There are three components in the equation that yields the optimum amount of light for plant growth. The first two involve the strength of the light (called candlepower) and the distance of the fixtures from the plants. As discussed, the needs of different plants depend largely on their relative size. The third element in the equation is time, the number of hours at a stretch that the bulbs should be on. The sun doesn't shine 24 hours a day (except in summer above the Arctic Circle) and your plant lights should not be in continuous use either.
Interestingly, plants don't only need light for proper growth-they also need darkness. The cycle is called the photoperiod, and it varies from variety to variety. Many vegetables require 14 to 18 hours of artificial light and 6 to 10 hours of darkness. Among these long-day plants are beets and spinach. Intermediate-day plants, such as celery, require 12 to 15 hours of light, while short-day plants, such as some types of onions, need only 10 to 13 hours of light It's important to pay attention to photoperiods. If you leave your lights on too long over, say, tomatoes, you'll find that your plants will produce lots of attractive green foliage, but few tomatoes. If you don't have the kind of schedule that will enable you to control these lights manually, use one of the many good automatic timers on the market
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