Greenhouse , a name commonly applied to any glass structure in which plants are raised, but by professional gardeners restricted to houses in which a comparatively cool temperature is maintained. A cool greenhouse is one intended simply to protect tender plants during the winter season, and the temperature may be as low as 35° or 40°; but plants arc not expected to grow in such a house. Where flowers are desired, the day temperature must be at least 60°, with a minimum of 40° at night. Where a higher temperature than this is kept up, the house is called a stove, and is heated to 70° or 80° or more, according to the character of the plants it contains. A conservatory is a greenhouse attached to a dwelling, and is designed more for the effective display of plants than for raising them. Houses for special cultures are graperies, ferneries, orchard houses, orchid houses, etc. In designing a greenhouse, light, heat, air, and water have to be considered. The simplest form of a greenhouse is a "lean-to," in which the back is formed by its being placed against some other building, a fence or a wall erected for the purpose.
The roof slopes at an angle of 45 from the back wall to the front one, which is usually provided with a row of lights between the bench upon which the plants are placed and the plate upon which the roof rests. The span-roofed house, being intended to receive light upon all sides, usually stands apart from other buildings; its roof is straight or curvilinear, and its height and other dimensions are governed by the kind of plants for which it is intended; houses of this kind are frequently made ornamental by means of architectural embellishments. Large houses are often built with a central dome and span-roofed wings, and some are built with a half span, one portion of the roof being shorter than the other. In commercial establishments, where the greatest economy is studied and little regard is paid to appearances, the houses are usually low span-roofs. Sashes 6 ft. long are used for the roof; these meet above on a central ridge pole, and rest below on a plate which has its upper surface hollowed to form a water conductor. An excavation is made deep enough to allow head room beneath the glass, and walled up with brick to a foot or two above the surface of the ground. Such houses are about 11 ft. wide, with a bench 4 ft. wide upon each side and a central path of 3 ft.
This brings the plants upon the benches near to the glass, the position most favorable to growth, and gives sufficient room to work in; as the plants are intended to be sold while small, less space is required than in houses for other purposes. Heating was formerly accomplished entirely by means of flues running the whole length of the house, crossing the end, and returning upon the other side, the furnace being in a pit and accessible from the outside. This method of heating is sometimes still employed, as the outlay is less than in any other plan. The flue is sometimes built of brick its whole length, or is of brick for a few feet nearest the furnace, while the remainder is of pipes of earthenware or cement. The disadvantages of flues are the danger from leaks which may allow injurious gases to escape, and the difficulty of heating all parts of the house equally. On these accounts heating by means of hot water is preferred. There are numerous styles of greenhouse boilers, but they are all built upon the same principle, viz.: a reservoir of water with a pipe beginning near its upper portion, running the whole length of the house, and returning to the boiler, which it enters near its lower part.
When a fire is lighted under the boiler, the water immediately begins to circulate, the lighter warm water passing-out by the upper outlet, through the pipe, and returning to it through the inlet. In its passage through the pipes it gives off its heat by radiation, and with a well constructed apparatus the heat will be evenly distributed. At the end of the house furthest from the boiler is the expansion tank, an upright iron cylinder of somewhat larger diameter than the heating pipes; the flow and return pipe are both connected with this, which is open at the top or loosely covered; this prevents the expansion of the water when heated from exerting any pressure upon the pipes, and allows the air liberated from the water to escape, as well as any steam that may be formed when the apparatus is working to its fullest capacity. There is much difference in the internal construction of the boilers, each inventor striving to expose the greatest possible heating surface to the action of the fire. To economize heat, a flue to carry off the products of combustion from the boiler is sometimes run through the house. Ventilation is accomplished in various ways: in small houses by lifting or sliding the sashes, and in large ones by raising a portion of the upper part of the roof by proper machinery.
A water cistern is generally built under the floor of the greenhouse, into which the water from the roof is conducted. Shading is required as spring approaches, which is commonly accomplished by washing the glass with ordinary lime wash, or with whiting and milk; in some cases a screen of muslin is used, or a lattice work of narrow strips of wood. - For the construction and management of commercial greenhouses, Henderson's "Practical Floriculture" may be consulted. "Choice Stove and Greenhouse Plants," by B. S. Williams, is one of the most recent English works; and the amateur will find useful hints in "The Greenhouse as a Winter Garden," by F. E. Field.