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Grow Light Duration
Provided by Advanced Nutrients
Duration of light and dark periods, or photoperiodism, is also important to a plant's growth and reproduction. During the propagation stage, lights should be left on as long as possible, up to 24 hours per day. Strong initial growth will make the young plants healthier and more resistant to disease, ultimately providing better yields. As the plant begin to mature, however, light duration should be gradually reduced.
Most plants do well with fourteen to sixteen hours of light per day, during the vegetative stage. In a greenhouse setting, supplemental lights can be used on cloudy days or to extend daylight hours in the winter. With supplemental lighting, plants can be kept actively growing all year long, making out of season fruits possible. Dormancy can be prevented in some crops, and winter fruiting can be induced by providing extra light in the fall. If you have a sealed grow room, all that is meaningless. You control the seasons, you control night and day.
Plants use periods of darkness, too. During dark periods, plants continue to take in oxygen, and give off carbon dioxide in a process called respiration. During this time, the plant uses some of the energy that was stored as sugars during photosynthesis, producing proteins, hormones, and other complex products. Therefore, lighting mature plants more than seventeen and a half hours per day will not increase production significantly, since photosynthesis will slow down after a certain number of hours. In fact, too many daylight hours can actually be counterproductive, delaying or preventing flowering in some crops.
Most plants are sensitive to day length. Some plants flower during the spring and summer, when days are longer than nights, and some flower in the autumn, when nights are longer than days. Therefore, plants are classified as short-day, long-day, and day-neutral.
A hormone called "florigen" controls budding and flowering. Long day plants require about 14 to 18 hours of light to produce just the right amount of florigen to flower and reproduce. Short day plants require about 10-13 hours of light. If short day plants are exposed to too many hours of light, florigen can be destroyed, preventing blooming.
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