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Moving away from 12:12: The effect of different photoperiods on medicinal cannabis

"The standard practice to initiate flowering in medicinal cannabis involves reducing the photoperiod from a long-day period to an equal duration cycle of 12 h light (12L)/12 h dark (12D). The fact that this blanket rule is assumed to be optimal for all varieties is quite remarkable considering the diverse latitudinal origins of cannabis and known variation in photoperiod-dependent and -independent (a.k.a auto flower trait) flowering time control in cannabis." In a recent study, researchers sought to identify the effect of nine different flowering photoperiod treatments on the biomass yield and cannabinoid concentration of three medicinal cannabis varieties. "The results show that the assumption that a 12L:12D photoperiod is optimal for all lines is incorrect. In some lines, yields can be greatly increased by a lengthened light period during flowering."

The study 
The three varieties used in the study were: 'Cannatonic,' a high CBD-accumulating line, as well as 'Northern Lights' and 'Hindu Kush,' two high THC varieties. "The nine treatments tested, following 18 days under 18 h light/6 h dark following cloning and propagation, included a standard 12L:12D period, a shortened period of 10L:14D, and a lengthened period of 14L:10D. The other six treatments started in one of the aforementioned and then 28 days later (mid-way through flowering) were switched to one of the other treatments, thus causing either an increase of 2 or 4 h or a decrease of 2 or 4 h."

The results showed differences between the three varieties in their response to different light treatments, with some significant yield increases in response to 14L. "Most significantly, the high-CBD line (Cannatonic) showed cannabinoid yield increased to more than double when an early 14L photoperiod was applied compared to the standard 12L. In contrast, one of the two high-THC lines tested (Northern Lights) only showed a 50% yield increase in the 14L < 12L treatment, whereas the second high-THC line tested (Hindu Kush) did not show any significant yield effects."

According to the researchers, all three varieties showed a positive response to 14L in the early flowering phase with regard to height and flower biomass, which can be explained as a result of the extra energy available for photosynthesis. For two varieties (Cannatonic and Hindu Kush), the 14L treatment tended to lead to more flower biomass than 14 > 12 or 14 > 10, indicating that the reduction in energy later in the flowering phase negatively affected flower initiation and/or development. "The absence of this effect for Northern Lights is possibly a result of earlier flower maturation in this variety, with the more-developed inflorescences less affected by the reduction in available light energy. Alternatively, stored carbohydrate reserves could have been remobilized closer to harvest, acting as a buffer against the reducing photoperiods in the second half of flowering."

"Our results showed that distinct varieties can exhibit markedly different responses to changes in photoperiod length, and the standard photoperiod for the flowering phase of 12L:12D is not optimal for all varieties," the researchers say. "In particular, cannabinoid yields can be more than doubled by increasing the photoperiod during the flowering phase from 12 h to 14 h, as demonstrated by the Cannatonic line, with the increase in cannabinoid yields driven by gains in both flower biomass and flower cannabinoid concentration. A 14L > 10L photoperiod also achieved a strong yield benefit which utilizes the same number of light hours as 12L and therefore incurs no extra electricity costs. For one high-THC line, a 14L > 12L photoperiod increased THC yields by 49%, driven by a gain in biomass only (no change in % THC), whereas a second high-THC line did not show any significant differences. As this treatment also benefitted Cannatonic, this may be the best 'all-round' treatment optimal for mixed cultivation and untested varieties," the researchers conclude.

To read the complete study, go to

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