Greenhouse
A Greenhouse is usually understood to be a structure specially devoted to the cultivation or exhibition of plants that never require a very high temperature. It is distinguished from a conservatory by the occupants being almost exclusively grown in pots and tubs; whereas, in the other instance, many are permanently planted out. Greenhouses have a wide application, ranging from a single house possessed by an amateur, to a large structure set apart for the exhibition of plants that are previously grown to the flowering stage in other houses or pits. Subjects which are available for Greenhouse decoration throughout the year, are almost innumerable, and include a large proportion of the most beautiful plants in cultivation. An important essential for their general well-being is plenty of light; consequently, this is one of the first structural conditions to be secured. Secondly, provision should be made for admitting any quantity of air whenever required, as is the case throughout the summer. The best houses of modern construction are far before those of former years in these respects, the general substitution of large for small panes of glass, and glass roofs for slates, having effected great improvements. In well-arranged Greenhouses, where sufficient plants are at command, a fine display may be insured throughout the year by having a varied selection, and hastening and retarding to keep a succession. Nearly all the improved types of florists' flowers and select annuals are available for pot culture if desired. Exotic plants are extremely numerous and attractive, particularly those from Australia, the Cape of Good Hope, the Himalayas, China, and Japan. Many hardy flowering plants and shrubs may also be lifted from the open ground and forced in early spring-a time when Greenhouses are better furnished and more interesting than at any other season of the year. Where there are other houses and pits devoted to the preparation of flowering plants, each subject may be much better provided with its special requirements, than when space for cultivation is limited to the Greenhouse only. Here a certain temperature is maintained which suits a number of plants in flower, but may not be warm enough for others that are making their annual growth. If one house can be devoted more especially to the exhibition of those plants in flower and others with ornamental foliage, and they are changed as becomes requisite, the interest in Greenhouse subjects will be rendered more certain. This is, however, frequently impracticable. Shape and Aspect. Plenty of light and air being essential conditions for keeping in view in the construction of a Greenhouse, it follows that the site chosen should be an open one. The best shape is a span roof, as light is admitted on all sides, and the plants are not so likely to draw or grow in any one direction. Fig. 148 represents a section of a span-roofed house well adapted for Greenhouse plants. A centre stage is shown that may be made flat if desired, for accommodating vigorous-growing subjects. The side stages are on a level with the walls, and upright sashes above these (a, a) are hung so as to open in the way indicated. Top ventilation must also be provided, either by sliding sashes, as shown in Fig. 149 (which, however, have the disadvantage of admitting rain if open), or by a more modern method which prevents this, by raising a portion of the roof with a lever. A small lean-to house well adapted for amateurs or others with only a limited quantity of plants, is shown in Fig. 149. This might be sufficiently heated by a very small hot water apparatus. Hip-roofed houses usually admit light, but are not so convenient for attending and arranging the occupants as the full span. Again, shelf stages are not so favourable to good cultivation as flat ones, where a moist bottom of ashes or small stones may be secured, on which to stand the pots. Ventilation in hip-roofed houses must be obtained by sliding or other movable sashes at the apex, and by small doors inserted in the front wall. The aspect best suited is one nearly south; but, with the full span, the ends should run north and south, so that all possible light may be admitted, and the sun's rays in summer somewhat obstructed in the middle of the day. Greenhouse plants are divided into two general groups, hard-wooded and soft-wooded.
Hard-wooded Section. This includes all plants of a shrubby habit, and the majority of climbers. A large proportion of them are difficult to cultivate, particularly if their requirements are insufficiently understood, and suitable positions are not provided. The majority flower in spring and summer, and proper treatment varies according to the condition of the plants in such as the growing, resting, and flowering periods. Healthy and floriferous hard-wooded plants in spring, notably the numerous and beautiful species from Australia and the Cape, are only secured by constant attention throughout the preceding summer and winter. Nearly all repotting should be attended to just after the annual growth begins, this season varying with different species. A somewhat closer atmosphere, and more moisture, may be allowed for a month afterwards, to encourage the emission of roots into the new soil. Afterwards, more air may be gradually admitted, and, in bright summer weather, a thin, temporary shading applied. The aim, with hard-wooded plants, should be to encourage summer growth to the fullest extent, and to insure its thorough ripening in autumn. Without this, the results subsequently obtained in the production of flowers will be but secondary compared with what is possible under good cultivation. There are also numerous evergreen shrubs and small trees of an ornamental character, well adapted for intermixing in Greenhouses as permanent decorative subjects, and a large proportion may be cultivated in comparatively small pots. When any require larger sizes, they should be shifted in spring or early autumn, and, if possible, kept a little closer for a few days after-wards. Examples of hard-wooded flowering plants are: Acacia, Azalea, Boronia, Camellia, Epacris, Erica, and Pimelia. Soft-wooded Section. This includes all that have stems and leaves of a more or less sappy growth, and are mostly propagated from cuttings in that state, or from seeds. A large number of florists' flowers are included in this section, and, as many of them are growing throughout the winter months, a position where all possible light is obtainable, should be allotted them. Many soft-wooded subjects are easily cultivated; but they are very sensitive to improper treatment, such as allowing too close an atmosphere, or too much heat or shade. A number of beautiful Greenhouse plants are annually raised from seed, and good strains or selections of such varieties as Calceolarias, Celosias, Cinerarias, Mignonette, Primulas, Rhodanthes, &c., should be secured. These should be sown at different periods, in order to prolong the flowering season. Immunity from insects, and a continued growth without check, are important conditions in the successful cultivation of all. Other plants of a soft-wooded nature, but which become somewhat hard with age, are either propagated each year, or, in some cases, treated as perennials. Examples of these are: Chrysanthemums, Eupatoriums, Fuchsias, Pelargoniums, and Salvias. A selection from each is indispensable for Greenhouse decoration, and all are easily cultivated where space admits. An open and somewhat rich soil is a general requirement, and plenty of air and water in summer, after the plants are become established. Lilies of the Valley, Solomon's Seal, Deutzia gracilis, Dicentra spectabilis, &c., are amongst the most attractive and useful subjects for forcing.