This is the practice of encouraging plants to flower or fruit early. The simplest form of forcing is done by inverting a tub over rhubarb crowns to encourage etiolated growth. Similarly rhubarb, sea-kale, dandelion and chicory can be forced under the greenhouse staging in mild heat and darkness, which also blanches these crops, rendering them more palatable.
Another very simple method of forcing is to cut sprays of winter or spring‑flowering shrubs such as Jasminum nudiflorum and forsythia in bud and place them in a warm room to bring out the flowers before those out of doors bloom. The sudden change from very cold to warm encourages quick growth and is employed in the forcing of flower bulbs. It has been found that some bulbs make better and speedier growth under conditions of forcing if they are pretreated by cold storage.
Daffodils, in particular, respond to this treatment, and are put in cold storage for several weeks during August and September before being forced. Pre-treated bulbs such as ‘Golden Harvest’ and ‘Carlton’ are available to amateurs. Hyacinths, too, are offered as pre-treated bulbs, such varieties as `Ostra’, ‘Pink Pearl’ and ‘Jan Bos’ responding well. They should be planted as soon as they are received, otherwise normal temperatures will nullify the cooling treatment.
The importance of keeping newly planted bulbs for forcing in really cool conditions initially, cannot be overemphasized. A good root system has to be developed before quick growth can be expected, and it is in very cool conditions that root formation takes place. Whether pre-cooled or not, bulbs can be forced by artificial light, where natural light can be excluded completely and the usual temperatures for bulb forcing be maintained 65-75°F (18-24°C).
Light is giver, for any consecutive twelve hours of the twenty-four and one 100-watt lamp is sufficient for 1 square meter (yard) of bulbs when mounted 90cm (3ft) above the bowls or boxes. Commercially the use of such growing rooms is increasing, and artificial light is used in the cultivation of chrysanthemums to provide flowers all the year round, by a system of forcing and delaying. Considerable progress is also being made in the use of carbon dioxide to enrich the atmosphere of both growing rooms and greenhouses, to encourage speedier growth, once leaves are formed, of such crops as lettuce, cucumber, tomato and chrysanthemum.
Many shrubs can be brought into flower early for greenhouse display (or once they are flowering they can be transferred to the house for room decoration) in late winter or early spring. Young plants can be lifted from the ground or, bought specially, potted up and brought into the greenhouse about New Year. Keep a buoyant atmosphere and cool conditions for two or three weeks then increase the temperature gradually, watering and feeding as required. Such plants as Deutzia gracilis, Prunus triloba, Indian azalea, forsythia, philadelphus, syringa, hydrangea and spiraea, respond well to this type of cultivation but it is wise not to force the same plants year after year.