Last fall, my husband and I replaced the old windows in our house with double-paned glass, so we had several beautiful old wooden windows lying around. They happen to make perfect covers for a cold frame. A project was born.
The other essential element of a cold frame is the actual frame: the sides that support the cover. Since the sides come into contact with the ground, it’s preferable to use wood that’s rot- and termite-resistant (or you’ll be rebuilding your frame every few years). You don’t want to use pressure-treated lumber for this, because it’s actually treated with much more than pressure. It contains toxic chemicals that you don’t want to be leaching into the ground where you’re growing your food. What you’re looking for is a naturally rot- and termite-resistant wood, like cedar or locust.
It’s not that easy to come by cedar or locust lumber in hardware stores, except for finishing boards that are quite pricey and that only come as 1″ rather than the 2″ boards that would be better for a cold frame project. Happily, however, my husband just finished building some new cedar shutters for our house and had quite a few scraps left over. These were the thin, 1″ boards, but he had enough scraps that I was able to simply screw them together to make the equivalent of 2″ boards.
You’ll see in the picture the results. The outside dimensions are 55″ by 40″. I added the center support for extra strength when moving the frame and because it supports the edges of the two windows where they come together in the middle and keeps wind from getting through any gap.
You’ll also see that I don’t do anything fancy in constructing this cold frame. Some people like to attach their windows with hinges so that they can prop them open on warmer days for ventilation. I find it’s more convenient just to set the windows on top of the frame and slide them over a bit when ventilation is required. Then, when the weather is too warm to need the cover even at night, I can remove the windows entirely and just place some lattice over the frame to keep the deer and rabbits out. Not attaching the windows to the frame also makes it easier to move, since it doesn’t have to be moved as one heavy, unwieldy object.
Some people also like to make the wall of their cold frame that’s farthest from the sun taller, so that the windows sit at an angle. I think that strategy would be more advantageous if the back wall of my cold frame were brick or stone, so that having more wall area enclosed in the frame would retain more heat overnight. With this style wooden frame, however, I don’t think the heat gain is worth the extra trouble to construct a sloped top. But perhaps some of you have experience that will prove me wrong.
In any case, I’ve just sown lettuce seeds in my new frame. We have about 5 weeks until our first frost here in Zone 7, so that will give it plenty of time to get established before the cooler weather hits. I won’t put the windows over the frame until around the time of the first frost, though. With lettuce, it’s better to err on the side of keeping the plants too cool rather than too hot!
Protected in this frame, lettuce will grow right through the winter in this area. And who doesn’t love a fresh green salad in January?