A cold frame is a small greenhouse used to acclimatize plants to outdoor conditions before planting, to sow seeds of vegetables and flowers, to harden off seedlings started indoors and often also to grow vegetables that would otherwise struggle to mature in colder or short-season climates, like eggplant or okra (gumbo). You don’t have to be much of handyman to throw together a rectangle with an angled top (to let more sun it and let heavy snow slide off) using a few pieces of wood and an old window frame.Usually the cold frame is placed against the house, facing south, since the foundation will provide even more protection against the cold. By definition, a cold layer is heated only by the sun.
A hotbed is identical to a cold frame, but heated. In the old days, it would have been heated by a thick layer of fresh horse manure covered in sand; nowadays most gardeners use electricity to heat theirs. You can install heating mats or heating cables or just a lamp with an incandescent bulb. Such bulbs may seem a bit antiquated, but they do give off a lot of heat.
Since a hotbed is heated, it can be used right through the winter, especially for growing vegetables that love cool temperatures, such as spinach, lettuce, Swiss chard and radish.
Oddly enough, keeping plants cool in a cold frame or hotbed is more complicated than keeping them warm: on a sunny day, heat can build up tremendously! To manage the temperature in either hotbeds or cold frames, you’ll need to start opening the lid part way on sunny days even in late winter. In most climates, by April there will be days when you can leave the top fully open on warm, sunny days. During the summer, the frame will likely be open more than it is closed: simply close it when night temperatures risk dropping below 50˚F (10˚C).
The Most Laidback Model of All!
Even though cold frames and hotbeds are simple to assemble, there is an even easier model you can make. Simply form a box of straw bales and place as an old window frame on top. It will be ready to use in only a few minutes!