Cold frames tend to include a wooden frame or box with a lid that can be raised and lowered. They use no artificial heat source so the temperature inside the structure is typically only 5 to 10 degrees warmer than the outside temperature.
The cover for a cold frame should be transparent and can be made from an old storm window, polycarbonate or polyethylene film. The size depends on the intended use of the frame. It can be placed over a raised bed or directly on the ground to protect early plantings of cool season crops such as spinach. They can also be used to harden off transplants before planting in the ground or to protect tender perennials during winter.
A hotbed is a cold frame with a heat source. It could be an electric heater or even a natural heat source such as manure. Hot beds can be considered miniature greenhouses and are used to get a jump on warm season vegetables.
On sunny days, cold frames and hotbeds can heat up quickly. It is important to be able to vent these structures so plants do not get overheated. Storm windows can be attached with hinges so they can be propped open on sunny days to allow heat to escape.
When extreme cold temperatures are predicted, the season extenders can be insulated with additional layers such as straw or blankets over the top of the structure.
Position cold frames and hotbeds facing south to get the maximum winter sun exposure. Make sure there is good drainage so moisture is not trapped inside. If possible, position the structure close to the house for ease of monitoring and maintenance. For hotbeds with electric heating consider where the power source is and locate the structure in that vicinity.