Both cold frames and hotbeds allow a gardener to grow plants that would not normally be allowed to grow due to cold temperatures. They are most useful in the springtime, however, they can be used in the fall as well. Cold season vegetables like, lettuce and spinach are ideal to start in a coldframe, as they can get an extra head start.
First let me explain the difference between a hotbed and a cold frame. While some people use the names interchangeably, there is actually a difference between the two. A cold frame uses only the sun as its source of heat. Generally a coldframe can maintain a temperature of about 5 to 10 degrees different that the surrounding air. While this might not seem like much, it can be the difference between an unexpected frost killing your crop, and having your plants survive. A hotbed has heat supplied from an outside force besides the sun. Therefore a hotbed is better suited for warmer season vegetables such as tomatoes, peppers, and eggplants. Many people think of a hotbed as a miniature greenhouse.
If you are building a new cold frame it is best to divide it into two sections. This will allow you to control the temperature separately for the two sections that you have created. It will also allow you to ventilate one section differently from the other section. Generally a cold frame is pointed in a southern orientation to allow for the maximum sunlight. It is also recommended that the inside of the cold frame be either white or silver to allow for the maximum reflection of the available sunlight.
A hotbed may be heated by several means. Some of the more popular methods include electric cables, hot water, and manure. Manure heated beds can be difficult to control, thus this method is not generally recommended to beginners. Thermostats are used to control the heat generated by the electric cables, as you don’t want the hotbed getting too hot or too cold. Generally you want the soil temperature of your hotbed kept at about 70 to 75 F for germination. After that the temperature required will depend upon the type of plants you are growing.
It is best to water the plants in your cold frame and hotbed in the mornings. Make sure you do not overwater your plants especially if you have a hotbed, as the higher temperatures can lead to disease as well as spindly growth. You must remember to ventilate your cold frames and hotbeds to control heat buildup during the day, as well as to prevent damping-off disease. Make sure you raise the side that is opposite to the wind, as young tender plants are very susceptible to wind burn. You will also use ventilation to harden off your plants as the season grows warmer.