By March, Indiana gardeners are anxious to get their hands into the soil, which often remains too cold and too wet for digging. That is, unless you’re one of the lucky gardeners who has a cold frame.
Essentially miniature greenhouses, cold frames take advantage of passive heat trapped from the sun. Average nighttime temperatures in a cold frame are 7 to 10 degrees warmer than outside; on a cloudy day, the temps can be 10 to 15 degrees higher than the surrounding area. On sunny days, the heat can soar; open the frame to vent it.
Creating a Cold Frame
The simplest cold frame is a square box without a bottom that sits directly on the soil. The top is a sheet of glass or plexiglass called the light; recycled windows or storm doors are ideal.
In the Northern Hemisphere, you want your cold frame oriented east to west to catch light from the sun’s southern orientation. To trap even more sun, build the frame so that it’s slightly higher at the back than the front. A slope of 20 degrees is ideal.
On sunny days, prop the light up a bit with a notched stick or a bit of brick to allow the heat to escape.
Using the Cold Frame In Spring
Start Cool-Weather Veggies: Direct sow your salad greens and cole crops in the cold frame, and grow them until they’re big enough to transplant later in the season. As a bonus, you free up space inside the house to start the warm-weather crops that can’t handle spring temperatures.
Harden Off Tender Plants: Once you’ve moved those cool-weather crops out of the cold frame, use it for hardening off tender vegetables and annuals. Just tuck the flats and pots inside, and be sure to open the light during the day to let some heat escape. Leave the frame open for longer periods each day until your plants are ready to be planted in the garden.
Extending Your Harvest in Fall
Grow Hardy Vegetables into Winter: Direct sow your fall crops in the cold frame; good choices are mache, spinach, sorrel, and parsley. Leave the frame open while they grow through the fall. When frost threatens, close the light, but leave it propped during sunny days. You can harvest right past Christmas!
Store Root Crops: Sow fall carrots, and when winter arrives, place a cold frame on top of them. Add 1 inch of compost, then fill the space to the light with straw. You can pull incredibly sweet carrots all winter.
For more on cold frames, check out this article from Fine Gardening. You can get complete instructions on building and using a coldframe, as well as other season-extending tricks, in Eliot Coleman’s excellent book Four-Season Harvest.