The material for the hotbed is fresh horse manure mixed with leaves. This is thrown into a heap to heat. As soon as steam is seen coming from the heap the manure is turned over and piled again so that the outer part is thrown inside. When the whole is uniformly heated and has been turned two or three times, it is packed firmly into the pit already dug.
A frame six feet wide, twelve inches high on the north side and eight inches on the south side and as long as the bed is to be, is now made of plank. This is set upon the heated manure, thus leaving six inches on each side outside the frame. More manure is then banked all around it, and three or four inches of fine light and rich soil are placed inside the frame.
The frame is then covered with hotbed sashes six feet long and three feet wide. These slide up and down on strips of wood let into the sides of the frame. A thermometer is stuck into the soil and closely watched, for there will be too much heat at first for sowing seed. When the heat in the early morning is about 85°, seeds may be sowed. The hotbed is used for starting tomato plants, eggplants, cabbage plants, and other vegetables that cannot stand exposure. It should be made about eight or ten weeks before the tender plants can be set out in the locality. In the South and Southwest it should be started earlier than in the North. For growing the best tomato plants, and for such hardy plants as lettuce and cabbage, it will be better to have cold-frames in addition to the hotbed; these need not be more than two or three sashes.
Cold-Frames. A cold-frame is like the frame used for a hotbed, but it is placed on well-manured soil in a sheltered spot. It is covered with the same kind of sashes and is used for hardening the plants sowed in the hotbed. The frame must be well banked with earth on the outside, and the glass must be covered on cold nights with straw, mats, or old carpets to keep out frost.
Fig. 85. Frame to carry the Sash of a Hotbed or Cold-Frame
Care of Hotbed and Cold-Frame. If the sun be allowed to shine brightly on the glass of a cold-frame or hotbed, it will soon raise the temperature in the hotbed to a point that will destroy the plants. It is necessary, then, to pay close attention to the bed and, when the sun shines, to slip the sashes down or raise them and place a block under the upper end to allow the steam to pass off. The cold-frame also must be aired when the sun shines, and the sashes must be gradually slipped down in mild weather. Finally, they may be removed entirely on sunshiny days, so as to accustom the plants to the open air, but they must be replaced at night. For a while before setting the plants in the open gardens, leave the sashes off night and day.
Fig. 86. Greenhouse and Cold-Frames
While the hotbed may be used for starting plants, it is much better and more convenient to have a little greenhouse with fire heat for this purpose. A little house with but four sashes on each side will be enough to start a great many plants, and will also give room for some flowers in pots. With such a house a student can learn to manage a more extensive structure if he gives close attention to airing, watering, and keeping out insects.