Every indoor grower has heard it a thousand times: flip to 12/12 to flower for photoperiod strains. But what if that long-held rule, twelve hours of light followed by twelve of darkness, isn’t quite the sweet spot we think it is?
We have another fascinating piece of research to share this week. A 2023 study from the University of Guelph asked the question head-on: Is twelve hours really the optimum photoperiod for promoting flowering in indoor cannabis? Here’s what the researchers found, and what it means for anyone growing under LEDs today.
Background: The 12/12 Tradition
When cannabis cultivation moved indoors, the 12-hour dark cycle became gospel. The reasoning was simple: most drug-type C. sativa strains behave as short-day plants. Once day length drops below a critical threshold, they stop vegetative growth and begin flowering.
Twelve hours of light became the industry standard because it was easy to program, it reliably triggered flowering across most genetics, and it allowed multiple cultivars to share the same room without light-response conflicts.
But that convention was never rigorously tested on modern indoor hybrids. Today’s genetics often combine high-latitude and equatorial lineages, producing a spectrum of photoperiod sensitivities, from strict short-day cultivars to semi-autoflowering phenotypes. It’s entirely possible that some strains could handle (or even prefer) a longer day length, benefiting from more photosynthetic light before the lights go out.
The Experiment
Researchers Ashleigh Ahrens, David Llewellyn, and Youbin Zheng grew ten popular THC-dominant cultivars, each with typical potency above 19% THC, in a controlled-environment growth chamber.
The lineup included well-known varieties such as Blue Dream, Gorilla Glue, OG Kush, and Ghost Train Haze, spanning the full indica–sativa spectrum.
They exposed clones of each cultivar to six different photoperiods over a 24-hour cycle:
- 12 h light
- 12.5 h
- 13 h
- 13.5 h
- 14 h
- 15 h
Light intensity averaged ≈ 360 µmol m⁻² s⁻¹ from full-spectrum white LEDs, with careful shielding to eliminate stray light (less than 0.05 µmol m⁻² s⁻¹ at canopy level).
The plants grew for 3–4 weeks under these schedules. Researchers tracked:
- Elapsed Days to Flowering (EDTF) – the time until each plant produced at least three pairs of stigmas on the primary shoot.
- Inflorescence Metrics – fresh weight, volume, and total floral biomass.
- Harvest Index (HI) – the proportion of total above-ground mass allocated to flowers versus vegetative tissue.
What Happened When Light Hours Increased
1. Flowering Still Happened, Up to a Point
All ten cultivars successfully initiated flowering under photoperiods up to 14 hours. In some cases, flowering was slightly delayed, by zero to four days compared to the 12-hour control, but not halted.
A few cultivars even began to flower under 15 hours, but their floral tissues never progressed beyond initiation; stigmas appeared, then stalled. In other words, they entered the “idea” of flowering but never developed marketable buds.
Takeaway: 14 hours still worked for nearly every strain tested; 15 hours was too long.
2. Longer Days Didn’t Ruin Flowering, but Yields Varied
When researchers compared flower weights and volumes, patterns differed by cultivar:
- Blue Dream, Black Triangle, and Powdered Donuts showed quadratic responses, meaning their best yields occurred around 12.6 – 13 hours of light.
- Chem de la Chem, Legendary Larry, and OG Kush performed best at the traditional 12 hours, declining steadily beyond that.
- Others, like Ghost Train Haze and Gorilla Glue, showed no significant difference up to 14 hours.
The data suggest that for some genetics, stretching the day by even half an hour can slightly boost early floral biomass, likely because of the higher Daily Light Integral (DLI), the total light received per day.
3. The Harvest Index Told the Real Story
The Harvest Index (floral FW / total FW) revealed how each plant allocated energy between vegetative growth and flower formation.
Five cultivars reached their maximum HI between 11.8 h and 12.9 h, reinforcing that the “sweet spot” hovers close to 12 hours for most strains. Yet others maintained respectable floral ratios up to 13.5 h.
That nuance matters for commercial growers. A small extension in light hours could increase overall biomass (thanks to more DLI) without dramatically slowing floral development, provided your cultivar tolerates it.
Why It Matters: DLI, Energy, and Economics
For indoor cultivators paying by the kilowatt, light-hour decisions have real cost implications.
Every additional hour of light adds roughly 8% more DLI (and power cost) per hour. A 13-hour schedule yields ≈ 8% more light; 14 hours, about 17%. If that extra light meaningfully increases yield or potency, it might justify the energy expense. If it only produces more leaves, it’s wasted electricity.
This study hints that some strains could harness that extra energy efficiently, particularly those with sativa heritage, while others simply elongate vegetatively without proportionate bud gain.
In short: longer days = more photons = potentially more yield, but only if your genetics can translate light into flowers rather than stems.
Context from Other Research
The Guelph team’s results align with several recent studies:
- Peterswald et al. (2023) found that medicinal cannabis grown under 14 h of light achieved ≈ 30% higher floral yield than at 12 h, largely due to increased DLI.
- Zhang et al. (2021) observed minimal delay in flowering for hemp cultivars up to 13.5 h, with 1–2-day delays at 14 h.
- Moher et al. (2021) reported similar initiation timing between 12 h and 13.2 h in tissue-culture plants.
Taken together, these studies challenge the one-size-fits-all 12/12 formula. Flower initiation appears flexible in many modern hybrids, especially those bred from mixed geographic lineages.
Limitations to Keep in Mind
The trial lasted only three to four weeks, roughly the first third of a normal flowering cycle. The team stopped before full maturity, so we can’t say whether the modest early gains under longer light hours translate into heavier final yields or richer cannabinoid profiles.
Also, plants were grown at high density with moderate light (≈ 360 µmol m⁻² s⁻¹), far below commercial PPFD levels of 800–1000. Under stronger light, photoperiod effects might differ.
Still, the data establish that flowering initiation, the crucial flip point, can occur at day lengths up to 14 hours for many cultivars without catastrophic delay.
Practical Takeaways for Growers
1. Know Your Genetics
Photoperiod response is cultivar-specific. Indica-dominant lines such as ‘Chem de la Chem’ and ‘Legendary Larry’ stuck closely to 12 hours, while sativa-leaning cultivars like ‘Blue Dream’ tolerated longer days. If you run multiple strains in one room, the conservative 12/12 remains safest.
2. Experiment Cautiously with 12.5 – 13.5 Hours
If you’re growing a single cultivar and can track development precisely, try extending the light period by 30–90 minutes. Watch for:
- Delays in pistil formation
- Excessive vegetative stretching
- Slower trichome development
If flowering stays on schedule and buds form normally, the extra light may improve biomass and cannabinoid yield.
3. Mind the Energy Curve
Adding one hour of light increases both potential yield and electricity use by ≈ 8%. Compare grams per kilowatt-hour, not just grams per plant, to decide whether the longer day pays off.
4. Avoid 15 Hours Unless You’re Testing
At 15 h, flowering stalled entirely in most cultivars. Those plants produced stigmas but no developing buds, essentially wasting energy and time.
The Bigger Picture: Rethinking the “Flip”
For decades, the 12/12 schedule has been treated as biological law. This study reminds us it’s really an industry convention, not a universal constant. Cannabis evolved across latitudes, from equatorial day-neutral landraces to Himalayan short-day indica, and modern hybrids carry bits of both worlds.
A cultivar’s “critical photoperiod” may vary anywhere from 12 to 14 hours. Understanding where each strain sits on that curve allows growers to fine-tune DLI, potentially increasing yield without extending total production time.
As lighting technology advances and sensors make precision control affordable, photoperiod could become another optimization lever, right beside PPFD, CO₂, and nutrient management.
Conclusion: 12 Hours Is Reliable, Not Sacred
Ahrens et al. (2023) concluded that many indoor cannabis cultivars are capable of initiating flowering under day lengths up to 14 hours, though inflorescence size and early floral yield tended to decline beyond 13 hours. Future studies should follow crops through full maturity and analyze cannabinoid content under different light schedules.
For now, the takeaway is straightforward:
- 12 hours = the most consistent trigger.
- 12.5 – 13 hours = a promising frontier for light-efficient yield gains.
- 14 hours = risky but workable for some sativas.
- 15 hours = too long; flowering stalls.
If you’re running LEDs in a tightly controlled room, small photoperiod tweaks could become one of the simplest ways to squeeze more value from each watt.
So the next time someone insists that 12/12 is the only way, you can point to the data, and remind them that in cannabis, as in science, rules of thumb are made to be tested.
Citation:
Ahrens, A., Llewellyn, D., & Zheng, Y. (2023). Is Twelve Hours Really the Optimum Photoperiod for Promoting Flowering in Indoor-Grown Cultivars of Cannabis sativa?Plants, 12(14), 2605.
