Flagstaff, AZ – Earth Notes: Cold Frames
The short frost-free growing season in the Southwest can make growing crops through to harvest a tricky business. Savvy gardeners muster all the help they can and many use cold frames, which can extend the growing season by months.
Cold frames use free energy, relying on the sun's warmth shining through the clear cover. Be sure that your cover is made of PVC, plastic or polycarbonate, so that it's light enough to lift easily.
Here's why. An insulated cold frame keeps interior temperatures 10 degrees or more higher than the outside air on cold nights. But on a sunny day, even in winter, temperatures inside an airtight cold frame can quickly climb above 100 degrees Fahrenheit.
For that reason, cold frames need to be vented so that plants don't bake. It's easiest to do this automatically with thermostatically controlled lid openers. They contain mineral oil that opens the lid once the inside temperate reaches a preset limit, and closes it again when things cool off.
Insulation is important, too it helps retain heat through chilly nights. A thick layer of polystyrene works well, or heavy masonry blocks, but you can also add some partly filled plastic water bottles. They'll absorb heat during the day, then re-radiate it at night. They'll work even better if you paint them black.
Crops such as kale, beets, swiss chard and perennial herbs do well in cold frames through most of the year, providing fresh and healthful produce in a challenging landscape.