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"Forcing" is the very inappropriate name of the process by which roses and other plants are induced to bloom under glass in advance of their natural season. We say that the name is inappropriate, because one of the chief essentials to the success of the process consists in an abstinence from all that is violent or sudden, and in the gentle and graduated application of the stimulus of artificial heat.
Roses may be forced in the greenhouse, but not to advantage, because the conditions of success will be inconsistent with the requirements of many of the other plants. The process is best carried on in a small glass structure made for such purposes, and called a "forcing-pit."
A pit ten or twelve feet long and eight or ten wide will commonly be large enough. It may be of the simplest and cheapest construction. In a dry situation, there is advantage in sinking the lower part of it two or three feet below the surface of the ground. The roses may be placed on beds of earth, or on wooden platforms, so arranged as to bring the top of the plants near the glass; and a sunken path may pass down the middle. The pit may be heated by a stove enclosed with brick-work, and furnished with a flue of brick or tile passing along the front of the pit, and entering the chimney at the farther end. The lights must be movable, or other means provided' for ample ventilation; and if these are such that the air on entering will pass over the heated flues, and thus become warmed in the passage, great advantage will result. A pit may be appended to a greenhouse; in which case it may be heated by hot-water pipes furnished with means of cutting off or letting on the water.
The roses potted for forcing, as directed in the last section, should be kept in a dormant state till the middle of December. A portion of them may then be brought into the pit, and the young shoots pruned back to two or three eyes. The heat at first must be very moderate, not much exceeding forty-five degrees in the daytime: and, throughout the process, the pit should be kept as cool as possible at night; great care, however, being taken that no frost is admitted. With this view, the glass should be covered at sunset with thick mats. Syringe the plants as the buds begin to swell, and lose no opportunity to give air on mild and bright days. Raise the heat gradually till it reaches sixty degrees; which is enough during the winter months, so far as fire-heat is concerned. The heat of the sun will sometimes raise it to seventy or eighty degrees. Syringe every morning; and, if the aphis appears, fumigate with tobacco; then syringe forcibly to wash off the dead insects. As the plants advance in growth, they require plenty of water; and, as the buds begin to swell, manure-water may be applied once or twice. When the buds are ready to open, the pots may be removed to the greenhouse or drawing-room, and another supply put in their place for a second crop of flowers. When the blooms are faded, the flower-stalks may be cut back to two or three eyes, and the plants placed again in the forcing-pit for another crop. This, of course, is applicable to ever-blooming roses only.
The most common and simple way, however, of obtaining roses in winter, is to grow them on rafters in the greenhouse. Some of the Noisette, China, and Tea roses, thus treated, will furnish an abundant supply of excellent flowers. By pruning them at different periods during the summer and autumn, they will be induced to flower in succession; since, with all roses, the time of blooming is, to a great degree, dependent on the time of pruning.
Roses potted in the manner described for forcing may also be brought into bloom in the sunny window of a chamber or drawing-room. They will bloom much better if allowed to remain at rest in a cool cellar for a month or two after potting.