A 2024 study from the University of Guelph in Canada flowered two high-THC cannabis cultivars under 13 hours of light a day instead of 12. Flower yield per plant rose 1.35 times in one cultivar and 1.50 times in the other. The total daily light rose only about 8%. This part explains how to test 13 hours on your own farm without risking a whole crop, and what you lose if the test fails. It also covers a 2025 Australian trial that replaced the last two hours of light with far-red light.
A few terms are used throughout. The photoperiod is the number of hours of light in each 24 hours. PPFD is the light that plants can use for photosynthesis reaching one square metre each second, in µmol m⁻² s⁻¹. DLI (daily light integral) is the total of that usable light per square metre over a whole day, in mol m⁻² d⁻¹.
Part 1 explains the 2024 study in detail. It also shows how to work out the day length at your site in Thailand and how to find light leaks in a flowering room.
This is Part 2. Part 1: Thirteen Hours Beats Twelve: Flowering Photoperiod and Light Timing at Thai Latitudes
1. How to test 13 hours without risking a whole crop
Changing the photoperiod is one of the cheapest experiments in indoor growing. It is also one of the easiest to run badly. Each rule below has a reason.
Change only the photoperiod. Keep everything else the same: cultivar, feeding, light intensity, room, trellis net, and drying and curing. Changing the nutrients at the same time ruins photoperiod tests more often than anything else. If only the light hours change, any difference you see comes from the light hours.
Run both groups in the same cycle. Use two rooms or two tents at the same time. A 12/12 crop grown in March and a 13/11 crop grown in July cannot be compared. They grew in two different climates. In Thailand they also grew under two different natural day lengths.
Keep intensity the same, and record the DLI of both groups. If you raise the light hours and the intensity together, you learn nothing about the light hours. The main finding was that yield rose far more than DLI. You can only check this if you know the DLI of both groups. DLI = PPFD × hours of light × 3,600 ÷ 1,000,000.
Record the date flowering starts and the harvest date separately. Use the date the first white pistils (flower hairs) appear at the top of the plants. In the study, one cultivar started flowering about 1.5 days later at 13 hours. If your 13-hour group takes longer to finish, you need to know whether it started later or ripened for longer. The two cost different amounts to run.
Watch for late flowering, and know your genetics. Each cultivar has a critical day length, the longest light period under which it still flowers reliably. Most cultivars grown in this region have never been tested at 13 hours. Check the 13-hour group closely for the first two weeks after you switch to the flowering schedule (the flip). If flowers form late or unevenly, treat that as a warning, whatever the breeder’s label says.
Weigh dry flower per square metre, not per plant. The study reports figures per plant, and the researchers say these may not carry over to a full room. Grams per square metre shows whether the extra hour earns back its electricity.
Translator’s note: a mechanical timer stops during a power cut and then runs late by the length of the cut. Check the timer clocks after every outage, or use a digital timer with battery backup. Write each outage in the log for both groups, with the date and how long it lasted.
Translator’s note: in a Thai room without air conditioning, the 13-hour group gets one more hour of lamp heat each day. Log the highest room temperature in both groups. If the 13-hour room runs clearly hotter, the heat may cancel part of any gain (section 4).
A simple log, one row per group:
| What to record | Why |
|---|---|
| Latitude, month, cultivar | So that others can compare your result with theirs |
| PPFD at canopy and DLI | To check that intensity stayed the same |
| Flip date, date flowering starts, harvest date | To separate a later start from longer ripening |
| Highest room temperature, power cuts | To spot heat or timing problems |
| Dry weight in grams per square metre | The result that decides whether 13 hours pays off |
2. What it costs if the test fails, and what you can realistically gain
This section matters because “just try it” is easy advice to give when it is someone else’s crop.
The realistic risk is late or unreliable flowering in cultivars that are sensitive to long days. It shows up in the first two weeks after the flip. That gives you time to switch back to 12 hours before you lose the cycle. The mistake can be fixed. That is why a photoperiod test is reasonable on a commercial crop, unlike a change of growing medium, for example.
The realistic gain is much smaller than the headline figure. The study’s 35–50% is a gain per plant, measured on widely spaced plants. The researchers expect it to shrink in a full room. The editors’ own guess, not a measured figure, is a modest gain per square metre, somewhere between a few percent and the low teens. Treat anything above that as a bonus.
From about April to August, even a modest gain is worth having in this climate. It also comes with fewer hours of closed blackout screen in a light-deprivation greenhouse. From about November to February, first compare the gain with the cost of the extra lighting.
What to do: before the test, work out the extra electricity cost of the 13-hour group (Part 1, section 5). Then work out how many extra grams per square metre would cover it. If that figure is far above a modest gain, test in a different season.
3. A second timing change: far-red light for the last two hours
Who ran the trial. Peterswald T.J., Mieog J.C., Kretzschmar T. and Purdy S.J. published it in 2025 in Scientific Reports. They work at the NSW Department of Primary Industries and Regional Development, the farm research agency of the state of New South Wales in Australia, and at Southern Cross University, also in Australia. The effects of far-red light on medicinal Cannabis | Scientific Reports
Far-red is deep red light at the edge of what the human eye can see. Plants sense it through phytochrome, the light sensor in the leaves that reads red and far-red light.
How the trial was set up. The best treatment was called 10L_2D. It gave 10 hours of full light, then 2 hours of far-red light only, then 12 hours of full darkness. The control group was on a normal 12/12 schedule. Three genetic lines were tested: ‘Northern Lights’, ‘Hindu Kush’ and ‘Cannatonic’.
The far-red was not added after a normal 12-hour day. It replaced the last two hours of full light. So the treated plants received about one sixth less light for photosynthesis than the control group. This difference blurs the result, because two things changed at once.
What it found:
- ‘Northern Lights’: total cannabinoid yield was 0.43 g per plant, against 0.25 g in the 12/12 control, about 70% higher. THCA concentration rose about 25%. THCA is the acid form of THC in raw flower. The difference in dry flower yield was not statistically significant.
- Why that gain needs care: a 25% rise in concentration cannot produce a 70% rise in yield on its own. So the flower weight in that treatment must also have been higher, by roughly a third, even though the difference was not statistically significant. Read the gain as higher concentration plus a weight increase that the trial could not confirm, not as concentration alone.
- ‘Hindu Kush’: the two-hour treatment gave no gain. A four-hour far-red treatment only brought THC back to the level of the 12-hour control.
- ‘Cannatonic’ (a CBD-dominant cultivar): CBDA, the acid form of CBD, was lower in every far-red treatment. For a CBD crop, far-red made the product worse.
- More far-red was worse. The 12L_2_2D treatment gave 12 hours of light, with far-red added during the last two light hours and for two hours into the dark (4 hours of far-red in total). It produced the tallest plants and reduced trimmed flower weight in all three genetic lines.
- Electricity: lighting used about 5.5% less electricity than the 12/12 control. This covers lighting only, not the whole facility. The paper’s cost and emissions per kilogram are based on Australian electricity prices and Australia’s power grid, so they do not apply to Thailand.
Translator’s note: fewer hours of full-intensity light also mean less lamp heat in a room without air conditioning. This is reasoning only. The paper did not measure it.
This is not the same as far-red in your lamp’s daytime light. A far-red channel that runs during the day changes how the plant reads light quality. That affects stretch and plant shape (for background on light spectrum and the red to far-red ratio, see the topic on light spectrum and DLI). Far-red given as the day ends is a signal. It shifts phytochrome at the moment the day ends. This explanation is standard plant science, not a finding of this paper. If your lamp 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 lamp on its own timer. Set 10 hours of full light, then 2 hours of far-red, then 12 hours of dark. Run it against a 12/12 control in the same cycle, so that both groups share the same weather and day length.
- Get laboratory potency results for both groups. Without them you cannot see the result, because the main gain was in cannabinoid concentration. If you cannot pay for testing, do not run this experiment.
- Weigh the dry flower and track plant height in both groups. Extra height is the early warning that the plants are getting too much far-red.
- Record the cultivar clearly. Do not run this on a CBD-dominant crop you plan to sell until another trial repeats the Cannatonic test and shows that the CBD loss does not apply to CBD cultivars in general.
Translator’s note: to find a separate far-red lamp, ask the official distributor of your lighting brand in Thailand. For laboratories that test cannabinoid potency, ask other growers about their experience in the B2B Services section.
How much weight to give this trial. It is one study of three genetic lines in one facility. It has not been repeated and was not run in the tropics. The treatment 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.
4. How this fits with other lighting research
A research team at Wageningen University & Research, an agricultural university in the Netherlands, published a lighting study in 2025 (Holweg et al.). Raising PPFD from 600 to 1200 µmol m⁻² s⁻¹ increased flower dry matter in a straight line. The mix of specialized metabolites did not change. These are compounds such as cannabinoids and terpenes.
The same study found that high air temperature lowered cannabinoid concentration but did not change terpene concentration. Its effect on dry matter differed from one genetic line to another.
Put together, the studies point one way for a tropical room. More light intensity also brings more heat, so it needs more cooling. Changing the timing of light (13 hours, or far-red at the end of the day) costs much less. So far, though, both timing changes have worked only in some cultivars.
For related topics, see the vegetative stage and when to flip and the light-deprivation greenhouse topic linked in section 2.
What to do with this: before you buy more lamps, log the canopy temperature during the light period for one full cycle. If the room is already hot, test a timing change before you add light intensity.
Share your results
If you have compared 13/11 with 12/12 side by side in the same cycle, please post your results below this topic. These four facts are enough: grams per square metre for both groups, the date flowering started in both groups, your latitude, and the month.
This is one of the cheapest experiments a grower can run, and a result from the tropics would add the most. Here the extra hour can cost or save money, depending on the season and the blackout screen. Researchers in temperate countries have little reason to study that.
If you have a far-red lamp and access to a laboratory, results for the 10 hours + 2 hours far-red schedule are just as useful. Please name the cultivar. Results from anyone willing to retest a CBD-dominant cultivar are especially useful.
Please post data and photos only. In Thailand, advertising cannabis flower and selling it online are prohibited.
Legal note
This article gives technical growing information for readers who operate legally where they live. Every country regulates cannabis growing differently. In Thailand, cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. Research, export, sale and processing of cannabis flower require a licence.
The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569, in force since 30 April 2026, tightens the licence criteria for cannabis flower. Nothing here is legal advice.
References
- 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
- Peterswald T.J., Mieog J.C., Kretzschmar T., Purdy S.J. (2025). The effects of far-red light on medicinal Cannabis. Scientific Reports 15:17435. NSW Department of Primary Industries and Regional Development and Southern Cross University, Australia. The effects of far-red light on medicinal Cannabis | Scientific Reports
- Holweg et al. (2025). Environmental and Experimental Botany 230:106085. https://doi.org/10.1016/j.envexpbot.2025.106085