Set Your Site
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.