Twelve hours of light and twelve of dark is the most fixed rule in indoor cannabis growing. A 2024 controlled study grew two cultivars under thirteen hours of light instead. It got 1.35 times and 1.50 times the inflorescence yield. That increase was four to six times larger than the increase in daily light integral, so this is not simply “more light equals more yield.” For growers in Thailand the extra hour cuts both ways. From about April to August, natural daylength here is already above twelve hours: about 13 h 20 at sunrise-to-sunset in the north in June. So a thirteen-hour photoperiod needs less blackout-screen time than twelve. From about November to February, natural daylength falls to about 11 hours in the north. Then thirteen hours needs extra lighting and becomes the more expensive option. This article covers both seasons. It also covers a second light-timing change from 2025: replacing the last two hours of light with far-red.
1. What the study did and found
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. University of Guelph, Canada. https://www.mdpi.com/2223-7747/13/3/433
The study made one comparison: 12 hours against 13 hours of light, at about 540 µmol m⁻² s⁻¹ at canopy level. That gave a DLI of 23.8 vs 25.7 mol m⁻² d⁻¹, about 8% higher at 13 h.
| Cultivar | Inflorescence yield, 13 h vs 12 h | Total THC concentration (% of dry weight) | Flowering initiation |
|---|---|---|---|
| ‘Incredible Milk’ | ×1.35 | +9% (THC yield per plant up at least 38%, mostly from the extra biomass) | delayed ~1.5 days |
| ‘Gorilla Glue’ | ×1.50 | unchanged | not delayed |
This finding is more than a lighting-schedule tweak. The yield increase was four to six times larger than the relative increase in daily light integral. Getting 35–50% more inflorescence from an 8% DLI increase means the extra hour does more than deliver extra photons. It changes how the plant allocates what it produces.
The authors’ own caveat, which we repeat rather than bury: per-plant gains of this size may shrink in commercial canopies. There, plants are spaced tightly and compete for space and light. The authors warn against reading per-area gains from per-plant results. A 50% per-plant gain in a spaced research trial does not promise a 50% gain in a packed room.
2. Why the extra hour works differently at 7–20° north
The paper does not cover this part, because it was not written for this region.
In temperate indoor growing, 12/12 is simply a number you set on a timer in a sealed room. Choosing 12 or 13 hours costs nothing either way. In Thailand the number matters in two situations: light-deprivation greenhouses and outdoor growing.
Daylength by latitude. Thai daylength varies much less than temperate daylength, but it still varies.
- North (about 18–20°N: Chiang Mai, Chiang Rai): sunrise to sunset runs from about 11 h 00 in December to about 13 h 20 in June.
- South (about 7–8°N): from about 11 h 35 to about 12 h 40.
- Central Thailand falls between the two.
Plants also respond to the dim light of dawn and dusk. Civil twilight adds roughly 40–50 minutes a day. So in a northern Thai June, a plant senses something closer to 13.5–14 hours, not 13 h 20. Check the figures for your own site with a sunrise/sunset calculator such as NOAA’s (Solar Calculator - NOAA Global Monitoring Laboratory). Use your exact latitude, and include civil twilight.
Light-deprivation greenhouses: it depends on the month.
- About April to August: natural daylength plus twilight is above 13 h in the north and close to it in the south. A 13-hour photoperiod means the blackout screen stays closed about one hour less per day than for 12 hours. A closed screen traps heat and humidity, so every hour it stays open is an hour you are not fighting your own climate. When you plan the screen schedule, compare it with the daylength the plant senses (twilight included), not the sunrise-to-sunset figure. The screen has to cover the dim light of dusk and dawn too.
- About November to February: natural daylength is under 12 h everywhere in Thailand. That is about 11 h 00–11 h 40 in the north and about 11 h 35–11 h 55 in the south. A 13-hour photoperiod then needs supplemental light: about 1.5–2 hours a day in the north and about 1–1.5 hours in the south. The study added its extra hour at full intensity. To copy that, each fixture runs 1.5–2 hours longer at full power. A 600 W fixture would use about 0.9–1.2 kWh more per day, before the extra cooling. In these months 13 hours is the harder and more expensive option, not the easier one.
- March and September–October are shoulder months, when daylength is near 12 h.
Outdoor and semi-outdoor growing. For much of the year, natural daylength here is at or below the range where many drug-type cultivars start to flower. So photoperiod plants planted outdoors can flip to flower early. Whether a plant flowers under an effective 12 hours or 13 hours may partly depend on when in the year you plant. That lever costs nothing, but it is our own reasoning: we have found no field study that tests it.
3. What the study does not establish
Two cultivars. Both responded, which is encouraging. But two is two.
One extra hour, not a curve. The trial compared 12 h with 13 h only. Do not extrapolate upward. The point at which a photoperiod plant stops flowering reliably (its critical photoperiod) depends on the genotype. An earlier trial by the same Guelph group tested ten drug-type cultivars at 12 to 15 hours. All ten started flowering at up to 14 hours, with delays at 14 h of between zero and about four days depending on the cultivar. At 15 h, four cultivars did not start flowering at all. In several cultivars, the harvest measures peaked between about 12.6 and 13 hours. Those plants were harvested after only 3–4 weeks, so this is not final commercial yield (Ahrens, Llewellyn & Zheng, Plants 12(14):2605, 2023, https://doi.org/10.3390/plants12142605). How the equatorial, landrace or Haze-type lines grown in this region respond has not been tested.
Commercial canopy scale untested. The authors point this out themselves.
Nothing on terpenes. The study measured cannabinoids only.
No tropical replication. The trial ran in a growth chamber, not in a hot, humid room. High air temperature is known to reduce cannabinoid and terpene yield (see section 7). Whether the yield gain from the extra hour holds up in an unchilled Thai room is unknown.
4. How to test it on your own site without gambling a crop
A photoperiod change is one of the cheapest experiments in indoor growing, and one of the easiest to run badly. The rules:
Change only the photoperiod. Keep the same cultivar, feed, intensity, room, trellis and post-harvest handling. A nutrient change made at the same time ruins photoperiod experiments more often than anything else.
Run both arms at the same time if you possibly can. Use two rooms, or two tents, in the same cycle. A 12/12 crop in March compared with a 13/11 crop in July is not a comparison. It is two different climates, and here also two different daylengths.
Keep intensity constant, and record DLI. If you raise the photoperiod and the intensity together, you learn nothing about the photoperiod. The whole point of the finding is that yield rose far more than DLI did. You can only see that if you know the DLI in both arms.
Record the flowering start date separately from the harvest date. One cultivar in the study started flowering ~1.5 days later. If your 13-hour arm takes longer to finish, you need to know whether the cause is a later start or a longer ripening period. They have different operating costs.
Watch for delayed flowering, and know your genetics. The critical photoperiod varies by cultivar, and most cultivars grown in this region have never been tested at 13 hours. Watch the 13-hour arm closely in the first two weeks after the flip. If flowers form late or unevenly, treat that as the warning sign, whatever the breeder’s label says.
Weigh dry, per square metre, not per plant. The study reports per-plant figures, and the authors say these may not transfer. Grams per square metre is the number that pays your electricity bill.
5. What it would cost you to be wrong
This is worth stating, because “just try it” is easy advice to give about someone else’s crop.
The realistic downside is delayed or unreliable flowering in genetics that are sensitive to long days. This shows up in the first two weeks after the flip, which gives you time to go back to 12 hours before you lose a cycle. That is a recoverable error. It is why a photoperiod test is reasonable to run on a commercial crop, unlike, say, a substrate change.
The realistic upside, discounted honestly: the study’s 35–50% is a per-plant figure from spaced plants, and the authors expect it to shrink in a full canopy. Our own guess, not a measured figure, is a modest single-digit to low-double-digit gain per square metre. Treat anything more as a bonus. From April to August, even a modest gain plus fewer hours of closed blackout screen is a good trade in this climate. From November to February, set the gain against the cost of the extra lighting first.
6. A second timing lever: swapping the last two hours of light for far-red
Peterswald T.J., Mieog J.C., Kretzschmar T., Purdy S.J. (2025). The effects of far-red light on medicinal Cannabis. Scientific Reports. NSW Department of Primary Industries and Regional Development and Southern Cross University, Australia. The effects of far-red light on medicinal Cannabis | Scientific Reports
The design. The best-performing treatment, 10L_2D, used 10 hours of full light, then 2 hours of far-red only, then 12 hours of true dark. The control was a 12/12 schedule. Far-red was not added after a normal 12-hour day. It replaced the last two hours of full light. So the treatment plants got roughly a sixth less photosynthetic light than the control, and that confounds the result. Three genotypes were tested: ‘Northern Lights’, ‘Hindu Kush’ and ‘Cannatonic’.
What it found:
- ‘Northern Lights’: total cannabinoid yield was 0.43 vs 0.25 g/plant against the 12/12 control, about +70%. THCA concentration rose about 25%. Dry flower yield did not differ significantly. However, a 25% rise in concentration cannot produce a 70% rise in yield on its own. The flower weight in that treatment must also have been higher numerically (by roughly a third), even though the difference was not statistically significant. So treat the gain as concentration plus a weight increase that was not significant, not as “concentration alone”.
- ‘Hindu Kush’: no gain from the two-hour treatment. A four-hour far-red treatment only brought THC back up to the level of the 12-hour control.
- ‘Cannatonic’ (CBD-dominant): CBDA was lower in every far-red treatment. For a CBD crop, far-red made the product worse.
- More far-red was worse. 12L_2_2D used 12 h of light, with far-red during the last two light hours plus two hours into the dark (4 h of far-red in total). It produced the tallest plants and reduced trimmed flower weight in all three genotypes.
- Electricity: lighting electricity was about 5.5% lower than in the 12/12 control. That figure covers lighting only, not the whole facility. The paper’s per-kilogram cost and emissions figures rest on Australian tariff and grid assumptions and do not transfer to Thailand.
Why it matters here: fewer hours of full-intensity light means less heat in an unchilled room. That is our reasoning, not something the paper measured.
This is not the same as far-red in your growing spectrum. A far-red channel that runs during the day affects how the plant reads light quality, and with it stretch and plant shape (background on spectrum and R:FR in topic 1010). Far-red at the start of darkness is a signal to the plant. It shifts phytochrome, the plant’s light sensor, at the moment the day ends. That explanation is standard plant science rather than a finding of this paper. If your fixture runs far-red during the day, you have not tested this treatment.
If you want to test it:
- Use a separate, low-wattage far-red source on its own timer: 10 h full light, then 2 h far-red, then 12 h dark. Run it against a 12/12 control in the same cycle.
- Get lab potency results for both arms. Without them you cannot detect the result. If you cannot pay for testing, this is not the experiment to run.
- Weigh dry flower and track plant height. Extra height is your early warning that you are overdosing on far-red.
- Record the cultivar prominently. Do not run it on a CBD-dominant crop you intend to sell unless someone replicates the Cannatonic result and shows it does not apply generally.
This is one study, with three genotypes in one facility, no replication and no tropical trial. It worked in one cultivar out of three and made the product worse in another. It is cheap to trial, but it is not a recommended practice.
7. Where this sits against the rest of the light literature
In a 2025 Wageningen study, raising PPFD from 600 to 1200 µmol m⁻² s⁻¹ increased inflorescence dry matter in a straight line without changing the composition of specialized metabolites. The same study found that high air temperature reduced cannabinoid and terpene yield, with effects on dry matter that differed by genotype (Holweg et al., Environmental and Experimental Botany 230:106085, Redirecting). Taken together: in a tropical room, extra intensity is bought with heat. Changes to the timing of the photoperiod are the cheap levers, but so far both have been cultivar-specific. For spectrum and R:FR background see topic 1010; related discussions are in topics 1332 and 1280.
Has anyone here run 13/11 against 12/12 side by side in the same cycle? All it takes is grams per square metre and the flowering start date for both arms, plus your latitude and the month. This is the cheapest experiment on this forum and the one where a tropical result would matter most. Here the extra hour costs or saves money depending on the season and the blackout screen, and researchers at temperate latitudes have little reason to study that. If you already have far-red capability and lab access, results for the 10 h + 2 h far-red schedule with the cultivar named would be just as valuable, especially from anyone willing to retest the CBD-dominant result.
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation is regulated differently in every country. In Thailand, cannabis inflorescence is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis) B.E. 2568, and cultivation and sale require licensing. Nothing here is legal advice.