Start at the Top
The only essential for the frame’s cover is that light gets through. Almost any transparent material will work: glass, fiberglass, polyethylene, or flexible greenhouse coverings—the differences between them are insignificant.
Many people use old window sashes. Be aware, however, that some old frames may be covered with lead-based paint. If in doubt, pass on them, and keep looking until you find windows that the owner can assure you are lead-free. Also, make sure the wood isn’t rotting and the glass is secured firmly inside the frame. Be careful when handling the glass, and keep small children away from it.
In extreme northern areas, glass isn’t always the best option. “Here in Alaska, glass coldframes break under the weight of our heavy winter snow,” explains Jeff Lowenfels, a gardening columnist, “so we tend to rely on thick sheets of Lucite or other window-strength plastic instead.”
If you’re buying material to cover your coldframe, consider Lexan, an improvement over Lucite. Lexan stands up well to the elements (like rain, sleet, ice, or snow). And it insulates especially well.
Other gardeners prefer the corrugated fiberglass (4-by-8-foot panels) sold for greenhouse walls. Although it costs a little more than other plastics, it lets in a lot of light and doesn’t turn yellow with prolonged exposure to sunlight.
The simplest frame uses hay bales: Just arrange four bales of hay or straw into a square shape to make the sides of your coldframe. Put your glass or plastic cover on top of the bales. (Use the straw for mulch next spring, after you disassemble the frame.)
If you can’t get bales of hay or straw where you live, you can use other materials for your side walls. Cinder blocks are a good alternative; just be sure to turn them on their sides so the holes point up and down. Otherwise, air will pass through. Cover the top holes to keep the structure warmer.
To make a more permanent and easily vented structure, build the sides from wood and attach your top to it with hinges. Cedar, cypress, and redwood are naturally rot-resistant, but you can use almost any kind of wood—as long as it isn’t pressure treated (CCA, Wolmanized, and so on). Pressure-treated wood contains highly toxic substances, including arsenic. Secure the pieces of wood with elbow braces at each corner, glued and then screwed in with two 1- or 1 1⁄2-inch galvanized screws.
If you garden in an extremely cold area, you’ll need a more permanent and better insulated coldframe. Jan Scheefer, a high-altitude gardener in Gunnison, Colorado, made her coldframe walls out of 6-inch-thick poured concrete, which she painted black to absorb solar heat. She capped the frame with corrugated fiberglass framed with pine 2-by-4s.
Stone and mortar walls are another option. Building stone walls requires more labor and know-how than pouring concrete, but they can be much less expensive if you happen to have stone on your property.
By digging a pit beneath your coldframe, you can plant 6 to 8 inches below the surrounding soil level—so the soil will insulate your plants. But digging a pit requires moving a lot of soil, and it makes your coldframe more permanent than you may want.
If you do dig, be aware that rain can run off the frozen ground and into the unfrozen frame bed, causing flooding problems. To prevent this from happening, put a layer of gravel at the bottom of the pit, beneath the layer of soil.
There are other ways to insulate your plants that don’t require a pit. You can pile soil, leaves, or wood chips around the outside of an aboveground frame to hold heat. Or consider adding heat more directly. “Many Alaskans warm their coldframes by putting fresh manure or jugs of water inside,” says Lowenfels. If you go the manure route (creating a hotbed), don’t plant directly in the manure—your plants will burn. Instead, cover the manure with 6 to 8 inches of soil before planting.
If possible, orient your coldframe to the south, with the top angled about 25 to 30 percent from front to back. If that isn’t possible, at least make sure your coldframe is in a sunny spot. And angle the top enough for rain to run off.
If you garden in an area with severe northern exposure (such as in Alaska), you’ll need to angle your coldframe a bit more steeply because of the sharp angle of the sun during spring and fall at those longitudes.
Don’t Forget to Vent
Proper ventilation is probably the most important part of growing inside a coldframe! On warm or cool sunny days, heat can build up inside the sealed frame, so you’ll need to open the lid. Leave it closed and you risk cooking your crops before you harvest them.
The most basic venting tool is a sturdy stick or dowel that you use to prop open the top, late in the morning of any sunny day when outside temperature is expected to rise above 40ºF. (On a sunny 50ºF day, the temperature inside your coldframe can quickly soar to 80ºF.) Make notches in the stick so you can prop open the top at different heights, depending on the outside temperature. And always close the lid or vent by late afternoon so some of the insulating heat of the day is trapped inside to help protect against the night’s chill.
The most reliable solution, though, is to include an automatic vent in your frame design (unless you live in a very snowy region; the vents usually aren’t strong enough to lift the load). Such vents automatically open and shut your coldframe when specific temperatures are reached.
So what’s the real secret to successful coldframe use? Paying attention to the conditions and your plants—just like you do with everything else in gardening.
What’s in for Coldframes?
What can you grow in your coldframe? Anything you grow in your garden: In many areas, you can sow seeds of spinach, lettuce, kale, choys, and other salad greens in fall to enjoy in winter. Or, transplant heads of lettuce, cabbage, and cauliflower inside the frame. Consider transplanting a short pepper plant or two for extended production through another month or two.
In areas with an extra-short growing season, a coldframe may be the only way to grow warm-weather crops. High-altitude gardeners and Alaskans use coldframes through summer to raise tomatoes, green beans, and cucumbers.