Photoperiod cannabis planted outdoors in Thailand often starts to flower about four to six weeks from seed. By then it has not built a proper branch structure. The cause is daylength. Thai days never get long enough to hold the plant in its leafy growth stage. This part covers three responses: growing autoflowering plants, planning a crop around early flowering, and using a blackout cover to set the date flowering starts.
It ends with a comparison of all four main responses and three more lessons from Thai outdoor forum threads. It also lists what is still unknown. Part 1 covers the cause in detail and the two ways to keep a plant in leaf growth: starting it indoors, and adding light at night.
This is Part 2. Part 1: Why Photoperiod Plants Flower at Six Weeks in Southern Thailand, and the Four Ways Growers Stop It
1. The cause, in brief
A photoperiod plant switches from leaf growth to flowering when the day gets shorter than a certain length. The leaf-growth stage is called the vegetative stage. A University of Florida (United States) trial in 2021 found thresholds between 13 h 45 min and 14 h. Those were the varieties with the shortest thresholds among 27 hemp varieties. In a 2024 University of Guelph (Canada) trial, two high-THC varieties still flowered under 13-hour days, one only about 1.5 days later. Many varieties start flowering once the day falls below about 13–14 hours.
Thailand’s longest days are about 12 h 35 min in Phuket and Hat Yai, 12 h 55 min in Bangkok and 13 h 15 min in Chiang Mai. So a photoperiod plant put outside without extra light is already under its flowering daylength. In the south, no month of the year keeps it in leaf growth.
2. Option 2: Grow autoflowering plants instead
How it works. Autoflowering plants start flowering at a certain age, not when the nights get long. Short equatorial days do not affect them, so the problem in this article does not apply to them.
Why it fits Thai conditions. It is the only option that needs no equipment, no strict routine and no schedule. It also explains some confusing results. Many strain names exist in both a photoperiod version and an “Automatic” version. Kalashnikova from Green House Seeds, a Dutch seed company, is one example. The Khanom grower in Part 1 did not say which version he grew. Check which version you have before you decide why a plant finished fast.
Cost: only the price of the seeds.
What can go wrong:
- You cannot move the harvest date. An autoflower has a short, fixed life cycle. You cannot delay its harvest to avoid a stretch of heavy rain. In a monsoon climate, being able to shift a harvest by two weeks is worth a great deal.
- Heat stunting. Autoflowers cope less well with the stress of tropical growing. A plant stunted by heat has no extra leafy-growth time to make up the loss. Growers report this as a problem in high heat. See also choosing genetics for rooms above 30 °C.
Translator’s note: trying autoflowers in Thailand. What you get is a crop with no lamps, timers or covers, finishing on a predictable date. In Thailand, the main things to plan for are rain and heat. Count the plant’s expected life cycle forward from the sowing date. Then compare the harvest date with the usual heavy-rain weeks in your area. Before you buy seeds, read what is legal when buying cannabis seeds in Thailand.
What to do: in one season, sow a few autoflowers next to your usual plants. Write down the sowing date, the harvest date, the weather in the final weeks and any heat damage. This shows whether autos suit your site before you commit a whole crop. Autos suit a small grow with no equipment. They suit a grower less well if the harvest date has to fit around the weather.
3. Option 3: Accept early flowering and plan the crop around it
How it works. Stop trying to prevent early flowering. Accept that a photoperiod plant put outside will flower early, and plan for it. Grow shorter plants and more of them. Or start from a large, well-rooted clone grown indoors (Option 0 in Part 1), so that a small plant still gives a useful harvest.
Why growers do it. It costs nothing. Some growers report running this method on purpose, not by accident. (“12/12” means 12 hours of light and 12 hours of darkness, close to the natural Thai day.)
Cost: nothing in equipment, but a real cost in yield per plant.
What can go wrong. Each plant only grows during its single flowering period. It never gets weeks of leafy growth to build a branch structure first. Yield per plant will be a fraction of what the same genetics give after eight weeks of leafy growth.
Planting many small plants close together makes up for this. This method is called sea of green (SOG). In Thailand, the number of plants and the growing site must stay within your permit and within the conditions of the Controlled Herbs (Cannabis) notification B.E. 2568 (explained here).
What to do: before planting, estimate how many small plants you need for the harvest you want. Check that number against your permit. Then plant on purpose. A short crop that you plan gives a usable harvest, while the same crop by accident usually disappoints.
4. Option 4: Use a blackout cover to set the flowering date
How it works. A blackout cover is a lightproof cover put over the plants each evening. It sets the date flowering starts and gives the plants a clean, regular long night. It does not create a leafy growth stage. In Thailand, the natural day already triggers flowering, so a blackout cover cannot keep a plant in leaf growth. Only extra light at night (Option 1) or an indoor start (Option 0) can do that.
When it is worth using. The useful months are the long-day months, roughly May to mid-August in the north. Days there run at about 12 h 45 min to 13 h 15 min. Some varieties respond to those days slowly or unevenly. A daily blackout cover then makes flowering start on the day you choose, usually after a leafy stage kept going with a lamp.
From November to February, days across Thailand are no longer than about 12 hours. The natural night is already long enough, so a blackout cover adds almost nothing. In the far south, it is rarely worth the work in any month.
You may not need a full 12 hours of darkness. In the 2024 Guelph trial, two varieties flowered well under 13-hour days. Nobody has tested whether this holds for your genetics outdoors in Thailand.
Cost: moderate for a home-built cabinet over a few pots. It is high for a greenhouse with a motorised curtain. The bigger cost is the daily work.
What can go wrong:
- Heat and humidity build up. A closed blackout cover traps heat and humidity. In the Thai wet season, it turns into a humidity chamber unless air can move through it while closed.
- Male flowers on female plants. Irregular dark periods and light leaks are known causes of stress. They can raise the risk of hermaphroditism, where a female plant also forms male flowers that can pollinate the buds.
Translator’s note: Thai heat inside the cover. In Thailand, the cover goes on in the late afternoon, when air temperatures are often 30–35 °C. Put a thermometer–hygrometer with minimum/maximum memory inside the cover. Read it each morning when you uncover the plants. It shows how hot and humid the night inside really was. You can then decide whether the vents are working.
Building a home-made blackout cabinet for potted plants
For a few pots, a cabinet does the job without a greenhouse:
- Frame: use PVC pipe. Size it for the plant and its pot, with room above for growth. Weigh down the base so wind cannot tip it over.
- Cover: use black-and-white “panda film”, plastic sheet that is white on one side and black on the other. Put the white side out to reflect sun and the black side in. Plain black plastic in Thai afternoon sun turns the cabinet into an oven.
- Vents: put one vent low down and another at the top. Cover each vent with an overlapping flap of the same film, folded so air passes through a bend that blocks light. Gaps let light in, and missing vents let heat and humidity build. Either mistake can cost you the crop.
- Light-leak test: on a sunny day, put a phone inside the closed cabinet, recording video. Watch the footage for any glow. This finds leaks you cannot see from outside.
- Timing: cover in the late afternoon before sunset. Uncover at the same time every day, keeping both the start and the end fixed. Pick times you can really keep and set alarms. Make sure a second person knows the schedule. A cheap pulley driven by a timer removes most human error.
- Commitment: plan to cover the plants every day through the whole flowering period, usually 8–10 weeks. If you cannot keep that schedule, do not start.
- Mould: check inside the cabinet and the densest buds regularly. Humidity trapped at night is the extra risk in this climate, and it can lead to bud rot. Where you can, choose varieties with looser buds.
The idea of a pot-sized cabinet comes from the book Organic Marijuana, Soma Style. The book was written for Dutch conditions. In the Netherlands, starting the blackout in summer brings flowering forward so the harvest comes before the autumn rain. That calendar does not work in Thailand. Here, the main rains fall in the very months that a summer start would flower in.
What to do: build a cabinet only if you can cover and uncover at the same times for 8–10 weeks. Plan the airflow before you build, not after. Use the cabinet to set the flowering date, not to keep plants in leaf growth. For blackout at greenhouse scale, see Greenhouse Growing: Light Deprivation and Tropical Structures.
5. The four options compared
Option 0 from Part 1 (starting plants indoors) is not in this table. It is usually combined with one of the four options.
| Cost | Gives a real leafy growth stage | Controls the harvest date | Main thing that goes wrong | |
|---|---|---|---|---|
| Extra light at night | low | yes | yes | missed nights; weatherproofing |
| Autoflowers | seeds only | not needed | no | cannot move harvest away from rain; heat stunting |
| Accept early flowering | none | no | no | low yield per plant |
| Blackout cover | moderate to high | no (timing only) | yes | traps heat and humidity when closed |
What to do: pick the row that matches your equipment, budget and time. If you want both a leafy stage and a harvest date you control, you need extra light at night. You can add a blackout cover in the long-day months.
6. Three more lessons from the Thai outdoor threads
Three other points come up again and again in Thai outdoor forum threads. Imported grow guides usually miss them, for the same reason they miss the daylength problem.
Shade cloth. Growers in an ASEAN NOW outdoor-growing thread recommend shade cloth under direct Thai sun to prevent leaf curl. This comes from grower experience, not trial data. Full equatorial sun is not the same as full sun in northern Europe. What to do: shade some plants and leave others in full sun, then compare the leaves. This tells you whether shade helps at your site.
Soak, then let it dry. The same thread describes soaking the growing medium, then letting it dry for a day or two. This replaces watering on a fixed daily schedule. Again, it is grower practice, not trial data. What to do: lift each pot just after a soak and again as it dries. Learn the difference in weight, so you water when the pot is light, not by the calendar.
Plants grow taller than expected. Growers report outdoor plants growing far taller than expected when they were not trained enough. Varieties from near the equator, and lines with a lot of Haze in them, flower for a long time and grow tall by nature. Topping, low-stress training and supercropping (explained here) are the techniques to use. What to do: start training earlier than a temperate grow schedule suggests. For pots, drainage, staking and outdoor feeding, see topics 1325, 1341 and 1345.
7. What is still unknown
- No field trial in Thailand. No published field trial of any of these responses in Thailand has been found. All of them are grower practice reported on forums, not tested methods.
- No critical daylength figures for local genetics. Nobody has measured it variety by variety for the genetics sold in this region. Which lines tolerate a long day and still flower well is known only from growers’ stories.
- No measurement of humidity inside a closed blackout cover in a monsoon climate. The physics is clear, but nobody has measured how large the effect is.
- Most reports come from English-language forums. Thai growers’ own experience is under-represented. Replies in Thai are very welcome.
Share what you run
Please reply below this topic with which of these responses you use and what it actually cost you. For extra light at night, the details that matter are the lamp wattage, the schedule, the weatherproofing, and what happened the night the timer failed. If you have run a blackout cover through a Thai wet season, say how you ventilated it and what humidity you measured inside. The forum threads above still lack exactly these details.
Related reading on this forum
- Greenhouse Growing: Light Deprivation and Tropical Structures
- Outdoor Cannabis in Southeast Asia: Seasons, Strains, and Pest Management Under Tropical Conditions and Outdoor Growing in Tropical Southeast Asian Climate
- Transplanting Cannabis and Grow Bags for Cannabis, on outdoor pots, staking and feeding
References
- Daylength figures: sunrise-to-sunset values for 21 June and 21 December 2026, from sunrise-sunset.org (https://sunrise-sunset.org/), rounded to the nearest 5 minutes.
- Zhang M., Anderson S.L., Brym Z.T., Pearson B.J. (2021). Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science. Frontiers | Photoperiodic Flowering Response of Essential Oil, Grain, and Fiber Hemp (Cannabis sativa L.) Cultivars
- 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. 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 - PMC
- ASEAN NOW, “Growing outdoors in the south” (April 2025). https://aseannow.com/topic/1358964-growing-outdoors-in-the-south/
- Organic Marijuana, Soma Style (book; source of the pot-sized blackout cabinet idea).
This article is technical growing information for readers who operate legally where they live. Growing cannabis is illegal in most Southeast Asian countries. In Thailand, cannabis flower 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 flower need a licence. Growers must also follow the conditions of that notification and of any permit they hold. Flower may be sold only with a PT 33 prescription, for at most 30 days, and advertising and online sale of flower are prohibited. Recreational use is not permitted. The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569, in force since 30 April 2026, tightens the licence criteria for cannabis flower. The Cannabis Control Act is still a draft: Cabinet approved it on 22 September 2026 and sent it to the House. Nothing here is legal advice.