A long night may be broken artificially using a technique called night-break lighting. Incandescent tungsten bulbs produce a high proportion of red light and are cheap to run. Hung about 1 m above the crop and spaced to give about 150 lux for four hours ensures that the Pfr730 critical level is not reached. Cyclic lighting saves electricity and uses a series of brief alternating light and dark cycles to replace one continuous break. High pressure sodium lamps are used where they are installed for supplementary lighting (see p114), this saves expense in providing two systems. Crops such as chrysanthemums can be induced to flower in the summer by imposing a long night regime artificially, using opaque black cloth or plastic curtains to cover the crop (see Figure 11.8). A night of nine to fifteen hours causes the Pfr730 level to drop below the critical limit and the flowering process to be initiated.
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