A grower in Khanom put Jack Herer and Kalashnikova outside in April 2025, and both flipped to bud at forty-five days. He posted asking what he had done wrong. If both were photoperiod plants, he had done nothing wrong. This is one of the most often repeated outdoor growing problems in Thailand, and it comes up across at least five separate forum threads. The cause is a fact of geography that most grow guides never mention: near the equator, the day never gets long enough to hold a photoperiod plant in veg. This article explains how that works, then goes through the responses growers here actually use, with the cost and the failure mode of each.
1. The problem, in growers’ own words
This is not a hypothesis. Growers in Thailand keep reporting it:
- THCFarmer, “The impossible outdoor Phuket grow”: a long thread running since December 2023. The thread concluded: “supplemental lights for sure… necessary every day.”
- ASEAN NOW, “Any tips on growing outdoors?”: general outdoor advice for Thai conditions.
- Rollitup, “Growing on balcony in Thailand”: one grower sums up the problem as “only about a 30 min difference in daylight hours throughout the year.” The idea is right but the figure is a grower’s underestimate. The real numbers are in section 2.
- THCFarmer, “Growing outdoors with consistent light of 12/12”: a thread that exists because growers here have to plan around it.
- ASEAN NOW, “Growing outdoors in the south”, April 2025: the Khanom case above, with Jack Herer and Kalashnikova flowering at 45 days and the grower asking why.
Five separate threads, one cause. If this has happened to you, you are not a bad grower and your seeds were not defective.
A note on the Khanom case. Green House Seeds sells Kalashnikova as a feminised photoperiod strain, and sells Kalashnikova Automatic as a separate product. The thread does not say which version he grew. If it was the regular feminised one, both plants did exactly what this article describes.
2. The mechanism
Photoperiod-sensitive cannabis starts flowering when the day becomes shorter than a critical length, which is the same as the night becoming longer than a critical length. That threshold depends on the cultivar. In a 2021 University of Florida trial of 27 hemp cultivars, the shortest critical daylengths found were between 13 h 45 min and 14 h. In a 2024 University of Guelph trial, two high-THC cultivars flowered normally under 13 h days. A fair working rule: many cultivars flower once daylength falls below roughly 13–14 hours, which means nights of about 10–11 hours or more.
In temperate countries this is a seasonal event. Northern Europe swings from roughly 17 hours of daylight in June to 7 hours in December. A plant put outside in May gets a long, clear vegetative season, and flowering is triggered by a predictable shortening of the days in late summer.
Thailand does not have that swing. Approximate sunrise-to-sunset daylength at the solstices:
| Location | Latitude | Shortest day (Dec) | Longest day (Jun) | Annual swing |
|---|---|---|---|---|
| Phuket / Hat Yai | ~7–8° N | ~11 h 40 min | ~12 h 35 min | under 1 hour |
| Bangkok | ~13.7° N | ~11 h 20 min | ~13 h 00 min | ~1 h 40 min |
| Chiang Mai | ~18.8° N | ~11 h 00 min | ~13 h 20 min | ~2 h 15 min |
No part of Thailand gets much more than about 13 h 20 min of daylight on any day of the year. That is below the threshold at which many cultivars flower.
The consequence: a photoperiod plant put outside anywhere in Thailand is already under its flowering daylength. There is no long vegetative season to grow into. The plant does what it is built to do and starts flowering as soon as it is mature enough to respond, which growers commonly report at around four to six weeks from seed.
North versus south. The south has it worst. At Phuket or Hat Yai the longest day of the year is only about 12½ hours, so there is no season in which an unlit plant will stay in veg. In the north, the May–July days of about 13 hours sit near the threshold of some cultivars. They can slow flowering a little but will not reliably prevent it. The responses below apply to the whole country, but in the south they are necessary rather than optional.
This is why “just put it outside after the rainy season” advice imported from Europe or California produces a 30 cm plant with a single cola. The plant never got a vegetative phase.
3. The responses, with honest costs
Response 0: Do the veg indoors, and pick your planting date
What it is: start seedlings or clones indoors under 18/6 and move them out only once they have reached the size you want. Outside, they will begin flowering soon after the move unless you also use Response 1, so the indoor phase is your vegetative phase. Large, well-rooted clones shorten the whole process.
Planting date: if you do put plants out unlit, plant around the longest days (May–July). This gives the most natural daylength the year allows. The gain is real in the north, where June days reach about 13 h 20 min. In the far south it is small, because the longest day there is barely longer than the shortest.
Cost: a small indoor space and light for a few weeks.
Verdict: the simplest way to get a bigger plant outdoors. It pairs well with Responses 1 and 3.
Response 1: Supplemental light to extend the vegetative day
What it is: a low-wattage light on a timer, run in the evening or as an interruption of the dark period, to keep the effective daylength above the flowering threshold until you choose to stop.
Why it works: you are not trying to grow the plant with the lamp. Sunlight does that. You are only trying to break the dark period so the plant does not read a long night. A signal needs far less light than photosynthesis does.
Practical settings (a starting point, not a tested protocol for Thai conditions):
- Evening extension: run the lamp from sunset so that sunlight plus lamp adds up to at least 16–18 hours.
- Or a night break: 1–2 hours of light near the middle of the dark period, for example roughly 23:00–01:00. Splitting the night is the aim, so a break placed right after sunset does far less than one in the middle.
- Intensity: low. Before you rely on it, take a hand-held light meter (or a phone light-meter app as a rough check) to the canopy at night with the lamp on. The reading should be clearly above zero across the whole plant, including the lower and far-side branches.
- Placement: above and aimed down at your own plants.
Evidence it is what growers here settle on: the Phuket thread concluded “supplemental lights for sure… necessary every day.”
Cost: low. A modest lamp per plant or per group, a timer, and weatherproofing.
Failure modes: it has to run every night. Missed nights, especially several in a row, can be enough to start flowering in a sensitive genotype. Once flowering has started, reverting a plant is stressful and slow. Weatherproofing in a monsoon climate is a real engineering problem, not an afterthought. Stray light can also affect neighbouring plants and people, which is one more reason to keep the lamp low, shielded and aimed downward.
Verdict: the standard answer, and the one that gives you a real vegetative phase outdoors. The timer and the weatherproofing are the parts that decide whether it works.
Response 2: Grow autoflowering plants instead
What it is: sidestep the problem. Autoflowers flower on age rather than photoperiod, so equatorial daylength does not affect them.
Why it fits here: it is the only response that needs no equipment, no discipline and no schedule. It also explains some confusing results. Many strain names, Kalashnikova among them, exist in both a photoperiod and an “Automatic” version. Check which one you have before you diagnose a fast finish.
Cost: genetics only.
Failure modes: a short, fixed life cycle means you cannot delay harvest to dodge a rain window, and in a monsoon climate being able to shift a harvest by two weeks is worth a great deal. Autoflowers also cope less well with the stress of tropical growing. A heat-stunted auto has no extra vegetative time to make up the loss, and growers report this as a problem in high heat (THCFarmer, “Autoflowers heavily stunted by heat?”).
Verdict: the right answer for a small personal grow with no infrastructure. A harder fit for anyone whose harvest date needs to work around the weather.
Response 3: Accept an effective 12/12 season and plan around it
What it is: stop fighting it. Accept that a photoperiod plant put outside will flower early. Plan for short plants and more of them, or start from a large well-rooted clone (Response 0) so that “small” still means productive.
Why growers do it: it costs nothing. The THCFarmer thread on growing outdoors under consistent 12/12 light shows that some growers run it on purpose, not by accident.
Cost: nothing in equipment; a real cost in yield per plant.
Failure modes: you get one flowering window’s worth of plant size and no chance to build a frame first. Yield per plant will be a fraction of what the same genetics give after an eight-week vegetative phase. Sea-of-green style planting makes up for this with more plants. In Thailand, the number of plants and the site must stay within your permit and the conditions of the 2025 Controlled Herbs (Cannabis) notification.
Verdict: honest, cheap, and much better than the accidental version of the same thing. Planning for it instead of being surprised by it is the difference between a crop and a disappointment.
Response 4: Blackout for harvest-window control only
What it is: a lightproof cover used to fix the date flowering starts and give a clean, consistent long night. It does not create a vegetative phase. In Thailand the natural day already triggers flowering, so blackout cannot give you veg. Only added light (Response 1) or an indoor start (Response 0) can.
When it is worth using: in the long-day months, roughly May to mid-August in the north, when days run at about 12 h 45 min to 13 h 20 min. Some cultivars respond slowly or unevenly to those days. A daily blackout then makes flowering start on the day you choose, typically after a lit veg phase. From November to February, days across Thailand are under about 11 h 50 min, so the natural night is already long enough and blackout adds almost nothing. In the far south it is rarely worth the work in any month.
You do not necessarily need to force a full 12 hours of darkness. In the 2024 Guelph trial, two cultivars flowered well under 13 h days. Whether that holds for your genetics outdoors in Thailand is untested.
Cost: moderate for a DIY cabinet over a few pots; high for a greenhouse with a motorised curtain. The bigger cost is daily labour.
Failure modes: a closed blackout cover traps heat and humidity, and in a Thai wet season it becomes a humidity chamber unless it is ventilated while closed. Irregular dark periods and light leaks are a known stress factor that can increase the risk of hermaphroditism.
A DIY blackout cabinet for potted plants
For a handful of pots, a cabinet does the job without a greenhouse:
- Frame: PVC pipe, sized for the plant and its pot with headroom for growth, and weighted at the base so it does not blow over.
- Cover: black-and-white “panda film”, white side out to reflect sun, black side in. Plain black plastic in Thai afternoon sun turns the cabinet into an oven.
- Ventilation: a vent low down and another at the top, each shielded by an overlapping, baffled flap of the same film, so air passes through a light-blocking bend. Gaps let light in, and missing vents let heat and humidity build. Either mistake can cost the crop.
- Light-leak test: put a phone inside recording video on a sunny day, then watch the footage for any glow.
- Timing: cover in the late afternoon before sunset and uncover at the same time every day, at both ends. Pick times you can actually keep, set alarms, and have a second person who knows the schedule. A cheap timer-driven pulley removes most human error.
- Commitment: plan for daily covering through the whole flowering period, typically 8–10 weeks. If you cannot keep the schedule, do not start.
- Mould: check inside the cabinet and the densest buds regularly. Trapped night-time humidity is the added risk in this climate. Choose varieties with looser bud structure where you can.
The idea of a pot-sized cabinet comes from Organic Marijuana, Soma Style. That book was written for Dutch conditions, where blackout from late July brings flowering forward ahead of the autumn rain. That calendar does not carry over to Thailand, where the main rains run through the months a late-July start would flower in.
Verdict: a timing tool, not a vegetative-phase tool. Do not build one without a plan for airflow while it is shut. Greenhouse-scale structures are covered in topic 1280.
4. The four, compared
| Cost | Gives a real veg phase | Controls harvest date | Main failure mode | |
|---|---|---|---|---|
| Supplemental light | low | yes | yes | missed nights; weatherproofing |
| Autoflowers | genetics only | not needed | no | cannot dodge a rain window; heat stunting |
| Accept 12/12 | none | no | no | low yield per plant |
| Blackout | moderate to high | no (timing only) | yes | traps heat and humidity when closed |
5. What the threads get right that the guides get wrong
Three other points come up again and again in the Thai outdoor threads. They belong here because they are part of the same problem of imported advice not fitting:
Fifty percent shade cloth. Growers in the ASEAN NOW outdoor thread recommend 50% shade to prevent leaf curl under direct Thai sun. This is grower experience, not trial data. Full equatorial sun is not the same input as full Northern European sun.
Flood-style watering. The same thread describes soaking the medium, then letting it dry for one to two days, rather than watering on a fixed daily schedule. This is grower practice, not trial data.
Height gets away from you. One grower’s 20-plant outdoor crop reached 12 feet (about 3.7 m) without enough training. Equatorial and Haze-heavy lines are long-flowering and tall by nature. Topping, low-stress training and supercropping are the relevant techniques. The point here is only that they need to start earlier than a temperate schedule would suggest. Pots, drainage, staking and outdoor feeding are covered in topics 1325, 1341 and 1345.
6. What nobody has established
- No published field trial in Thailand of any of these responses that we are aware of. All of them are grower practice reported on forums, not tested protocols.
- No cultivar-by-cultivar critical daylength data for the genetics sold in this region. Which lines tolerate a long day and still flower well is still anecdote.
- No measurement of the humidity penalty inside a closed blackout cover in a monsoon climate. The physics is not in doubt, but nobody has measured the size of the effect.
- Most reported experience comes from English-language forums. Thai growers’ own experience is underrepresented, and Thai-language replies are very welcome here.
Which of these are you running, and what did it actually cost you? Practical detail matters more than principle here, especially for supplemental light: what wattage, on what schedule, weatherproofed how, and what happened the night the timer failed. And if you have run a blackout cover through a Thai wet season, how did you ventilate it, and what humidity did you measure inside? That is the information the threads above are still missing.
Related reading on this forum: topic 1280 (Greenhouse Growing: Light Deprivation and Tropical Structures); topics 1325 and 1073 on outdoor growing in Southeast Asia; topics 1341 and 1345 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
- THCFarmer, “The impossible outdoor Phuket grow.” https://www.thcfarmer.com/threads/the-impossible-outdoor-phuket-grow.155969/
- ASEAN NOW, “Growing outdoors in the south” (April 2025). https://aseannow.com/topic/1358964-growing-outdoors-in-the-south/
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation 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, issued 26 June 2025. Cultivation needs a licence and must comply with the conditions of that notification and your permit. Flower may not be sold or advertised online, and recreational use is not permitted. 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.