Cold frames and Hotbeds
Fighting the forces of nature takes energy, growing cold tolerant winter veggies works with nature. A cold frame simply provides additional protection, allow you to get a jump on, or extend the season by up to 4 weeks at either end. In our zone 8 climate we can get by with row covers alone.
A hotbed or forcing bed uses the addition of heat to further increase the growing days.
Hotbeds have been used for many years to bring on delicate plants in cold weather. They where most commonly used by Victorian gardeners to force crops like Melons, Cucumbers, Strawberries, and Radishes. In fact any crop that was needed in the kitchen by the cook out of season could be grown in a Hotbed.
There are techniques known to gardeners years ago, which have been lost to us modern,’ advanced' gardeners. We are used to out of season fruits and vegetables from around the world, readily available in our grocery stores.
Fresh vegetables harvested from home, taste better and have not lost any nutrients in travel. Many winter vegetables are poor shippers; so if you want quality leeks or kale you have no choice but to grow them yourself. , many vegetables store more sugars when they are exposed to cold temperatures. Also, In addition, you know what has, and hasn't, been sprayed all over your own plants!
Plants grow better with protection in winter, you can choose to simply cover them when temperatures drop too low, below -5C, even without cover many cold hardy plants can tolerate freezing. and enjoy a very simple nearly protection free winter garden, or push the envelope depending on how much energy you would like to put into it.
Heat will be required to germinate many seedlings, but for instance you could sow carrots in Nov, mid Jan, and mid Feb with heat cables, or a manure hotbed. This may not be practical for the home gardener, as it would be easier to plant in Aug and store in the ground through winter. It may be more practical for getting a jump on the season with crops that need a long summer season, or to keep greens in production all winter.
The most effective, and cheapest coldframe is one which uses two layers of protection. This can be achieved with a high hoop or framed structure with plastic stretched over it, and another layer of row cover or plastic suspended directly over the plant. You can keep the lower layer off the plants with stakes, or # 9 wire frames bent to suit.
There are many plans for coldframes available in the library or on line. They should be located in a sunny South or West sheltered exposure. Flat top models should have a minimum of 10 % angle on the cover, and be at least 12" to 18" deep. Heat may be added by cables or setting the frame over a manure hotbed.