's or 1x12's, concrete blocks, ends of 2x4's, and a few hinges. Before building the frame, you must have the windows or at least know their dimensions. Double glazed windows, if you can find them, have the best insulating properties and are ideal. I always see them at my local salvage yard (my favorite place to shop). Start by finding enough wood to build a lopsided wooden box that is taller on the back side than on the front and open on the top and bottom. Plywood scraps, 1 x 6, 1 x 8, or 1 x 12 boards can be screwed or nailed to 2 x 4 uprights in the corners. For example, a cold frame could be 24 inches high on the back wall and 12 inches high on the front wall. The two side walls will be 24 inches tall on the back and 12 inches tall on the front. Now, remember that geometry class you took? Use the Pythagorean Theorem to calculate the dimensions of the sides of the box. Given any right triangle, the sum of the squares of the sides is equal to the square of the hypotenuse. "Yeah, whatever". Before giving up on me, just relax, we can get through this. For example, if the window you found was 36 inches wide by 48 inches long, then the back wall could be 24 x 48 inches and the front wall could be 12 x 48 inches. The 36 inch side of the window will require a diagonal cut. Here comes the geometry: the hypotenuse of the right triangle is 36 inches and the vertical side is 12 inches. Here is how to calculate the mystery length: 36 squared equals 1,296 (square of the hypotenuse). 12 squared equals 144 (square of one side). To get the length of the remaining side, subtract 144 from 1,296 to get 1,152. Take the square root of 1,152 to get roughly 34 inches. If you survived this article so far, the sides of this cold frame are going to be 24 (back) x 34 (bottom) x 12 (front) inches. The sloping edge should measure 36 inches. If you find windows that are some other dimension, then one of three things could happen: (1) you understand the Pythagorean Theorem and can calculate the new dimensions, (2) you do not understand the Pythagorean Theorem but you know someone that does and they can help you, or (3) you could care less how the cold frame looks and start cutting boards until they fit fairly well and seal the gaps with putty, bailing wire, and/or duct tape. In all seriousness, if you truly need help, contact me at the Prescott Cooperative Extension office (520-445-6590) and I will do my best to assist you. To finish the cold frame, screw or nail the box together and attach the hinges to the window frame and the back wall. Bury the cold frame about two inches deep. When installing, orient the frame with the low side to the south to capture the most light (remember that during the winter, the tilt of the earth=s axis causes the sun to be lower in the southern sky). You will also need to have some various lengths of wood: a long one to prop the cold frame open when you are working or harvesting inside and some various shorter pieces to prop the window open during the day to prevent overheating. If you made it this far without giving up, yet have no intention of building a cold frame yourself, then you may want to look through gardening supply catalogs and nurseries. Most of them are expensive (over$100). You can also lay a storm window over four straw bales arranged in a square for a simple, well insulated design. Many people design elaborate cold frames complete with thermal mass and ventilation systems. These models approach a greenhouse in size and scale. The public libraries often have greenhouse books that discuss cold frame design. The Solar Greenhouse Book edited by James C. McCullagh is one such book. The Internet also has various resources. The University of Arizona Cooperative Extension, Yavapai County has a new web site with horticulture information: http://ag.arizona.edu/yavapai/anr/hort/. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |