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 University of Guelph study found that a 13 hour photoperiod grew 35 to 50 percent more bud per plant in two strains, with no drop in THC.

This post breaks down what the study actually did, why a 13 hour photoperiod gave 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, one from the same Guelph group and one from Australia, had already questioned 12/12, with mixed results; the 2024 paper tested a 13 hour photoperiod all the way to harvest on two high-THC cultivars.

13 on, 11 off. In a controlled 2024 trial, clones given one extra hour of light a day produced 35 to 50 percent more flower per plant.

The Quick Answer: A 13 Hour Photoperiod Beat 12/12 by 35 to 50 Percent Per Plant

In a 2024 controlled trial published in the journal Plants, two THC-dominant cannabis cultivars (Incredible Milk and Gorilla Glue) were grown to harvest under either a 12 hour or a 13 hour photoperiod. The 13 hour photoperiod produced 1.35 times the dried bud per plant in Incredible Milk and 1.50 times in Gorilla Glue. THC concentrations either held steady or improved. Both treatments shared one walk-in growth chamber, split into curtained compartments, with the same nutrients and the same light intensity (about 540 µmol/m²/s). 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. A 35 to 50 percent gain from an eight percent light bump (the authors call it 4 to 6 times the increase in light) 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 switches to flowering when the days get shorter than a certain length. Growing guides were already recommending 12/12 by the late 1970s, and it stuck because it reliably flips almost every photoperiod strain into flower fast, and lets growers run many strains in one room on one timer.

What got lost is that 12 hours is a safe, conservative day length, not a magic number. Each strain has its own cutoff: once the day is shorter than that cutoff, the plant flowers. Twelve hours sits below the cutoff of almost every strain, so it works everywhere, but it isn’t necessarily the best day length for yield.

In a 10-strain test, every strain started flowering at 13 hours of light, and about half did so with little or no delay at up to 13.5 hours. But because every grower assumed 12/12 was the answer, almost no one was running side-by-side trials with longer days. Researchers in Australia and at Canada’s University of Guelph have started to change 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 high-THC photoperiod cultivars: Incredible Milk and Gorilla Glue. Two treatments: 12 hours of light vs 13 hours of light per day, run side by side in curtained compartments of the same growth chamber, three compartments per treatment. The 13 hour photoperiod started on day one of flower, straight from an 18 hour veg. Both groups were harvested on the same days, when the 12 hour plants were ripe, so the 13 hour plants were noticeably less ripe at harvest: their pistils browned and trichomes ambered more slowly in both strains.

Illustration: in the 2024 study, plants on a 13 hour photoperiod finished with 35 to 50 percent more dried bud per plant than plants on 12/12. Same strains, same nutrients, same light intensity.

The headline numbers from the 13 hour photoperiod treatment:

Incredible Milk: 1.35× the dried bud weight per plant of the 12/12 control. Its top buds were bigger but about a third less dense.
Gorilla Glue: 1.50× the dried bud weight per plant 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: Measured in the top bud at harvest, total THC was about 9 percent higher in Incredible Milk under the 13 hour photoperiod (roughly 25.7% to 28.1%) and unchanged 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. Peterswald and colleagues in New South Wales, Australia (NSW Department of Primary Industries and Southern Cross University; 2023, Plants 12:1061) tested nine photoperiod schedules on one high-CBD and two high-THC lines. They found 12/12 isn’t optimal for every strain, but a static 14-hour day cut THC concentration by about 40 percent in both THC lines (Northern Lights and Hindu Kush). That study had no 13 hour treatment.

Ahrens, Llewellyn and Zheng at the University of Guelph (2023, Plants 12:2605) asked directly “Is twelve hours really the optimum?” across ten cultivars, growing them for the first 3 to 4 weeks of flower, and concluded there is potential to raise yield in some strains with a slightly longer day. The 2024 paper is the first of these to compare a 12 and a 13 hour photoperiod all the way to harvest.

Why a 13 Hour Photoperiod Pulled So Much Extra Yield in the Study

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. An extra hour of light at the same intensity is about 8 percent more light per day, and more light generally means more growth in cannabis.
Slower start, bigger plant. The authors’ leading explanation: the longer day slowed the switch into flower slightly, so plants kept building leaves and branches during the early-flower stretch. More leaf catches more light, and a bigger frame carries more bud sites. (Extra leaf and stem weight was measured in Incredible Milk, not in Gorilla Glue.)
Same harvest dates, more bud. Both groups were cut on the same days, so the 13 hour photoperiod gain did not come from a longer flower run.
Cultivar-specific response. Gorilla Glue gained more (1.50×) than Incredible Milk (1.35×). The authors stress that photoperiod responses are “strongly cultivar-dependent”, and in their earlier 10-strain test, which only ran the first 3 to 4 weeks of flower, most strains put on less early bud as the day got longer. That early result doesn’t predict harvest, though: Gorilla Glue and Incredible Milk were among the strains with smaller early buds, and both out-yielded 12/12 at harvest in 2024.

The honest answer is that we do not yet know exactly what mechanism is doing most of the work. The result is clear for these two strains; whether a 13 hour photoperiod does the same for yours is something you have to test.

The Photoperiod Studies Side by Side

The 2024 paper gets most of the space in this post, but it is one of four controlled studies this post draws on that changed the day length in flower, and on a quick read they seem to disagree. Here they are side by side:

Study and strains	Day lengths tested	How far it ran	Yield on the longer day	THC on the longer day
Peterswald 2023, Australia: Northern Lights and Hindu Kush (THC), Cannatonic (CBD)	10, 12 or 14 h, steady or switched halfway through flower (nine schedules)	To harvest; all plants cut the same day	Steady 14 h: more flower than 12 h in all three (39% more in Hindu Kush). 14 h for the first half, then shorter: more flower in Cannatonic and Northern Lights	Steady 14 h: about 40% lower in both THC strains (12% to 7%, 10% to 6%). 14 h for the first half, then shorter: same as 12 h
Ahrens 2023, Guelph: 10 high-THC strains, Gorilla Glue and Incredible Milk among them	12, 12.5, 13, 13.5, 14 and 15 h	First 3 to 4 weeks of flower only	Early bud weight peaked at 13 h or less in every strain; at 15 h, flowers that started did not develop	Not measured
Ahrens 2024, Guelph: Incredible Milk and Gorilla Glue (THC)	12 or 13 h	To harvest; all plants cut when the 12 h plants were ripe	1.35× and 1.50× the dried bud per plant	About 9% higher in Incredible Milk, unchanged in Gorilla Glue
Sae-Tang 2025, Netherlands: White Russian (THC)	12 h, with some weeks stretched to 18 h	8 weeks of flower, to harvest; all plants cut after the same 8 weeks	18 h for only the last 2 weeks, extra hours as bright as the rest of the day: 12.5% more flower (a 4.9% rise at a brighter setting wasn’t statistically significant). 18 h from week 2 or 4, using blue light: plants went back to growing leaves on top of the buds	Last 2 weeks only: no change. Longer blue-light stretches: lower THC

Our read: on yield, the studies mostly agree. Every study that carried a longer day to harvest got more flower from at least one strain, and the 2024 authors call their gain similar to the Australian one at 14 hours. Guelph’s 10-strain test points the other way, but it stopped at 3 to 4 weeks, and as covered above, its early result didn’t predict harvest. The real split is THC, and it tracks how far past 12 hours the day went and for how long: a steady 14 hours cut it, while a steady 13 hours, 14 hours for only the first half, and 18 hours for only the last two weeks did not.

One catch runs through all three harvest studies: the longer-day plants were cut on the same schedule as the 12-hour plants, and the pistils on the 13- and 14-hour plants aged more slowly. So those comparisons likely aren’t at equal ripeness (our inference).

Of the steady schedules carried to harvest, 13 hours is the only one that grew more flower in both THC strains without costing THC. The two split schedules also kept THC, but each raised yield in only one THC strain: Northern Lights in the Australian study, and White Russian, the only strain in the 2025 study. So 13/11 is the best-supported longer day to test, but each harvest result rests on one or two THC strains.

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. A caveat the authors spell out: their plants were spaced 35 by 70 cm (about 14 by 28 inches), and they warn against reading the per-plant gain as a per-area gain. In a sea-of-green setup with very high planting density, bigger plants may shade each other out, capping the per-area yield.
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 strong enough. In the 2024 study the plants went straight from 18 hours to 13 hours and flowered with at most a 1.5-day delay, but in the earlier 10-strain test some strains slowed down as the day got longer, and at 15 hours none of the 10 strains developed real buds.
Expect a later finish if you harvest by ripeness. Every plant in the 2024 study was cut when the 12/12 plants were ripe, while the 13 hour plants were still behind. If you wait for your 13/11 plants to ripen, plan on a longer flower run; nobody has measured how much longer.
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 Ahrens 2023 paper, which tested ten cultivars, saw cultivar-specific responses.
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. A photoperiod strain you already know is the best test plant, because you know how it normally finishes — for example Gorilla Glue 4, Bruce Banner, Chemdawg, White Widow, AK-47. Gorilla Glue 4 shares its name with one of the study’s two strains (the paper just calls it “Gorilla Glue”, so it may not be the same cut). None of the others was part of any of these studies.
Decide how you’ll start. The 2024 study ran 13/11 from day one of flower, straight from 18/6 veg, and that is the only version with harvest data. Some growers prefer to start at 12/12 for a week or two and then move to 13/11 as a safety net for strains that are slow to flip. Nobody has tested that two-step version, and it may give up part of the gain, since the study’s authors think the extra growth comes in the first weeks of flower.
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 two separate light-tight tents set up the same way. They can’t share a room or tent: the 12/12 plant would get an hour of light during its dark period, and even faint stray light at night can delay flowering. 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 plant size. Plants on a 13 hour photoperiod can finish noticeably bigger (in the 2024 study, Incredible Milk plants were about 80 percent bigger by the authors’ height-and-width measure). Train and top accordingly. If your tent is short on headroom, 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 — so a longer day isn’t covering for problems elsewhere in the grow.

The Bottom Line on a 13 Hour Photoperiod for Cannabis

One extra hour of light a day: in the 2024 study’s two strains, more bud with no drop in THC.

12/12 has been the indoor flowering default for decades. The 2024 Ahrens, Llewellyn and Zheng study suggests it isn’t always the best choice: a 13 hour photoperiod grew 35 to 50 percent more bud per plant in two strains, with no drop in THC. The wider evidence is mixed — a 14-hour day cut THC in the Australian study, and early-flower results varied a lot by strain in Guelph’s 10-strain test — so treat a 13 hour photoperiod as a promising test, not a new rule.

This is a low-risk experiment to run. It costs an extra hour of light a day, possibly a longer flower run, and, for a fair comparison, a second light-tight tent. If your strain responds like the two in the study, you could see noticeably more dried bud per plant. 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
Peterswald, T. J., Mieog, J. C., Azman Halimi, R., Magner, N. J., Trebilco, A., Kretzschmar, T., & Purdy, S. J. (2023). 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
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. PMC full text
Sae-Tang, W., Heuvelink, E., Kappers, I. F., Contreras-Avilés, W., Bernal Cortes, D., Groen, H., Marin Gomez, J., Nicole, C. C. S., & Marcelis, L. F. M. (2025). Long-days during the last two weeks before harvest applied to short-day medicinal cannabis can improve inflorescence yield. Industrial Crops and Products, 233, 121442. DOI

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. Many auto growers run 18/6 or 20/4 from start to finish — see our autoflower light schedule guide. 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 Ahrens 2023 paper found cultivar-specific effects across ten cultivars. 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. In Guelph’s 10-strain test (Ahrens 2023), early bud growth was generally smaller above 13 hours, and at 15 hours some strains’ flowers never developed. In the Australian study (Peterswald 2023), a 14-hour day grew more flower but cut THC by about 40 percent in both THC strains. Of the steady schedules run to harvest, only 13 hours raised yield in both THC strains tested without lowering THC, and only two strains were tested. A 2025 study of medicinal cannabis found another route: going from 12 to 18 hours only for the last two weeks, with strong extra light, raised yield without lowering cannabinoids; extending the day earlier sent plants back into leafy growth.

Does a 13 hour photoperiod hurt potency?

The 2024 study found total THC was unchanged in Gorilla Glue and about 9 percent higher in Incredible Milk under a 13 hour photoperiod. That isn’t true of every longer day: in the Australian study, a 14-hour day cut THC by about 40 percent in both THC strains (from 12% to 7% and from 10% to 6%), even though it grew more flower. So 13 hours looks safe for potency in the two strains tested; 14 hours did not.

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 some strains: extra electricity, slower flowering, or less dense buds. The upside, if your strain behaves like the two in the study, is a lot more bud per plant.

Does this study apply to outdoor grows?

Outdoors you can’t set the day length — latitude and the calendar do it. Photoperiod plants flip to flower once the days drop below their own cutoff, so the start date shifts with latitude and strain. These studies were all indoors, so they don’t tell you much about outdoor yield. If you grow in a greenhouse with blackout curtains, the study authors note a longer day lets you keep the curtains open longer in summer. A Blue Dream clone was one of the 10 strains in the Guelph test (it may not be the same cut as our Blue Dream); indoors it started flowering fastest at about 12.6 hours of light. Nobody has tested how that translates outdoors.

Is 13/11 the same as 11/13?

No. 13/11 means 13 hours of light and 11 of dark, the longer day tested in the 2024 study. 11/13 is a shorter day, 11 hours of light, that some growers switch to late in flower. No study we found has tested 11/13. The closest test is the Australian study, which cut plants from 12 to 10 hours of light halfway through flower: THC was no higher than on a steady 12/12, and the shortest-day treatments tended to yield the least in two of the three strains, though that difference wasn’t statistically significant.

Will a 13 hour photoperiod cause hermies?

The 2024 paper didn’t mention hermaphrodites at all, and with nine plants per strain per treatment it was too small to tell. Controlled studies haven’t shown which stresses cause hermies, and no seed is hermie-proof, so keep checking your buds through flower whatever light schedule you run.

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