Indoor-grown cannabis is commonly transitioned to a 12-hour daily photoperiod to promote flowering. However, previous research has shown that some indoor-grown cannabis cultivars can initiate strong flowering responses under daily photoperiods longer than 12 hours. Since longer photoperiods inherently provide higher daily light integrals (DLIs), they may also increase growth and yield. Therefore, in a recent study, two THC-dominant cannabis cultivars, 'Incredible Milk' (IM) and 'Gorilla Glue' (GG), were grown to commercial maturity at a canopy level PPFD of 540 µmol·m−2·s−1 from white LEDS under 12 h or 13 h daily photoperiod. "A 12 h flowering-stage photoperiod may not be optimized for maximizing the yield of all cultivars. Hence, cultivators who use a 12 h photoperiod for all cultivars may be 'leaving yield on the floor,'" the researchers say.

Results
In the 13 h treatment, the researchers observed a 35% increase in the total inflorescence yield. Yet the observed yield increases under longer photoperiods were disproportionately higher than the increases in DLIs. "Other developmental and morphophysiological responses to longer photoperiods may be contributing to the yield enhancements," the researchers say.

While some indoor-grown cannabis cultivars can flower under photoperiods longer than 12 h, increasing the photoperiod may moderate the speed and intensity of the transition to reproductive growth. This was seen in the study by the delayed time to visible inflorescences in IM and slower early floral development in both cultivars. However, delayed or repressed flowering initiation in the 13 h photoperiod led to enhanced rather than repressed inflorescence biomass when plants reached commercial maturity. "Since much of the vegetative growth after switching to the flowering photoperiod occurs during the first few weeks after the transition to short days, enhancements in vegetative growth during this period increase foliar biomass, probably enhancing light interception and thus growth potential. Enhanced vegetative growth during the early phases of the flowering stage may also increase the number of potential flowering sites and capacity for structurally supporting higher floral biomass. The plants of both cultivars in the 13 h treatment in the present study had higher growth indexes and appeared to be larger."

"Simple and cost-effective"
Despite the early delays in inflorescence development, by the time the plants in the 12 h treatment reached commercial maturity, the total inflorescence yield and the size of the apical inflorescences were markedly higher in the 13 h treatment in both cultivars. "Given that the increases in inflorescence yield were disproportionately higher than the increase in DLI, flowering photoperiod management may be one of the most efficacious cultural practices available to commercial indoor cannabis cultivators for increasing yield that is both simple and cost-effective to utilize. Furthermore, despite the plants in the 13 h treatment appearing to have delayed inflorescence maturation rates (e.g., slower stigma browning and reduced trichome ambering), the cannabinoid composition in the apical inflorescences was of comparable quality in both treatments in both cultivars," the researchers conclude.

The researchers add that cannabis' photoperiod responses are strongly cultivar-dependent. "Growers must investigate the effects of photoperiods with their own specific cultivars and cultivation systems."

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