Light is a primary driver of plant growth. For many cannabis genotypes, the photoperiod plays a crucial role in regulating various physiological processes, including determining when a plant will flower. When growing these genotypes indoors, it is important to know how long the photoperiod should be in different phases of their development.
By managing the duration and intensity of light exposure, growers can control the flowering phase, extend the growing season to facilitate year-round production, and increase yields in doing so.
What is photoperiod?
A photoperiod is the duration of light exposure a plant receives within a 24-hour period. But plants don’t just need light; they need light particles (photons) within 400 to 700 nanometers (nm) in wavelength, which is the range required for photosynthesis. The amount of photons in this range that hit one square meter per second is measured as photosynthetic photon flux density (PPFD).[1]
Together, the photoperiod and the PPFD influence the daily light integral (DLI), which is the amount of photosynthetic light plants receive in a day. DLI is measured in moles (mol). Cannabis is a light-loving crop with a proposed DLI of 40 to 50 mol.
Why managing the photoperiod is necessary
Cannabis varieties are either photos or autos. Photos—short for photoperiod-sensitive—align their development with the amount and timing of light available, while autos develop and reproduce independently of light.
Cannabis is a short-day[2] photoperiod plant, meaning it needs decreasing daylight to trigger reproductive growth. If you’re growing a photo variety indoors, knowing when to shorten the photoperiod, or “flip” the plant from veg to flowering, is crucial.
Studies show that cannabinoids accumulate in cannabis leaves and floral matter differently between growth periods. Vegetative biomass (i.e., leaves and stems) slowly accumulates small amounts of cannabinoids over the plant’s lifespan, while cannabinoid concentrations in inflorescences intensely increase the longer the plant flowers. [3]
If a plant stays in the veg stage because it never receives a shortened photoperiod, it will increase in vegetative biomass and rarely flower.[4]
Timing the veg stage and when to flip
To understand when to shorten the photoperiod, let’s define the three stages of the cannabis life cycle:
- Germination: Seeds germinate and seedlings sprout.
- Vegetative (or “veg”): Young plants grow vegetatively, marked by the proliferation of roots, stems and leaves.
- Flowering: Plants bud and flower until they reach maturity.
During the veg stage, plants should receive at least 18 hours of light to promote vibrant stem and leaf growth. Plants will need to stay in this stage for a few weeks—academic cultivators recommend 14‒21 days—so they can build a canopy framework and increase biomass. Vegetative biomass correlates with floral biomass, so growers may opt for a longer veg stage to maximize plant size and branching, thereby ultimately increasing the floral harvest.
Once plants achieve the desired vegetative size—a factor largely dependent on growing style—growers should shorten the photoperiod to 12 hours to initiate flowering. Continuous darkness for 12 hours ends the veg stage. The vast majority of indoor cannabis growers use a 12-hour light to 12-hour dark cycle to induce rapid and robust flowering.[5] However, research differs on whether this 12:12 cycle is the ideal photoperiod.
One study compared how 10-, 12- and 14-hour photoperiods affect biomass yield and cannabinoids in medicinal cannabis. Researchers found that cannabinoid concentrations more than doubled for a variety under a 14-hour photoperiod during the flowering phase, which also improved floral biomass. Thus, they concluded that the standard 12-hour photoperiod is not optimal for all varieties.[6]
Another study examined how in vitro cannabis plants responded to six different photoperiod treatments—12, 13.2, 13.8, 14.4, 15 and 16 hours per day—conducted in two trials over four weeks. Only the 13.2- and 12-hour photoperiods saw 50% of the plants flower in less than 28 days, and the 12-hour photoperiod had the fastest flowering response, taking only 13 days in the first trial and 19 days in the second. It took 19 days for 50% of plants under the 13.2-hour photoperiod to flower in the first trial and 22 days in the second. The researchers concluded that a photoperiod of less than 13.2 hours should be used for optimal flowering.[7]
Indoor cannabis growers must meticulously plan the timing of photoperiod reductions to meet the day-length requirements of photoperiod-sensitive plants. Although the standard practice is to initiate flowering by reducing the photoperiod from a long-day period to an equal-duration cycle of 12 hours light and 12 hours darkness, there is variability between strains.
Emerald Harvest Team
[1] Kelly, Nathan, Qingwu Meng, and Erik Runkle. 2022. “Photoperiod, light, intensity, and daily light integral.” Produce Grower, March 2022. https://www.producegrower.com/article/photoperiod-light-intensity-and-daily-light-integral/.
[2] Some photoperiod-sensitive plants are long-day, which means they need increasing daylight amounts to trigger reproductive growth.
[3] Dang, Michelle, Nishara Muthu Arachchige, and Lesley G. Campbell. 2021. “Optimizing Photoperiod Switch to Maximize Floral Biomass and Cannabinoid Yield in Cannabis sativa L.: A Meta-Analytic Quantile Regression Approach.” Frontiers in Plant Science 12: 797425. https://doi.org/10.3389/fpls.2021.797425.
[4] Moher, Melissa, Max Jones, and Youbin Zheng. 2020. “Photoperiodic Response of In Vitro Cannabis sativa Plants.” HortScience horts 56 (1): 108-113. https://doi.org/10.21273/HORTSCI15452-20.
[5] Ahrens, Ashleigh, David Llewellyn, and Youbin Zheng. 2023. “Is Twelve Hours Really the Optimum Photoperiod for Promoting Flowering in Indoor-Grown Cultivars of Cannabis sativa?” Plants (Basel) 12 (14): 2605. https://doi.org/10.3390/plants12142605.
[6] Peterswald, Tyson James, Jos Cornelis Mieog, Razlin Azman Halimi, Nelson Joel Magner, Amy Trebilco, Tobias Kretzschmar, and Sarah Jane Purdy. 2023. “Moving Away from 12:12; the Effect of Different Photoperiods on Biomass Yield and Cannabinoids in Medicinal Cannabis.” Plants 12 (5): 1061. https://doi.org/10.3390/plants12051061.
[7] See footnote 4.
