Cannabis plants need light to absorb and thus, to grow. Some growers believe that exposing and subjecting them to a 24-hour light source indoors will help them cultivate faster in their vegetative state. The more light they absorb and photosynthesize, the more they grow, right? True. Is this the most efficient and suitable for them as short-day plants? No.
Cannabis undergoes two growth phases, vegetative and flowering. Both phases require different photoperiods. Cannabis, as a short-day plant,requires at least 12 hours without light to prompt its flowering phase(1). On the other hand, when undergoing its vegetative growth,growers experiment with different light cycle combinations such as 18/6 (18 hours with lights on, 6 hours of darkness) and 20/4. These photoperiods are claimed to provide abundant time for the plants to receive light and also time for their much-needed dark cycle. Thus, 24 hours of continuous light for Cannabis is not as advantageous as it seems.
Disadvantages of 24 hours of Exposure
We can look into and categorize these downsides in two different perspectives, the plants and the grower’s.
As for the plants’, a study entitled “Plants under Continuous Light: A Review” (2010) concluded that while there are plants that benefit from continuous light exposure, there are types of plants that can attain physical or external damages from the over-exposure(2). “Growing Indoor Plants with Success” authored by Bodie V. Pennisi (2017) seems to support this notion and even gave an example of a plant suffering from scorched leaves because of overexposure (3). Though both studies did not cover cannabis, we can take it with a grain of salt that cannabis, as it is classified as a short-day plant (SDP) as pointed out earlier, does not fully gain from the 24-hour exposure. For one, 24/0 prevents them from transitioning to their flowering stage.Problems may also arise if we force the plants to grow as fast as possible, testing the limits of their genetics.
As for the grower’s perspective, keeping your electrical light sources plugged on throughout the day for weeks does not sound wise on the budget. The “benefits” of exposing the plants 24-hour do not compensate for the additional electricity expenditures it may cost you.
Why Plants need Dark?
Anything too much is bad. That is how the saying goes and it pretty much sums up what you need to know about growing cannabis plants indoors. As much as the plants grow faster with continuous grow light in their vegetative state, we do not want them to indefinitely be stuck in that stage. Cannabis plants need at least 12 hours of uninterrupted darkness to induct the flowering phase.
“Photoperiodic Control of Flowering in Plants” (2014) explains how photoperiodism is a mechanism of organisms to adapt to changes in light and darkness(4). The variance in the duration of light and dark periods trigger certain internal processes such as flowering for plants. This study also elaborates on the concept of the circadian clock in plants, which allows them to anticipate seasonal and environmental changes. This leads us to the fact that without darkness to jumpstart the flowering process for Cannabis, it would not happen at all.