Cold frames are sometimes piped and heated with steam or hot water (162). They may then be used at any season, and with their use the grower can have good control of all conditions which count for success. It is a recognized fact, however, that heated frames are not so satisfactory as greenhouses.
Cold frames are used to a far greater extent than hotbeds. Plants started in the greenhouse, hotbed or kitchen window are often transplanted into cold frames. This is perhaps their most common use. They are also employed extensively in the hardening of plants and in the forcing of fall and spring crops to maturity. Lettuce and radishes are especially popular for frame culture, while many other crops are often grown in cold frames. Many market gardeners own from 1,000 to 4,000 sash, and some growers confine their operations entirely to frame culture.
There are various methods of making forcing boxes. The most common plan is to make a frame 10 to 12 inches square and 6 inches deep, and cover with a pane of glass. These frames are especially valuable in starting melons and other cucurbits in regions where the summers are too short and cool to grow a satisfactory crop without the aid of glass. The frame is placed over the hill after planting seed in the open, covered with glass, and ventilated when necessary. Hundreds of them are used by some gardeners.