Various small and portable coldframes may be used about the garden for the protection of tender plants or to start them early in the spring. Pansies, daisies, and border carnations, for example, may be brought on very early by setting such frames over them or by planting them under the frames in the fall. These frames may be of any size desired, and the sash may be either removable, or, in case of small frames, they may be hinged at the top. Figs. 200-203 illustrate various types.
A hotbed differs from a coldframe in being provided with bottom heat. This heat is ordinarily supplied by means of fermenting manure, but it may be obtained from other fermenting material, as tanbark or leaves, or from artificial heat, as flues, steam pipes, or water pipes.
The hotbed is used for the very early starting of plants; and when the plants have outgrown the bed, or have become too thick, they are transplanted into cooler hotbeds or into coldframes. There are some crops, however, that are carried to full maturity in the hotbed itself, as radishes and lettuce.
The date at which the hotbed may be started with safety depends almost entirely on the means at command of heating it and on the skill of the operator. In the northern states, where outdoor gardening does not begin until the first or the last of May, hotbeds are sometimes started as early as January; but they are ordinarily delayed until early in March.
The heat for hotbeds is commonly supplied by the fermentation of horse manure. It is important that the manure be as uniform as possible in composition and texture, that it come from highly fed horses, and is practically of the same age. The best results are usually secured with manure from livery stables, from which it can be obtained in large quantities in a short space of time. Perhaps as much as one half of the whole material should be of litter or straw that has been used in the bedding.
The manure is placed in a long and shallow square-topped pile, not more than four or six feet high, as a rule, and is then allowed to ferment. Better results are generally obtained if the manure is piled under cover. If the weather is cold and fermentation does not start readily, wetting the pile with hot water may start it. The first fermentation is nearly always irregular; that is, it begins unequally in several places in the pile. In order to make the fermentation uniform, the pile must be turned occasionally, taking care to break up all hard lumps and to distribute the hot manure throughout the mass. It is sometimes necessary to turn the pile five or six times before it is finally used, although half this number of turnings is ordinarily sufficient. When the pile is steaming uniformly throughout, it is placed in the hotbed, and is covered with the earth in which the plants are to be grown.
Hotbed frames are sometimes set on top of the pile of fermenting manure, as shown in Fig. 204. The manure should extend some distance beyond the edges of the frame; otherwise the frame will become too cold about the outside, and the plants will suffer.