Use Cold Frames to Grow More Food
(Page 2 of 4)
December 2007/January 2008
By Barbara Pleasant, Illustrations by Elayne Sears
After seeds have begun to grow inside the frames, the plants are surprisingly cold tolerant. I have watched lettuce seedlings sail through 10-degree nights when the frame was covered with a thick polyester-filled blanket, and framed-up spinach never sulks no matter how cold it gets. Yet these and other winter-sown vegetables will complain if a frame is allowed to overheat, so it’s crucial that frames be opened to vent out excess heat. When in doubt, it is always better to vent than to risk frying your plants. If you can’t be around to open and close your frame and a warm sunny day is in the forecast, covering the top of a closed frame with a light-blocking blanket for a few days is your safest strategy. If blustery winds threaten to sabotage your venting plan, place a board over the box, between the frame and the top, to keep it from slamming shut. Or use hooks and eyes to fasten the open top to posts sunk into the ground alongside the frame.
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Climate-controlled Frames
Any cold frame will harness solar energy for your plants’ benefit, and there are several low-tech ways to help your frames retain solar warmth. Black antifreeze containers, milk or kitty litter jugs painted flat black can be filled with water and tucked into the corners. Or you can cover the spaces between plants with flat stones painted black or “solar pillows” — used freezer bags painted black and filled with water. If you want to get more sophisticated, check into the Solar Pods and Solar Cones developed by New Hampshire gardeners Leandre and Gretchen Poisson. Described in detail in their book, Solar Gardening, and in Mother’s online Archive, the Poissons’ devices are probably the best you can build, if you can afford the materials. The superior performance of these garden appliances comes from the use of Sun-Lite flexible fiberglass, which costs about $80 for a 4-by-8-foot piece.
Historically, gardeners have used the warmth generated by rotting manure to turn cold frames into hot beds. To make a hot bed, dig a hole inside your frame at least 12 inches deep and fill it with fresh horse manure mixed with straw, and topped with 6 inches of soil. As the manure decomposes, it releases heat into the frame.
But you don’t have to have fresh manure to build a hot bed — or at least a warm one. For example, let’s say you want to winter-sow broccoli, spinach or another crop that needs abundant nutrients. If you dig out a bed and refill it halfway with compost mixed with the cheapest dry dog food you can find (a sure-fire compost activator), and then top it with 6 inches of soil, the compost will generate enough heat to keep the little plants from freezing and thawing quickly — if they freeze at all. In spring, when the plants’ roots find the buried treasure deepin the bed, you may be looking at the biggest, best plants you have ever grown.
