A 13 Hour Photoperiod in Cannabis Flower: What 2024 Research Actually Showed

A 13 Hour Photoperiod in Cannabis Flower: What 2024 Research Actually Showed

Twelve hours on, twelve hours off. That has been the default flowering schedule for indoor photoperiod cannabis since at least the 1970s. It is in every grow guide, every nutrient calendar, every grow tent manual. The number is so universal that almost nobody questions it. But a small group of cannabis researchers has been quietly testing whether 12/12 is actually the right number, and a 2024 paper out of the University of Guelph just delivered the cleanest answer yet: a 13 hour photoperiod produces 35 to 50 percent more bud per plant with no loss in THC.

This post breaks down what the study actually did, why a 13 hour photoperiod gives so much more yield than the extra hour of light alone would predict, and how a home grower might cautiously test it without blowing up their entire flowering tent. Two earlier peer-reviewed papers from the same group already pointed in this direction; the 2024 paper closed the loop on two of the most popular high-THC cultivars on the planet.

The Quick Answer: A 13 Hour Photoperiod Beats 12/12 by 35 to 50 Percent

In a 2024 controlled trial published in the journal Plants, two THC-dominant cannabis cultivars (Incredible Milk and Gorilla Glue) were grown to commercial maturity under either a 12 hour or a 13 hour photoperiod. The 13 hour photoperiod produced 1.35 times the inflorescence yield in Incredible Milk and 1.50 times the inflorescence yield in Gorilla Glue. THC concentrations either held steady or improved. The grow rooms were identical in every other way — same room, same nutrients, same light intensity per hour. Only the day length changed.

That is a substantial result. A single hour of extra light per day raised total daily light integral by only about eight percent, but yield rose 35 to 50 percent. The disproportion is the interesting part. If you simply got eight percent more bud you could write it off as more photons in equals more flower out. Forty-plus percent gain off an eight percent light bump suggests a 13 hour photoperiod is doing something to the plant beyond just feeding it more light.

Why 12/12 Became the Cannabis Default in the First Place

The 12/12 rule is a holdover from how indoor cannabis growing was first reverse-engineered. Cannabis is a “short-day” plant — it flowers when nights become long enough to trigger a hormonal shift. Indoor growers in the 1970s wanted to force flowering on demand, so they picked 12 hours of darkness as a safe number that would reliably trigger the response in any cultivar. It worked. It became orthodoxy.

What got lost is that 12/12 is the minimum photoperiod that triggers flowering, not the optimum one for maximizing yield. The plant flowers fine at 13 hours of light. It also flowers fine at 13.5 in some cultivars. But because every grower assumed 12/12 was the answer, almost no one was running side-by-side trials with longer days. The University of Guelph group changed that.

What the 2024 13 Hour Photoperiod Study Actually Did

Ahrens, Llewellyn and Zheng published “Longer Photoperiod Substantially Increases Indoor-Grown Cannabis’ Yield and Quality” in Plants, volume 13, article 433. Two cultivars: Incredible Milk (a high-THC photoperiod strain) and Gorilla Glue (one of the most widely grown high-THC cultivars in North America). Both grown to commercial harvest maturity. Two treatments: 12 hours of light vs 13 hours of light per day, run side by side in matched rooms.

The headline numbers from the 13 hour photoperiod treatment:

• Incredible Milk: 1.35× the inflorescence biomass of the 12/12 control.
• Gorilla Glue: 1.50× the inflorescence biomass of the 12/12 control.
• Daily light integral (DLI): 23.8 mol/m²/day at 12h vs 25.7 mol/m²/day at 13h. About eight percent more total light.
• Cannabinoids: Major cannabinoid concentrations in the apical bud tissue at maturity were either unchanged or higher under the 13 hour photoperiod. Total equivalent THC went up in Incredible Milk and held steady in Gorilla Glue.
• Flowering initiation: Gorilla Glue flowered on schedule under both treatments. Incredible Milk flowered roughly 1.5 days later under the 13 hour photoperiod, but more than made up for that delay in final biomass.

The paper builds on two earlier studies from the same lab. Moher and colleagues in 2022 (Plants 12:1061) tested a range of shorter and longer photoperiods on a CBD-dominant cultivar and found 12/12 was nowhere near optimal. Peterswald and colleagues in 2023 (Plants 12:2605) asked directly “Is twelve hours really the optimum?” and answered no for several cultivars. The 2024 13 hour photoperiod paper is the cleanest two-cultivar test in the series.

Why a 13 Hour Photoperiod Pulls So Much Extra Yield

The eight percent more DLI does not explain the 35 to 50 percent more bud. Something else has to be going on. The authors and the broader photoperiod literature point to a few likely contributors when you run a 13 hour photoperiod in cannabis flower:

• Longer photosynthetic window per day. The plant has an extra hour to do work. Cannabis is a C3 plant and continues to fix carbon at light intensities much higher than 12/12 typically delivers. An extra hour at peak light is direct carbon gain.
• Delayed but not repressed flowering. Some cultivars take an extra day or two to commit to flowering under a 13 hour photoperiod. That short delay means more vegetative biomass at the moment of stretch, which translates into a larger frame to put bud sites on. The plant ends up bigger before it puts on weight.
• More efficient resource allocation in late flower. The 2024 paper noted enhanced inflorescence biomass at commercial maturity rather than just longer flowering time. Plants under the 13 hour photoperiod packed more weight into the buds rather than just running longer to make the same buds.
• Cultivar-specific response. Gorilla Glue gained more (1.50×) than Incredible Milk (1.35×). The size of the 13 hour photoperiod gain depends on genetics. Some cultivars will probably barely respond. Others may respond even more dramatically than Gorilla Glue did.

The honest answer is that we do not yet know exactly what mechanism is doing most of the work. The phenotypic result is clear; the underlying physiology is still being pulled apart. What growers can take to the bank is the result: when you run a 13 hour photoperiod on photoperiod cannabis, you tend to get more bud without losing potency.

Important Caveats Before You Change Your Light Schedule

The Ahrens paper is careful to flag what the result does and does not mean. A grower planning to test a 13 hour photoperiod should sit with these caveats first:

• Per-plant yield, not per-square-foot yield. The biggest caveat the authors raise. A 13 hour photoperiod made each individual plant grow a lot bigger. In a sea-of-green setup with very high planting density, those bigger plants may shade each other out, capping the per-area yield. The full benefit shows up most clearly when plants have room to fill out.
• Tested on photoperiod cultivars only. Autoflowers were not part of this study. Autoflowers do not depend on photoperiod to flower at all and respond differently to day length — extending light on autos is a separate question entirely.
• The starting cue still has to be reliable. Some cultivars need a confident shift into shorter days to even commit to flowering. Going straight from 18/6 veg into 13/11 flower instead of 12/12 may delay or weaken flowering initiation in sensitive strains. The 2024 paper saw a small (~1.5 day) delay in Incredible Milk; other cultivars could behave worse.
• Two cultivars is two cultivars. Generalize cautiously. Gorilla Glue and Incredible Milk both responded well. A landrace sativa or a stretchy haze-dominant strain may not behave the same way. The earlier Peterswald 2023 paper saw cultivar-specific responses across a wider panel.
• Power and heat. An extra hour of light per day is also extra electricity, extra heat load on the room, and extra wear on the bulbs or LED drivers. Budget for it before you switch.

How a Home Grower Could Test a 13 Hour Photoperiod

If you want to try a 13 hour photoperiod without risking your whole flower run, the conservative way to do it is one strain at a time, side by side if possible. A practical home protocol:

• Pick a known photoperiod strain. Heavy yielders that already finish strong are good test candidates — Gorilla Glue 4, Bruce Banner, Chemdawg, White Widow, AK-47. These are the closest analogues to the cultivars in the paper.
• Trigger flowering at 12/12 first. Run two weeks of 12/12 to make sure the plants commit to flowering. Then shift to 13/11 for the remainder of flower. This protects against weak flowering initiation on a borderline strain.
• Keep light intensity the same. A 13 hour photoperiod is about day length, not light intensity. Do not also turn the dimmer up — you want to test one variable at a time.
• Run a 12/12 control if you can. Two clones of the same plant, one on 12/12, one on 13/11, in the same room or two matched rooms. This is the only honest way for a home grower to get a real read on whether a 13 hour photoperiod helps your specific strain in your specific room.
• Watch the stretch. A 13 hour photoperiod tends to give plants a bit more vegetative time before flower commits. Train and top accordingly. If your tent is space-limited, this could backfire.
• Track yield by dry weight, not wet weight. Wet weight is misleading. Dry, cured, jarred weight is the only number worth comparing.

Pair this with the rest of the late-flower playbook — appropriate nitrogen feeding, no need to flush before harvest, the right drying temperature — and the gain you pick up from the 13 hour photoperiod compounds with the rest of the grow.

Frequently Asked Questions

Will a 13 hour photoperiod work on autoflowers?

The 2024 paper did not test autoflowers. Autoflowers do not depend on photoperiod to begin flowering, so the underlying mechanism is different. Most auto growers run 18/6 or even 20/4 from start to finish for max DLI. If you grow autos like Bruce Banner Auto or Moby Dick Auto, ignore the 13 hour photoperiod conversation — your light schedule question is a different one.

Will every photoperiod strain respond the same?

Probably not. The two cultivars in the Ahrens 2024 paper both responded well, but the magnitude differed (1.35× vs 1.50×). The earlier Peterswald 2023 paper found cultivar-specific effects across a wider panel. Some strains will likely respond strongly, others weakly. Test on one strain at a time before rolling out a 13 hour photoperiod across your whole flower room.

What about 13.5 or 14 hours? More is better, right?

Not necessarily. Pushing past 13 hours starts to weaken flowering signal in many cultivars. The Moher 2022 paper tested several long photoperiods and found a sweet spot rather than a linear “longer is always better” curve. 13 hours appears to capture most of the benefit while still being safely within the flowering window.

Does a 13 hour photoperiod hurt potency?

The 2024 study found that THC was either unchanged or slightly higher under the 13 hour photoperiod. The earlier related papers in the same series have generally found cannabinoid concentration holds up under longer photoperiods, with biomass scaling up. So the gain is real bud, not watered-down bud.

Should I switch my whole grow tent over to a 13 hour photoperiod tomorrow?

Not without testing. Run one strain side-by-side with a 12/12 control first. If the 13 hour photoperiod works on your specific cultivar in your specific room, then expand. The downside risk is real for sensitive strains — wasted electricity, weak flowering, or hermie pressure if the strain is borderline. The upside risk is, well, 35 to 50 percent more bud.

Does this study apply to outdoor grows?

Outdoor photoperiods are set by latitude and the calendar — you cannot dial them in. But it does inform timing: in regions where natural day length lingers above 13 hours longer into late summer, photoperiod cannabis will keep filling out before committing fully to flower. Strains like Blue Dream and Jack Herer can take advantage of those slightly longer late-summer days.

Will a 13 hour photoperiod cause hermies?

The 2024 paper did not report hermaphroditism in either cultivar tested. That said, light-stress hermies are most common in already-borderline strains, and pushing photoperiod is technically a stressor. Stable feminized seed from a careful breeder is the best insurance. Borderline genetics will break down under any number of stresses, not just light schedule.

The Bottom Line on a 13 Hour Photoperiod for Cannabis

The 12/12 default has been the orthodoxy of indoor cannabis flowering for fifty years. The 2024 Ahrens, Llewellyn and Zheng paper, on top of two earlier studies from the same lab, makes a strong case that a 13 hour photoperiod is a better default for many photoperiod cultivars — at least at the per-plant level, with no measured cost to potency. The mechanism is not fully understood, but the result is consistent across multiple papers and multiple cultivars.

This is one of the cheapest experiments a home grower can run. It costs an hour of electricity per day and one strain swap. If it works on your cultivar, you walk away with 35 to 50 percent more dried bud at harvest. If it doesn’t, you go back to 12/12 next round. Browse the full feminized seed catalog to pick a strain to test, or read our guides on light spectrum, nitrogen feeding, and drying temperature to dial in everything else around it.

Sources

• Ahrens, A., Llewellyn, D., & Zheng, Y. (2024). Longer photoperiod substantially increases indoor-grown cannabis’ yield and quality: A study of two high-THC cultivars grown under 12 h vs. 13 h days. Plants, 13(3), 433. PMC full text
• Moher, M., Llewellyn, D., Jones, M., & Zheng, Y. (2022). Moving away from 12:12; the effect of different photoperiods on biomass yield and cannabinoids in medicinal cannabis. Plants, 12(5), 1061. PMC full text
• Peterswald, T. J., et al. (2023). Is twelve hours really the optimum photoperiod for promoting flowering in indoor-grown cultivars of Cannabis sativa? Plants, 12(14), 2605. PMC full text