Forcing of Plants

scientific agricultural procedures used to produce flowers, vegetables, and fruits out of season. Forcing of plants is based on a knowledge of their biology and the use of special methods that make possible the regulation of their growth and development in different stages of morphogenesis. Regulation of light, temperature, and carbon dioxide content is very important for most forced crops. Plants are usually forced in the fall, winter, and early spring in special areas (greenhouses, hothouses, hotbeds, and cellars) where it is possible to change environ-mental conditions in the necessary ways.

Among the forced decorative crops are such attractive blooming bushes as the rose, lilac, and hydrangea, and among the decorative perennials are the tulip, hyacinth, narcissus, pink, chrysanthemum, and cyclamen. In the forcing of decorative bushes, plants prepared in advance with well-developed root systems and above-ground parts are used. Warm-water baths in which the above-ground parts of the plant are immersed are used to accelerate their blooming. A 12-16 hour treatment with water at a temperature of 30° to 35° C is usually sufficient. A similar effect is exerted by warm steam baths or by the presence of vapors of ether and acetylene in the air surrounding the plant. Preliminary seasoning at low temperatures is favorable for many plants. When forcing decorative plants under protected soil conditions, supplementary artificial light and carbon dioxide supplements are very important. Growth stimulators are used to change the blossoming times of decorative plants. Plants for forcing are used for different periods of time: roses, five to six years; pinks, two to three years; and bulbs, one year.

Vegetable crops used in forcing include turnip greens, Swiss chard, and chicory for salad and, for greens, parsley and celery. Root crops, bulbs, or rootstocks are planted in winter hothouses and hotbeds of all types during the fall-winter and early spring seasons. Early spring forcing of sorrel, Welsh onions, and rhubarb on open soil with temporary glass or plastic-sheet shelter is also used. The plants are planted in hothouses and hotbeds in a loose nutrient mixture and also in peat, sand, gravel, or keramzit (a porous clay mixture), the use of which requires the application of mineral fertilizers in solution (hydroponics). The soil mixture should be 15-25 cm thick. Forced plants demand lighting only after most of the leaves have appeared. Asparagus and chicory are forced in complete darkness because in light they produce bitter, coarse leaves and sprouts. Planting material for forcing is raised on open soil. During the fall-winter period onions, beets, and rhubarb are planted closely, whereas during the winter-spring period they are planted 2-3 cm apart. During the fall and winter months the forcing period for turnips is 35-40 days, whereas during the winter and spring months it is 25-30 days, with a harvest of 12-16 kg per sq m. For sorrel the forcing period is 35-40 days, and the yield is 3-5 kg; corresponding figures for chicory are 20-30 days and 5-7 kg, and for rhubarb, 30-45 days and 4-6 km per sq m. Strawberries, grapes, and plums are sometimes forced on protected soil.