Last updated: May 18, 2021

What Does Photoperiod Mean?

Photoperiod refers to the time that a plant or animal is exposed to light in a 24-hour period. Many types of plants require certain lengths of light exposure to enter various life cycle stages.

Growers frequently control the photoperiod in a plant’s life cycle through the use of grow lights to encourage the plant’s vegetative state, early flowering, bud phase, and ultimate harvest. Some plants also respond favorably to a longer than natural photoperiod by producing a more abundant harvest yield.

In cannabis cultivation, plants in the vegetative stage are typically given an 18/6 or 24/0 photoperiod, with the first number representing how many hours of daylight per 24 hours to provide, and the second number representing the amount of darkness hours per 24-hour period.


Maximum Yield Explains Photoperiod

In photoperiod-dependent plants, exposure to very specific periods of light is what triggers various plants to enter their life cycle phases. Some plants are considered long-day plants, requiring require days that are longer than their critical day length time to flower. Short-day plants will flower on shorter day lengths. In the middle, there are day-neutral plants, requiring equal amounts of light and darkness.

For example, cannabis is a short day plant. In the vegetative stage, cannabis plants should ideally receive at least 18 hours of light and 6 hours of darkness in a 24-hour time period. This is represented as 18/6. Some professional growers follow a 24/0 photoperiod where they leave their lights on for 24 hours a day. When indoor-grown plants move into the flowering stage, the light period, or their photoperiod needs, change to 12 hours of light, 12 hours of darkness. These are only examples; some plants require more or less light or darkness and growers often experiment with different light schedules.

Plant cultivars have commonly been bred to require very specific photoperiod times in order produce a bountiful harvest. Some plants also only respond to a photoperiod when they are young and not upon maturity. Such plants simply produce flowers based on their age and not light exposure, such as autoflowering cannabis.

Photoperiodism and photoperiod are two related terms that are used interchangeably.




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