Plant-houses are garden structures designed for the protection and cultivation of the plants of warmer climates than our own. Apart from the style of architecture, a plant-house must be so constructed as to admit a maximum of light to the interior; there must also be ample provision for ventilation, and means for maintaining such atmospheric temperature as is necessary to the plants that are to be cultivated in it. Glass, wood, and iron are the materials of which plant-houses are made. Masonry is not essential in the erection of plant-houses, but it is very generally employed to give stability, durability, and architectural effects. Glass obviously is the most important material: the larger the amount of it that enters into the structure of a plant-house the better adapted will it be for the cultivation of plants; the means for securing the maximum of light is thereby provided, and its regulation is then under the control of the cultivator.
Under the term plant-house is included every kind of horticultural glass erection employed in the culture of flowering and ornamental plants, as distinguished from those which are devoted exclusively to the culture of fruit-trees or other plants that are grown solely for the sake of their fruit. They are broadly divided into three classes—viz. hothouse or plant-stove, intermediate house, and greenhouse. The structure of each class may be the same in all respects except in the power of the heating apparatus. In the hothouse it must be adapted to create and sustain tropical temperatures irrespective of the temperature of the outer air; in the intermediate house the heat of extra-tropical and temperate countries must be provided; and in the greenhouse all that is required of the heating apparatus is the exclusion of frost or the maintenance of a minimum temperature of 40° F. Hothouses are either dry or moist, according to the class of plants to which each may be devoted; the natives of dry tropical regions and those of maritime lake and river districts severally requiring special adaptations in connection with the heating apparatus for providing atmospheric humidity. Thus there may be tropical orchid-houses, tropical fern-houses, tropical aquatic-houses—the latter being fitted with tanks of heated water, in which Victoria regia, Nymphaea, and other aquatic plants of the tropics are cultivated; but the more common class of stove is that in which the internal arrangements are made with the view of accommodating a large variety of plants, having considerable diversity of constitutional requirement. The intermediate house may be subdivided in the same way into the cool orchid-house, the cool fernery, &c.; but more commonly it is adapted to the wants of miscellaneous plants, and very often indeed it is used temporarily for tropical plants during their period of rest, when a lower temperature and less humidity than those of the stove are desirable. The greenhouse may be a heat-house if exclusively devoted to the culture of Cape heaths (Erica) and kindred plants, or it may be a New Holland house, if its inhabitants are chiefly composed of the interesting natives of New Zealand, Tasmania, and other temperate parts of Australasia.
The conservatory is a plant-house in which a miscellaneous collection of plants, after having been grown elsewhere, is placed in order to display the beauty of flowers and foliage. It may be either cool or hot, according to the classes of plants accommodated in it. Being usually a place of resort, or a lounge accessible from the drawing-room or some other part of a mansion, its architectural features should be in harmony with those of the building of which it forms a part; but due regard should also be had, in deciding upon architectural details, to the providing of ample light, and the means of securing perfect ventilation. The propagating house is a plant-house devoted to the purpose of rearing the several classes of plants indicated in the foregoing, either by seeds, cuttings, grafting, or any other mode that may be required in particular cases. It may be heated or cool, and differs from the ordinary plant-house chiefly in being more restricted in atmosphere. It is fitted with close glass-cases, fixed or portable, for the purpose of preventing exhaustion, by the atmosphere, of cuttings and other subjects temporarily destitute of roots. An essential feature of the internal arrangements of the propagating house is a bed or beds filled with sand, cocoa-nut fibre, or any other cleanly material, in which the pots containing cuttings may be plunged at will, to prevent evaporation from their sides and fluctuations of temperature in their contents. These beds are usually provided with hot-water pipes, tanks, or flues below, for the purpose of giving bottom heat when required.
See also FORCING, GARDENING, HOTBED, ORCHIDS, PEACH, VINE, &c.; S. Wood, Forcing Gardening (1881); Fawkes, Horticultural Buildings (new ed. 1886); Rivers, The Orchard House (1881); and other works by Baines (1885), Hibberd (new ed. 1880), May (new ed. 1888), and Williams (new ed. 1883).