A cold frame is a small greenhouse used 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 a 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 sash or two or three. Usually the cold frame is placed against the house, facing south, since the foundation will provide even more protection against the cold, but it could be independent. By definition, a cold layer is heated only by the sun.
My own cold frame is built against my house, on the south side. It has four glass panels, but is rather unusual in that I placed up against my basement windows (my office windows, in fact, as my office is in the basement). As I result, I actually have access to the cold frame from inside my house, all winter. That might not strike you as particularly useful, but we average about 4 feet (1.2 m) of snow buildup each winter (nearly 6 feet/1.8 m this year), so it would normally be inaccessible over the winter. Inside, I just have to slide open a window from indoors for access in any season. I use a thermometer to check the temperature and, on the rare occasions where temperatures drop too far below freezing—I tolerate down to about -25? F (-4? C)—, I simply open a window for a short time and let some heat in. Thus I can use it to force bulbs and overwinter plants that like a cold winter but won’t take deep freezing, like agapanthus and rosemary. And of course, to harden off seedlings in spring and to grow melons in summer.
Some Like It Hot
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 radishes.
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 as early as 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 54? F (12? 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!