Most growers flower cannabis under 12 hours of light and 12 hours of darkness each day. In a 2024 Canadian study, two high-THC cultivars were flowered under 13 hours of light instead. They produced 1.35 and 1.50 times more flower. This part explains that study, how to work out the day length your plants get in Thailand, and how to keep the dark period fully dark.
The extra hour raised the total light the plants received by only about 8%. The yield gain was four to six times larger than that. So the result is not simply “more light gives more yield”.
In Thailand, the extra hour saves money in some months and costs money in others. From about April to August, natural day length is already above 12 hours. In the north in June it is about 13 h 20 from sunrise to sunset. In those months a light-deprivation greenhouse keeps its blackout screen closed for less time at 13 hours than at 12. This is a greenhouse where a blackout screen is pulled over the crop to make the day shorter.
From about November to February, day length falls to about 11 hours in the north. Then 13 hours needs extra lamps and becomes the more expensive choice. Part 2 covers three things: how to test 13 hours on your own farm, what it costs if the test fails, and a 2025 trial that replaced the last two hours of light with far-red light.
1. What the 2024 study did and found
The trial was run by a research team at the University of Guelph in Canada. They grew the plants in a growth chamber, which is a closed cabinet where light, temperature and humidity are fully controlled. They changed only the photoperiod, which is the number of hours of light in each 24 hours.
- 12 hours of light against 13 hours of light during flowering.
- Light intensity at the top of the plants was about 540 µmol m⁻² s⁻¹ in both groups. This figure is called PPFD. It measures how much light that plants can use for photosynthesis reaches one square metre each second.
- The daily total rose by about 8%. Light intensity multiplied over the whole day gives the DLI (daily light integral), the total usable light per square metre per day. It was 23.8 mol m⁻² d⁻¹ at 12 hours and 25.7 mol m⁻² d⁻¹ at 13 hours.
The results are in the table below. “Flower” here means the inflorescence, the flower cluster (bud) that is harvested. “Total THC” counts both THC and THCA. THCA is the acid form in raw flower that turns into THC when heated.
| Cultivar | Flower yield, 13 h compared with 12 h | Total THC concentration (% of dry weight) | Start of flowering at 13 h |
|---|---|---|---|
| ‘Incredible Milk’ | ×1.35 | +9% (THC yield per plant up at least 38%, mostly because the plants made more biomass) | about 1.5 days later |
| ‘Gorilla Glue’ | ×1.50 | no change | not delayed |
Why the result is important. The plants made 35–50% more flower from only 8% more light. So the extra hour did more than deliver more light. It also changed how the plant divides what it produces between its parts.
The researchers’ own warning. Gains this large per plant may shrink in commercial rooms. There, plants stand close together and compete for space and light. The researchers warn against reading a gain per square metre from a gain per plant. A 50% gain per plant in a widely spaced trial does not promise a 50% gain in a full room.
What to do with this: before you test anything, measure the PPFD at the top of your canopy with a PAR meter (a meter for the light plants can use). You need this number to know the DLI of each test group (Part 2, section 1).
2. Photoperiod and critical day length: what the plant responds to
Most cannabis grown from regular or feminized seed is a photoperiod plant. While days are long, it keeps growing leaves and stems. It starts to flower when the daily light period becomes short enough. Indoors, growers trigger this by switching the timer to the flowering schedule. This switch is often called “the flip” (see the topic on the vegetative stage and when to flip).
Each cultivar (a named, bred variety) has a longest light period under which it still flowers reliably. This limit is called its critical photoperiod, or critical day length. It differs from one cultivar to the next. If the light period is longer than this limit, flowering starts late, starts unevenly or does not start at all.
The same Guelph team measured this limit in an earlier trial, published in 2023. They grew ten high-THC cultivars at 12 to 15 hours of light:
- All ten started flowering at up to 14 hours of light.
- At 14 hours, flowering started between zero and about four days late, depending on the cultivar.
- At 15 hours, four of the ten cultivars did not start flowering at all.
- In several cultivars, the harvest measurements were highest between about 12.6 and 13 hours. These plants were harvested after only 3–4 weeks, so the figures are not final commercial yields.
The cultivars common in this region have not been tested this way. This includes equatorial lines, Southeast Asian landraces (old local varieties) and Haze-type lines.
What to do with this: if you test a longer day, stay at 13 hours. These numbers are not a reason to try 14 or 15 hours. The safe limit depends on the cultivar, and nobody knows the limit for yours.
3. Working out the day length your plants actually get
Translator’s note: this section was added for growers in Thailand and Southeast Asia. The 2024 study ran in a closed chamber and does not cover natural day length.
Day length in Thailand changes much less through the year than in temperate countries, but it still changes. The table gives approximate times from sunrise to sunset.
| Region | Latitude | Shortest day (around December) | Longest day (around June) |
|---|---|---|---|
| North (Chiang Mai, Chiang Rai) | about 18–20° N | about 11 h 00 | about 13 h 20 |
| Central | between the two | between north and south | between north and south |
| South | about 7–8° N | about 11 h 40 | about 12 h 35 |
Plants also respond to the dim light before sunrise and after sunset. This period is called civil twilight. It adds roughly 40–50 minutes to each day. So in June in northern Thailand, plants sense a day of about 14 hours, not 13 h 20.
How to work it out for your own site:
- Find your exact latitude and longitude, for example in the map app on your phone.
- Open a sunrise and sunset calculator that also lists civil twilight, such as timeanddate.com: https://www.timeanddate.com/sun/ (search for your town). The NOAA calculator shows only sunrise and sunset, not civil twilight.
- For each week of your planned flowering period, write down the times of civil dawn and civil dusk, not only sunrise and sunset.
- The day length the plant senses is the time from civil dawn to civil dusk.
- Compare it with your target (12 or 13 hours) and with the 12–14 hour range from section 2.
You do this to learn three things. It shows how long a blackout screen must stay closed and how many hours of extra light you need. It also shows whether outdoor plants are likely to start flowering early.
What to do with this: make a table for your site with one row per week of the year. Each row needs civil dawn, civil dusk and day length. Keep it with your grow records.
4. Why the dark period must stay dark: phytochrome and light leaks
Plants detect the end of the day with phytochrome, a light sensor in the leaves. It reads red and far-red light. When the light stops at the end of the day, phytochrome changes state, and the plant registers that night has begun.
The plant then times how long the dark period lasts. Cannabis is a short-day plant. In such plants, a long, unbroken night is the main signal to flower. A light leak in the middle of the night can split one long night into two short ones. In sensitive cultivars, even very dim light left on through the night has delayed flowering. The usual result is flowering that starts late or unevenly.
How to find light leaks in an indoor room or tent:
- When the lights switch off, go inside, close the door and wait 5–10 minutes until your eyes adjust.
- Check door edges, tent zips, air inlets and outlets, duct joints, and holes for cables or pipes.
- Look for small indicator lights on controllers, dehumidifiers, air conditioners, timers and power strips.
- Cover gaps with black tape, foil tape or blackout cloth. Cover indicator lights with black tape.
- Do not seal air inlets shut. Fit a bent duct or a light-trap hood so that air can pass but light cannot.
How to find light leaks in a light-deprivation greenhouse:
- Check the screen at dusk and at dawn. At these times it must block the dim twilight.
- Check where screen panels overlap, the edges along the walls, and the gap at ground level.
- Look for outside light at night, such as street lamps, a neighbour’s security light, or car lights from a nearby road.
Translator’s note: power cuts are common in parts of Thailand and Southeast Asia. A power cut during the dark period does not break the dark. The risk comes from backup equipment. If you run a generator or emergency lights, make sure nothing lights up inside the flowering room during the dark hours.
Translator’s note: try not to enter the flowering room during the dark period. If you must, keep the visit short and use the dimmest light you can. Do not assume that a green work light is safe; this has not been shown for cannabis.
What to do with this: check the room in the dark before every flip, and again after any repair to the room, screen or ducting. A leak found before the flip is fixed with a roll of tape. A leak found after flowering starts may already have delayed it.
5. What 13 hours means in Thailand, month by month
Translator’s note: this section was added for Thailand and Southeast Asia. The study was not written for this region and does not cover it. The advice below is the editors’ own reasoning from the day-length figures in section 3.
In a sealed indoor room, 12/12 is just a timer setting. The day length outside makes no difference there, so choosing 12 or 13 hours only means changing the timer. In Thailand, the choice matters most in two places: light-deprivation greenhouses and outdoor growing.
About April to August
In these months, natural day length plus twilight is above 13 hours in the north and close to 13 hours in the south. With a 13-hour photoperiod, the blackout screen stays closed about one hour less per day than with 12 hours. A closed screen traps heat and humidity. So each hour it stays open is one hour less of heat and humidity building up under the screen.
Plan the screen times from the day length the plant senses (section 3), not from sunrise and sunset. The screen also has to block the dim light of dusk and dawn.
Translator’s note: these months include the hottest part of the year and the start of the rainy season. Humid air trapped under a closed screen raises the risk of botrytis (grey mould) in dense flowers. Log temperature and humidity under the screen to see how much difference one hour less of closed screen makes.
About November to February
Natural day length is under 12 hours everywhere in Thailand. It is about 11 h 00–11 h 40 in the north and about 11 h 40–11 h 55 in the south. So a 13-hour photoperiod needs supplemental (extra) lighting. You need about 1.5–2 hours a day in the north and about 1–1.5 hours a day in the south.
The study gave its extra hour at full light intensity. To do the same, each lamp has to run 1.5–2 hours longer at full power. A 600 W lamp would use about 0.9–1.2 kWh more per day, before counting the extra cooling. In these months, 13 hours is the harder and more expensive option, not the easier one.
What to do: multiply 0.9–1.2 kWh by the number of 600 W lamps. Then multiply by the number of flowering days and by the price per kWh on your electricity bill. The result is the extra lighting cost to weigh against the expected gain (Part 2, section 2).
March and September–October
These are transition months, when day length is close to 12 hours.
Sealed indoor rooms
Translator’s note: outdoor temperatures in Thailand often reach 30–35 °C. In a sealed room, the 13th hour adds one more hour of lamp heat each day. Many growers run the light period at night, when outside air is cooler and the air conditioner works less. For cooling capacity, see sizing air conditioners and dehumidifiers for a Thai flowering room.
Outdoor and semi-outdoor growing
For much of the year, natural day length here is at or below the range where many high-THC cultivars start to flower. So photoperiod plants grown outdoors can start flowering early (see why photoperiod plants flower at six weeks in southern Thailand). The planting date may partly decide whether a plant flowers under an effective 12 hours or 13 hours. Changing the planting date costs nothing, but this is only reasoning. No field study has tested it.
What to do: for each outdoor crop, record your latitude, the planting date and the date the first flowers appear. After two or three seasons, these records show which planting months give you the flowering day length you want.
6. What the study does not prove
- Only two cultivars were tested. Both responded, which is encouraging. But two cultivars cannot tell you how yours will respond.
- The study tested one step, not a range. It compared 12 hours with 13 hours only. Do not extend the result to 14 hours or more (section 2).
- Commercial-scale rooms were not tested. The researchers say so themselves.
- Terpenes were not measured. The study measured cannabinoids only. Terpenes are the compounds that give cannabis its aroma.
- The study has not been repeated in the tropics. The trial ran in a growth chamber, not in a hot, humid room. A 2025 Wageningen study found that high air temperature lowered cannabinoid concentration (covered in Part 2). Nobody knows whether the gain from 13 hours holds in a Thai room without air conditioning.
What to do with this: treat 13 hours as a trial in one room or tent, not as a new standard for the whole farm. Part 2 gives the rules for a fair test.
If you have measured the day length your plants sense at your site, post your latitude, the month and the figure below this topic. Other growers can use these numbers to plan their blackout and lighting times.
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
- Ahrens A., Llewellyn D., Zheng Y. (2023). Plants 12(14):2605. https://doi.org/10.3390/plants12142605
- Holweg et al. (2025). Environmental and Experimental Botany 230:106085. Redirecting
Continued in Part 2: Testing a 13-Hour Flowering Schedule on Your Own Farm, and the 2025 Far-Red Trial (Part 2 of 2)