For those of you who do not have a greenhouse, cold frames are a simple way to grow veggies during the shoulder season. Early in the season, cold frames allow us bring hardy greens to the table in abundance. In Europe, farmers have been building structures to extend the growing season since the 17th century, but their efforts picked up in the 19th century. They experimented with cloches (bell-shaped, glass row-covers), hot beds, and glass frames to grow local vegetables during the early spring, late fall, and into the winter. As the industrial revolution made glass affordable, farmers developed greenhouses and the cold frame.
Cold frames are used all over the world to extend the growing season, and for good reason. They are simple to build, require no additional heat, and are easily managed. From a plant’s perspective, cold frames provide a protected microclimate that allows them to endure chilly temperatures and snowfall. As days get shorter in the fall, plants enter a sort of torpor and slow their growth. This means that they should be sown early enough in the fall to allow for harvesting into mid-December in our area. In the spring, plants get a head start sensing temperatures warm enough for germination and grow more and more quickly as days get longer.
We are still experimenting with hardy crops that can sustain cool temperatures. Nevertheless, we know that kale, spinach, Asian greens, arugula, beet greens, carrots, and lettuce all tolerate these temperatures just fine. For a more complete guide to planting dates for fall and winter harvest, consult Eliot Coleman’s The Winter Harvest Handbook.
About a week ago, my father and I built two 6’ x 4’ cold frames from recycled materials (paying only a couple bucks for fasteners). So many people are recycling windows that if you keep your ear to the ground, you may be able to find some really cheap (or even free) options. Scrap lumber is in almost every construction dumpster you can think of if you are adventurous enough to search through the heap. (Just be sure to ask permission before you dive in.) Hinges are worth buying if you can’t find something heavy enough, but even these are floating around in the world of excess building materials. Our design was simple and easy enough for the two of us to put it together in two days.
To provide enough insulation for our climate, we used 2”X lumber for the framing. If you have extra insulation available, feel free to experiment with lining the frame, but this step is not essential. The glass should provide enough heat gain to keep the soil above freezing. You can also add large stones to the corners of the cold frame to act as a heat sink, but this limits growing space. The most important design feature, more so than insulation or a heat sink, is the angle of the glass. With too steep of an angle, growing space is lost. Too low and solar gain is diminished. Eliot Coleman recommends a 4” rise over 4’ for this altitude (a 7.5 degree angle). In other words, if your windows are 4’ long (from front to back), you will want the back wall to be 4” taller than the front wall.
Once you have constructed your cold frame, place it in a strategic location. To achieve maximum solar gain, it is important to have the frame angled south (approximate is fine but due south is best.) In addition to light, it is essential to place the frame on good soil or soil you can amend with compost. Because your frame will be made of untreated wood, it should be placed on top of the soil to avoid rotting. Also, place the frame somewhere you walk by every day. On the way to the driveway or out the front door would be ideal. It is surprisingly difficult to go outside and check on your plants in mid-March or early-December if they are not near the house or a place you go regularly.
The benefits of this structure are only truly appreciated when you harvest your first greens in March and your last crops into December. Winter is long enough in Teton Valley to go without fresh greens, and those from your own garden are unbeatable. With a little bit of handiwork and some old scrap materials, a cold frame goes a long way toward year-round greens on the table.
For a detailed construction plan, feel free to leave a comment and I’ll get back to you.