Great for starting seeds in advance of spring, or for hardening off plants for transition to a colder climate, a cold frame works by letting you fine-tune the amount of shelter and warmth plants receive in the harsher winter months. By staying low to the ground and out of the way of punishing winds, the frame can help protect plants from extreme cold. By propping the lid ajar during the peak heat of day, you can keep the the temperature from rising so high that the plants outpace the season. A blanket thrown over the lid will help keep warm air from escaping during the dead of night.
If you’re planning to build a cold frame, you’ll want to begin at ground level. Start by thinking about where you’ll put the frame, and how you’ll prepare the ground for the micro-climate it will provide. Below are some tips to get you started.
To give the plants in your cold frame the most opportunities for soaking up warmth and sunlight, it’s usually best to favor an unshaded location with a clear southern exposure. The key is plenty of sunlight, so be sure that there are no obstructions to cast shadows across your frame, particularly in the hours surrounding midday. Be mindful of the bed’s proximity to a water supply like a faucet or garden hose so that watering doesn’t become a hassle. Finally, choose a location with adequate drainage to ensure that your plants aren’t sitting in freezing, stagnant water all winter.
Sometimes, even the best available location provides less than ideal drainage. Placing gravel at the lowest layer of the bed will help in areas with poor natural drainage. As a last resort, you can build you cold frame on an elevated mound, but be forewarned that the increased exposure to wind may also decrease the amount of warmth the frame is capable of retaining.
Once you’ve decided on a location, it’s time to prepare the bed. Start by clearing away any grass or plants already there. Tilling the bed will make it more amenable to new plants. Be sure to level the planting surface. And if you’re building your cold frame as a permanent structure, you’ll want to dig beneath the level of the surrounding earth in order to settle the frame more snugly into place. Whatever you place beneath the frame, be sure to provide a top layer of at least six inches of soil for your plants.
The purpose of installing a cold frame is to protect plants from the ravages of winter weather, in part by providing a micro-climate several degrees warmer than the surrounding air. It’s possible to elevate that temperature several more degrees by digging 18 inches or more below the level of the frame and adding a layer of livestock manure underneath the soil layer. As the manure undergoes the process of fermentation, it releases heat which is caught by the frame.
Providing a windbreak to the north will also help keep the temperature from falling during high winds. Bales of hay make handy windbreaks, as they can be easily moved to adjust for changes in wind direction. Just make sure the windbreak doesn’t cast a shadow on the frame.