Cold frames are generally used without artificial heat. It is important that they be well sheltered from north and west winds. As a rule they are used later in the spring than hotbeds. The water requirements of the plants are therefore greater, hence the facilities for watering should be as convenient as possible. Spigots should be located so all the plants can be reached with hose and nozzles. If the cold frame plat is extensive, it will pay to install a line of Skinner irrigation in each frame. The pipe may be placed on the upper side of the frame under the crossbars, with the greenhouse type of nozzle provided at intervals of 3 feet (122). This is a great labor-saving device in the management of frames, and makes it passible to water more uniformly than can be done with hose or watering can. The suggestions on the location and management of hotbeds apply equally well to cold frames.
If the ground is uneven or sloping, it should be graded before placing or making the frames. Level ground is important for effective watering. When, for example, flats do not set level in the frames, much of the water applied with a hose will run off before it has time to percolate. This may also be true of solid beds, especially if the soil is clayey and lacking in vegetable matter. Excavation is not required for cold frames, as they are mostly placed on top of the ground and banked with some material that will afford additional protection.
Cold frames are less expensive to make than hotbeds. Their construction is more economical in three particulars: (1) Excavation is not necessary when vegetable plants are grown. (2) Heating material is not needed. (3) The frame itself requires less material.
Many of the points made in considering the frame and sash for hotbeds apply equally well to cold frames. As cold frames are placed on top of the ground, the sides do not need to be so high as for hotbeds, unless the hotbed frame is simply placed on top of the manure, in which case there would be no difference in this respect. The height of the sides, or, in other words, the depth of the frame, must be determined by the size of the plants to be grown. It takes a deeper frame to grow tomato plants than to mature a crop of forcing radishes. Ordinarily, a 12-inch board or plank is high enough for the north side and a 6-inch piece for the south side. An additional depth of 2 to 4 inches is an advantage in some lines of work. If flats or plant boxes are to be used in growing cabbage and tomato plants, the frame at the north side should be 16 inches deep and at the south side 10 inches. The frames may be movable or stationary. The most common plan is to nail the side boards, which should be not less than 1« inches thick, to stakes placed on the outside. Chestnut lumber, 1«-inch thick, makes very satisfactory material for this purpose. The end boards, held in place by cleats or other devices, may be removed and the soil of the frame cultivated in the manner shown in Figures 22 and 23. The crossbars should be dovetailed in the side boards so they can be removed quickly.
While portable frames are often used, they are not popular with extensive commercial growers, Portable frames may be dovetailed at the corners or held together by rods and bolts. They are usually made to accommodate either two or four 3 × 6-foot sash. A double frame in use at the Missouri Experiment Station is shown in Figure 24. The outside of frames when used in cold weather should be banked with soil, manure or sod. An excellent plan is to bank with soil and then sow grass seed on the slope, unless the alleys are to be cultivated during the summer.
When the frames are used only in starting plants in flats, the character of the soil is not considered. If used without flats, the greatest care should be exercised in preparing the proper soil. Unless the subsoil is brought to the surface, the land may be treated as for any other intensive garden work. Figure 25 shows plats of frame cucumbers at Norfolk, Va., with the side boards removed. The wide alleys will soon be completely covered with the cucumber vines.
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.