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Is twelve hours really the optimal photoperiod for cannabis?

"The use of a 12-h photoperiod has become widespread in the indoor cannabis cultivation industry due to its ability to consistently induce rapid and robust flowering responses in photoperiod-sensitive cannabis cultivars. Moreover, this protocol allows different cultivars to be grown concurrently in the same environment, further contributing to its almost ubiquitous use in the industry. However, the photoperiod responses of modern, indoor-grown cannabis cultivars are not well characterized," according to a new research conducted by the University of Guelph.

The researchers say that the hybridization of genotypes with various photoperiod responses has likely given rise to some modern cannabis cultivars with intermediate (i.e., >12 h) photoperiod responses. "Therefore, it is likely that photoperiods longer than 12 h could be used to transition some indoor-grown cannabis cultivars from vegetative to flowering stages. In so doing, daily crop exposure to photosynthetic light could be increased accordingly, potentially resulting in higher yields." Therefore, the study investigates the photoperiod responses of multiple indoor-grown cannabis cultivars and models the dynamics of flowering initiation and early inflorescence development.

The study found four cultivars (Blue Dream, Black Triangle, Incredible Milk, and Garlic Jelly) that initiated flowering most rapidly when grown under photoperiods between 12.4 h and 12.7 h. Additionally, they found four cultivars (Ghost Train Haze, Legendary Larry, Gorilla Glue, and OG Kush) with no delay in flowering initiation up to 14 h. Furthermore, two cultivars (Chem de la Chem and Powdered Donuts) demonstrated linear increases in EDTF with increasing photoperiod, but the delays were only 1.3 d and 2.3 d, respectively, at 13 h vs. 12 h. In addition, three cultivars (Blue Dream, Ghost Train Haze, and Gorilla Glue) initiated flowering under all photoperiod treatments, with only one of these cultivars (BD) having delayed flowering initiation under photoperiods longer than 13 h. "While these findings suggest that photoperiods longer than 12 h may provoke flowering initiation responses in indoor-grown cannabis, they also illustrate the cultivar-specificity of cannabis' photoperiod responses."

The photoperiod responses of the harvest parameters also varied considerably between the cultivars. "While flowering had initiated in three cultivars under 15 h (Blue Dream, Ghost Train Haze, and Gorilla Glue), they did not continue to develop beyond the flowering initiation stage. Contrasting with the 15-h treatment, floral tissues of all cultivars in the 14-h treatment continued to develop beyond the initiation stage into inflorescences. However, the floral tissues were generally smaller and less well developed than in plants under the shorter photoperiod treatments."

The harvest index (HI), which portrays the relative biomass allocation between vegetative and reproductive (i.e., floral) tissues, ranged from approximately 0.05 to 0.3, depending on the cultivar and photoperiod. "Three cultivars (Black Triangle, Garlic Jelly, and Powdered Donuts) all had maximum HI below 0.1 (i.e., <10% floral biomass). In comparison, the HI in three other cultivars (Blue Dream, Ghost Train Haze, and OG Kush) was higher than 0.2 in some treatments, illustrating considerable phenotypic variability in floral tissue development during the early part of the flowering stage. Five cultivars had negative linear responses for both floral biomass and HI to increasing photoperiod."

"When all of these responses are combined, they indicate that both vegetative and total aboveground biomass increased with increasing photoperiod in these cultivars. This is potentially due to the inherently higher DLIs under longer photoperiods," the researchers explain. "Since vegetative growth of indoor-grown cannabis normally virtually ceases after four to five weeks under a 12-h photoperiod, the HI of all tested cultivars should increase substantially during the remainder of the flowering stage. Therefore, if longer photoperiods promoted additional vegetative growth during the early phases of the flowering stage, either by changing the relative amount of vegetative vs. reproductive growth or simply from higher daily light exposure, this could result in higher yields at commercial floral maturity." The researchers advise cannabis growers to investigate the effects of photoperiods longer than 12 hours (up to 14 hours) on provoking robust flowering responses and increasing yields in their individual cultivars.

To read the complete study, go to