Many of us that use cold frames realize the need to provide a little extra heat for our seedlings on those cold nights when the temperatures drop below freezing. Here are three ways to provide that heat for free, without electricity for heaters, heat tape, or gas for heaters.
The first way is a very old and proven method. Simply put; manure. Provide a layer of manure about 2 inches under your soil.
Manure is composed of decaying plant material. As the material decomposes it releases heat. This heat from the decomposing manure will warm the soil above it (remember, heat rises) thereby providing warmth for your plants.
The second way has also been in use for centuries; line your cold frame with stone.
The stone on the bottom should, ideally, be raised an inch or so above the ground so the ground doesn’t leach the heat from your stone. Larger stones will hold heat longer than smaller ones but gravel will work better than nothing. I used 3 to 4 inch thick flag stone for mine. The stone absorbs heat from the sun all day and radiates it back out into your frame at night.
The third way is to place water containers (2 or 3 liter pop bottles work fine for this) around the sides and down the middle of your cold frame. If you have a larger area to warm, you might want to use larger water containers. The water works the same way the stones do by gathering heat all day and radiating it out at night.
I have personally used all three methods and I can tell you from experience they all work. They will not give you the reliable control of a thermostatically controlled electric heater but they will increase the temperature inside your cold frame significantly.
You will want to cover your cold frame at night to prevent the heat from escaping through the glass. I cover mine with an old discarded horse blanket because it is insulated. You can use anything to cover the glass including plywood or anything else you may have lying around unused. The better the insulating qualities of your cover the more heat it will hold inside your frame.
All three methods can be used at the same time to increase effectiveness. I use the stones and manure every year. The warmer soil resulting from keeping the heat loss down at night will keep your plants healthier and make them grow faster.
These are three simple tried and proven ways to heat your cold frame without the use of any energy at all except the free heat from the sun.