If you’ve ever wished your garden could produce into the frost, a cold frame might be the ticket. A cold frame is essentially a mini-greenhouse – a low, unheated structure (often built over a raised bed or planter box) with a clear lid that traps solar heat. In Colorado, where winters are cold but sunny, cold frames let hardy greens thrive under glass or plastic. In fact, one Colorado gardening expert calls them “a much less expensive alternative to greenhouses for growing i