Photoperiodic responses in plants are actually determined by the duration of darkness and not the period of light. Therefore, when the natural photoperiod is short (September through March), greenhouse growers can employ many different strategies to “break up” the night to promote flowering of obligate or facultative long-day. The same strategy is also employed to inhibit flowering of short-day plants like chrysanthemums. To promote flowering in long-day plants, greenhouse growers often provide day-extension lighting or night-interruption lighting. With either strategy, growers illuminate plants with low-intensity lighting (usually 1-2 µmol∙m-2∙s-1) usually from incandescent, compact fluorescent or light-emitting diode (LED) lamps.
Some growers use cyclic lighting, where lamps that are in a fixed location are turned on and off throughout the night break, in order to save energy and increase the lifetime of the bulbs. A common cyclic lighting strategy used by growers is to turn the lamps on for six minutes and then off for 24 minutes during the four-hour night break. With this strategy, the light should be on for at least 20 to 25 percent of the time during the four-hour lighting period. This strategy is effective on most short-day plants, but may not promote flowering of some long-day plants as much as a continuous four-hour period.
