Cannabis is a short-day plant, similar to poinsettias and chrysanthemums. Consequently, it requires a night cycle long enough to induce flowering. The critical night length, or the amount of time required for the photoperiod response in Cannabis to occur in order to stimulate flowering, is between nine and 10 hours, which corresponds to 14 to 15 hours of daylight, but it varies by cultivar. To induce Cannabis flowering in greenhouse production, plants receive 12 hours of darkness.
For clonal propagation (vegetative cuttings for rooting), cultivators must grow mother stock plants in the fall and winter to ensure there are enough cuttings for them in the spring and summer, or they must grow them indoors under artificial light. This presents a unique challenge in that the lack of natural light the plants are subjected to during the fall and winter would cause flowering. As a result, growers need supplemental lighting to reach a day-length extension of at least 16 hours or to perform night interruption to keep mother plants vegetative for cuttings. This range of external lighting should be adequate, though cultivars may have lower or higher lighting needs.
Keeping plants vegetative can be accomplished in two different ways (Fig. 1): The first is by extending the hours of light plants receive, giving plants 14 to 15 hours of light continuously. Ideally plants should have more than 16 hours of light to be on the safe side; and in fact, most cannabis operations provide 18 hours of continuous light. This method adds hours of daylight as the sun sets or slightly before. The second method is “night interruption.” This method adds a period of lighting in the middle of the night, breaking up the night period to prevent plants from flowering.
Night interruption is a common practice in floriculture production to retain the vegetative state of mother stock plants. However, we could not find research reports that address the suitability of using night-interruption lighting for retaining the vegetative state of Cannabis. Therefore, researchers at North Carolina State University conducted a study that compared 16-hour day-extension lighting to a truncated night-interruption lighting of 12+4 hours of lighting, and with 12-hour short-day lighting on the effects of flowering and vegetative growth of Cannabis. Here are the results:
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