Hotbed, in gardening, a bed of earth enclosed by a frame, which is covered by movable sashes, and heated from below by means of fermenting vegetable matter. In large establishments the hotbed is replaced by a glass structure heated by flues or by hot-water pipes. (See Greenhouse.) When vegetables are made to grow out of their proper season, they are said to be forced; large quantities of lettuce, radishes, etc, are forced for market in hotbeds during the winter months. The most general use of the hotbed is in starting such seeds as would germinate very slowly, if at all, in the open ground, and to forward plants for an early crop of those kinds that are later sown in the open air; by the use of the hotbed, plants six weeks old, of cauliflower and cabbage for example, may be had for planting out at the time when the outside soil is dry and warm enough to allow of the sowing of seeds, thus enabling the gardener to produce a much earlier crop. The hotbed allows us to extend the season of many vegetables about two months; for instance, the season of tomatoes would be a very short one if we depended upon plants from seed sown in the open ground, but with the aid of the hotbed the plants may be so far forward as to be ready to flower at the time when it is safe to put them out.
The usual heating material is horse dung; this is turned over a few times at intervals of a few days, and when in a state of active fermentation is laid up in a regularly formed bed 3 or 4 ft. thick, and a foot wider on each side than the frame of the hotbed; care is taken to have the manure evenly packed, and it is beaten with the fork to make it solid; the frame is then set upon the manure; fine, light, rich soil should be at hand, and when the thermometer shows that the heat of the bed (at first very violent) has receded to 90°, this is spread evenly over the manure to the depth of 6 or 8 in.; then the seeds may be sown. The use of one third or one half its bulk of forest leaves with the manure gives a more gentle and more lasting heat. The hotbed for a family garden is made in the manner described, and the frame, usually permanent, is large enough for two or three sashes. In market gardens the method is quite different. The regular hotbed sash is usually 6x3 ft.; the bars to hold the glass run longitudinally, there being no cross bars, but the glass is lapped at the edges about a quarter of an inch.
The width of the bed is the length of the sash, and the length of the bed is determined by the number of sashes; an excavation is made 2 1/2 ft. deep, and of the required size; this is boarded up with rough boards nailed to posts ; the boarding extends above the surface of the ground 12 in. in front and 18 in. at the rear; cross pieces are nailed from front to rear, upon which the sash can slide. The manure is then placed in this pit and the soil put upon it as before described. Mats of straw or shutters of thin boards are provided to protect the bed in cold nights, and to afford shading when needed. The hotbed should be in a sheltered place well exposed to the sun; if need be, shelter from cold winds is afforded by making a fence, or setting up a wind-break of brush. As soon as the young plants are up they require the same care in weeding, thinning, watering, and loosening the soil, as those in the open ground; besides this, the sashes must be opened more or less, according to the weather, to prevent injury from too great heat, and when open must be closed should the outer temperature fall, to prevent damage from cold. Unless the beds are carefully attended to in both particulars, an hour of neglect may destroy the contents.
Many plants require transplanting, when large enough, into other hotbeds before they are finally set out. Before setting in the open ground the plants are hardened by gradually exposing them by the removal of the sashes whenever the night temperature will allow. The usual night temperature for a hotbed is 55° to 65°, and that in the day 70° to 80°. - Where many varieties are to be sown in a bed, it is convenient, instead of sowing the seeds in the soil of the bed, to sow them in shallow wooden boxes 2 1/2 or 3 in. deep. Besides seeds, roots of various kinds are forwarded in hotbeds; sweet potatoes are buried in the soil of the bed in order to get sets for planting; dahlia roots are started, and such slow-growing bulbs as tuberoses are best forwarded in this way before putting them out. A little bottom heat will often resuscitate a languishing plant or start a backward one into growth, and a hotbed is often useful as a place in which to plunge the pots of such plants. Where a very gentle and long continued heat is required, what is called a bark pit is used; in this spent tanner's bark, or waste tan, as it is called, takes the place of manure.