History of cold frames
In hard to grow climate zones, gardeners throughout history have struggled to extend their season into the fall, get a jump start to the spring, or even grow through the winter. For hundreds of years, simple shelters have been created to make micro-climates giving these gardeners the ability extend their growing seasons. Simply protecting your plants from frost and hard rains can alone make a big difference. Adding a few degree increase to the surrounding air temperature makes even more of a difference. The Romans constructed cold frames using straw or stone walls and sheets of mica for lids. As the Roman army moved north, they found it more difficult to maintain a healthy balanced diet for the troops. There were workers whose job was to open the cold frames when needed and close them during colder times. Today the mica lids are replaced with glass or newer material like polycarbonate, 6mil. plastic, or plexiglas.
Traditional use for cold frames
In the past, cold frames were built attached to the outside south facing wall of a greenhouse. The greenhouse was heated and the "cold frame" was not. In the spring, the plant starts were moved from the greenhouse to the cold frame to acclimate (or harden off) to cooler temperatures.
In late summer the ground is amended with compost/manure and a layer of soil over the top to create a hot bed. Winter veggies are sown directly into the soil. In early fall, a cold frame is place over the bed. These cold hardy veggies would mature in late fall to early winter and continue to produce through the winter months. Manure hot beds continue to be a great way to grow through the colder season. During the winter the buried compost continues to generate heat, which keeps the root systems warm and increases the temperature inside the cold frame. This method takes a little trial and error but can produce great results. Because of the added heat, keeping an eye on venting is important.
Modern day cold frame
More recently, gardeners are re-discovering the use of cold frames. With both the availability of information and newer and stronger materials this gardening practice has become more practical. New technology like heat cables and heat mats have revolutionized the use of cold frames. Placing a heat mat in a cold frame for starting seeds has become a common practice for urban gardeners. When the outside temperatures warm enough, the heat mats are turned off and the plants are left inside to harden off. When the outside soil temperature is warm enough, the plant starts are transplanted to the garden beds. Installing a heat cable into the soil will allow you to grow directly in the soil for extending the growing season into the winter without using manure.