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.

201. A larger portable coldframe.

202. A commodious portable frame.

203. A low coldframe.

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.

It is preferable, however, to have a pit beneath the frame in which the manure is placed. If the bed is to be started in midwinter or very early in the spring, it is advisable to make this pit in the fall and to fill it with straw or other litter to prevent the earth from freezing deep. When it is time to make the bed, the litter is thrown out, and the ground is warm and ready to receive the fermenting manure. The pit should be a foot wider on either side than the width of the frame. Fig. 205 is a cross-section of such a hotbed pit. Upon the ground a layer of an inch or two of any coarse material is placed to keep the manure off the cold earth. Upon this, from twelve to thirty inches of manure is placed. Above the manure is a thin layer of leafmold or some porous material, that will serve as a distributor of the heat, and above this is four or five inches of soft garden loam, in which the plants are to be grown.

204. Hotbed with manure aa top of the ground.

It is advisable to place the manure in the pit in layers, each stratum to he thoroughly trodden down before another one is put in. These layers should be four to eight inches in thickness. By this means the mass is easily made uniform in consistency. Manure that has too much straw for the best results, and which will therefore soon part with its heat, will spring up quickly when the pressure of the feet is removed. Manure that has too little straw, and which therefore will not heat well or will spend its heat quickly, will pack down into a soggy mass underneath the feet. When the manure has sufficient litter, it will give a springy feeling to the feet as a person walks over it, but will not fluff up when the pressure is removed. The quantity of manure to be used will depend on its quality, and also on the season in which the hotbed is made. The earlier the bed is made, the larger should be the quantity of manure. Hotbeds that are intended to hold for two months should have about two feet of manure, as a rule.

205. Section of a hotbed built with a pit.

The manure will ordinarily heat very vigorously for a few days after it is placed in the bed. A soil thermometer should be thrust through the earth down to the manure, and the frame kept tightly closed. When the temperature is passing below 90°, seeds of the warm plants, like tomatoes, may be sown, and when it passes below 80° or 70°, the seeds of cooler plants maybe sown.

If hotbeds are to be used every year, permanent pits should be provided for them. Pits are made from two to three feet deep, preferably the former depth, and are walled up with stone or brick. It is important that they be given good drainage from below. In the summer-time, after the sash are stripped, the old beds may be used for the growing of various delicate crops, as melons or half-hardy flowers. In this position, the plants can be protected in the fall. As already suggested, the pits should be cleaned out in the fall and filled with litter to facilitate the work of making the new bed in the winter or spring.

206. Parallel runs of hotbeds with racks for holding sashes.

Various modifications of the common type of hotbed will suggest themselves to the operator. The frames should ordinarily run in parallel rows, so that a man walking between them can attend to the ventilation of two rows of sash at once. Fig. 206 shows a different arrangement. There are two parallel runs, with walks on the outside, and between them are racks to receive the sash from the adjacent frames. The sash from the left-hand bed are run to the right, and those from the right-hand bed are run to the left. Running on racks, the operator does not need to handle them, and the breakage of glass is therefore less; but this system is little used because of the difficulty of reaching the farther side of the bed from the single walk.

If the hotbed were high enough and broad enough to allow a man to work inside, we should have a forcing-house. Such a structure is shown in Fig. 207, upon one side of which the manure and soil are already in place. These manure-heated houses are often very efficient, and are a good make-shift until such time as the gardener can afford to put in flue or pipe heat. Hotbeds may be heated by means of steam or hot water. They can be piped from the heater in a dwelling-house or greenhouse. Fig. 208 shows a hotbed with two pipes, in the positions 7, 7 beneath the bed. The earth is shown at 4, and the plants (which, in this case, are vines) are growing upon a rack, at 6. There are doors in the end of the house, shown in 2, 2, which may be used for ventilation or for admitting air underneath the beds. The pipes should not be surrounded by earth, but should run through a free air space.

It would scarcely pay to put in a hot water or steam heater for the express purpose of heating hotbeds, for if such an expense were incurred, it would be better to make a forcing-house. Hotbeds may be heated, however, with hot-air flues with very good results. A home-made brick furnace may be constructed in a pit at one end of the run and underneath a shed, and the smoke and hot air, instead of being carried directly upwards, is carried through a slightly rising horizontal pipe that runs underneath the beds. For some distance from the furnace, this flue may be made of brick or unverified sewer pipe, but stove-pipe may be used for the greater part of the run. The chimney is ordinarily at the farther end of the run of beds. It should be high, in order to provide a good draft. If the run of beds is long, there should be a rise in the underlying pipe of at least one foot in twenty-five. The greater the rise in this pipe, the more perfect will be the draft. If the runs are not too long, the underlying pipe may return underneath the beds and enter a chimney directly over the back end of the furnace, and such a chimney, being warmed from the furnace, will ordinarily have an excellent draft.

The underlying pipe should occupy a free space or pit beneath the beds, and whenever it lies near to the floor of the bed or is very hot, it should be covered with asbestos cloth. While such flue-heated hotbeds may be eminently successful with a grower or builder of experience, it may nevertheless be said, as a general statement, that whenever such trouble and expense are incurred, it is better to make a forcing-house. The subject of forcing-houses and greenhouses is not discussed in this book.

208. Pipe-healed hotbed.

The most satisfactory material for use in hotbed and cold-frame sash is double-thick, second-quality glass; and panes twelve inches wide are ordinarily broad enough, and they suffer comparatively little in breakage. For coldframes, however, various oiled papers and waterproof cloths may be used, particularly for plants that are started little in advance of the opening of the season. When these materials are used, it is not necessary to have expensive sash, but rectangular frames are made from strips of pine seven-eighths inch thick and two and one-half inches wide, halved together at the corners and each corner reenforced by a square carriage-corner, such as is used by carriage-makers to secure the corners of buggy boxes. These corners can be bought by the pound at hardware stores.

209. Useful kinds of watering-pots. These are adapted Io different uses, aa are different forma of hoes or pruning tools.