With indoor production of cannabis on a rising steep curve, growers have continually been trying to find mechanisms through which production can be optimized and sizeable yields realized.
In cannabis strains sensitive to photoperiod, manipulation of photoperiod—the length of time per day that plants receive light—is one of the most powerful tools for controlling growth and flowering.
It has been common practice in the cannabis industry to abruptly induce flowers in photoperiod plants by transitioning them from an 18–24-hour vegetative photoperiod to a 12-hour photoperiod.
Several recent studies, however, indicate that small increases in photoperiod length without excessive flowering could result in disproportionately large increases in yield without loss of quality.
In a related study, Ahrens et al. (2024) conducted a trial in the journal Plants on the impact it would have to have a 13-hour flowering photoperiod instead of the standard 12-hour photoperiod for high-THC cannabis cultivars cultivated in indoor controlled environments.
Results were dramatic, showing 35 to 50% increases in inflorescence yields across treatments improved to a 13-hour photoperiod. Specifically, increased yields were evident, with all such observed increases in this range as large as four to six times the amounts expected by an increase in the total daily light integral the plants receive.
In addition, there were no negative impacts of the 13-hour treatment on cannabinoid potency or profiles at harvest.
“These findings suggest that the commonly employed practice of inducing flowering under 12-hour days is likely leaving significant yield potential on the table for most cannabis strains. Photoperiods slightly above 12 hours may then represent a simple, low-cost, and highly effective means through which indoor cannabis production can be enhanced.”
Flowering Time and Yield Effects
One of the most noteworthy findings in this study was the large effect size of the flowering yield increases under the longer 13-hour photoperiod compared to the 12-hour control.
On the same note, 35% more inflorescence yield weight was delivered to ‘Incredible Milk’ and 35% more to ‘Gorilla Glue‘.
The 13-hour photoperiod supplied only 8% more total daily light than the 12-hour treatment, yet inflorescence yields jumped by 35-50%.
Nutrient and Carbon Dioxide Revised Concentrations.
One such factor could be a slight delay in the time of flowering onset, allowing plants to accrue more vegetative biomass before floral initiation.
The authors established that the typeof transition to flowering was retarded by around 1.5 days in Incredible Milk with the 13 hour light regime, whilst Gorilla Glue did not show any similar delay.
Indeed at harvesting Incredible Milk and Gorilla Glue grown under the 13 hour days had developed vastly more above ground biomass, thicker stems and hence more vegetative growth than the same strains grown under 12 hour days.
However, larger plant size was not the only factor involved, because the proportional increase in yields was greater than the gain of greater plant sizes.
There were marginal changes in floral developmental trends as part of the cause and effects, which differed according to cultivars.
Incredible Milk showed a 36% reduction apical inflorescence density under 13-hour days, thus giving the assumption that the buds grew to be bigger but not denser, while there was no change observed in the case of Gorilla Glue.
The general study concluded that even small prolongations of the photoperiod at flowering proved to be worth it in terms of important improvements of yield.
While the relative effect was a little different in the two cultivars, both showed massive gains in bud production that far outstripped the increases in vegetative growth and light energy.
What is a clear takeaway from this is the fact that greater yields from both cultivars suggest the 12-hour photoperiod standard within the industry related to cannabis flowering is often less than optimal to realize maximum yield potential.
Cannabinoid Content and Quality Implications
A quality-related question might be whether such increases in yield seen under longer flowering photoperiods are actually achieved for free or at no such great divergence of the cannabinoid properties or profiles.
Fortunately, the findings from this research indicate that the cannabinoid content of inflorescence is not on the decline, and this might perhaps even be slightly better improved with marginally longer flowering days.
Major and minor cannabinoids were analyzed in the dry apical buds at harvest. In the Incredible Milk very high-THC cultivar, buds harvested at 13-hour photoperiod showed 9% higher total THC concentration compared to those at 12-hour, driven by THCA, which was increased by 10%.
In Gorilla Glue, there were no significant effects of photoperiod on total THC or THCA.
For the other measured cannabinoids, the buds of Incredible Milk with a longer photoperiod had 53% higher concentrations of CBGA and 19% higher CBDA, while there was no difference between the measurements of CBG and Delta 9-THC for either cultivar.
No significant levels were detected from the other minor cannabinoids, CBN or CBC, or very minor quantities of THCV in all the samples.
The lack of any major reductions in cannabinoid potency is encouraging, showing that the inflorescence chemical quality does not need to be sacrificed when elongating flower times to boost yields.
These differences could indicate that under 13-hour days, more precursor compounds like CBGa and CBDa are concentrated in higher levels in IM rather than in Gorilla Glue, indicating a slightly less mature profile until that point.
However, the high levels of total THC and the low levels of CBN content, often used as an indicator of overripeness or degradation, would lead to the suggestion that the 13-hour samples were still well within their appropriate window of harvest.
So, regarding key measures of cannabinoid potency and profile, there is no apparent decrease in quality but several possible gains by utilizing the extended, 13-hour photoperiod.
Higher yields did not seem to come at the cost of reduced quality or chemical fidelity.
Putting Findings into Action
This line of evidence indicates that most proprietary cannabis varieties should be able to achieve appreciably increased flowering productivity at photoperiods only modestly lengthened from the usual 12-hour photoperiod.
Extending the length of the day to 13 hours of flowering shows to be a very effective method to increase the yields of cannabis without any reduction of the profile of cannabinoids, and consequently without negative effects on the quality of the harvest.
For indoor growers, running slightly longer flowering days doesn’t require a change in lighting hardware or intensity; it’s a relatively simple adjustment. It’s an easy way to gain efficiency, allowing more output to be achieved from the same space and equipment.
With this range of individuals outperforming the control by between 35% and 50%, the increases in flower yields are highly significant with respect to improving space-use efficiency and reducing operating expenses.
Although the 13-hour photoperiod yield improvement was beneficial to all the tested cultivars, the magnitude of improvement from that increase was different between the two strains.
So, while examining the slight hints of delayed flower maturity in one cultivar, this suggests that optimal photoperiod may vary a little around the 13-hour target from strain to strain.
It would be appropriate for a grower to do trials of modestly longer flowering photoperiods to home in on the best balance of yield and quality. Timing of flowering and inflorescence density responses could be cultivar‐specific as well, so scheduling and effects on quality need to be evaluated in trials for a cultivar.
Also, changes in plant spacing or modification in training practices may be necessary as plants grow larger.
The potential for a dramatic increase in efficiency via photoperiod manipulation is an exciting finding for the industry in general.
This study is very important, in as much as hopefully it will teach us that the old dogma of 12 hour NDL [flowering on a 12-hour light/12-hour dark cycle] does not have to limit yield potential and that working with the plant’s photoperiod physiology is a powerful tool to really take indoor cultivation to the next level.
