The middle ground between a sealed room and an open field: polytunnels and glasshouses, light deprivation to control the flowering trigger, and the structures that make sense in a climate with heavy rain, high humidity and very strong sun.
A greenhouse buys most of the light for free and gives back partial control of the environment, which is why it is the common choice for licensed production at scale in this region. It also inherits the hardest parts of both approaches: outdoor pest pressure with indoor humidity, since a closed house on a still tropical night is exactly the condition grey mould wants.
Two decisions shape the build. Light deprivation, meaning a blackout system that shortens the day on demand, is what turns a greenhouse into something that can run more than one cycle a year at this latitude, and it has to be designed in rather than added later. And ventilation is the whole game: passive side and ridge venting sized for the worst month, supplemented by forced air movement inside the canopy, is the difference between a working house and a mould incubator.
There are no reference articles in this sub-board yet; it is the newest board on the forum.
Related elsewhere on the forum: Outdoor Cannabis in Southeast Asia: Seasons, Strains and Pest Management in Outdoor Growing.
Day Length at This Latitude, and Why Light Deprivation Matters
Days in Thailand never get very long. Over a year, day length (sunrise to sunset) runs from about 11.3 to 12.9 hours in Bangkok and about 11.0 to 13.3 hours in Chiang Mai. Cannabis is a short-day plant, so a photoperiod plant in an unlit greenhouse here tends to start flowering soon after it is planted, at almost any time of year, often while it is still small.
That gives a tropical greenhouse two separate tools to manage, and it needs both:
- Lights to hold plants in veg. A short burst of light in the middle of the night, or extending the evening, keeps mothers and young plants vegetative. Low-power lighting is enough for this purpose; it is a signal, not a source of growth.
- Blackout to flower on schedule. A blackout system gives a clean, complete dark period of at least 12 hours, so a whole block starts flowering together and finishes on a date you choose, for example before the heaviest rain.
Blackout must be complete. Gaps at curtain edges, doors, vents and cable entries, or a light left on inside, interrupt the night and can delay flowering or trigger hermaphrodite flowers. Inspect from inside in the dark during the first week of every crop. Manual covering depends on someone closing it at exactly the same time every day; at commercial scale, a motorised curtain on a timer removes the most common error.
Heat and Humidity in a Tropical House
- Heat. A closed greenhouse in full tropical sun overheats within hours. Roof and side vents sized for the hottest month, retractable shade cloth, and cross-ventilation (air in low on one side, out high on the other) are the first line of control.
- Ventilation imports humidity. In the wet season the outside air is often at 80 to 90% relative humidity, so venting removes heat but brings moisture back in almost as fast as it takes it out. Pad-and-fan evaporative cooling also loses effectiveness as outside humidity rises, and it adds water to the air.
- Mechanical dehumidification is the only reliable way to hold humidity down in a flowering house in the wet season, and it is expensive in a large, leaky envelope. That cost is the main reason many growers here flower in a smaller sealed space instead.
- Air movement inside the canopy. Horizontal airflow fans prevent still, humid pockets in the lower canopy where grey mould starts. They are cheap and should not be left out.
- Anti-drip film. Condensation dripping from the roof onto flowers creates infection points. Use covering film with anti-drip properties, made for tropical UV.
Very high humidity costs quality even when no disease appears: a 2025 controlled study in Frontiers in Plant Science found that elevated relative humidity markedly lowered cannabinoid concentrations and delayed flowering development.
Keeping Pests Out, and Biological Control
- Screen every opening. Insect netting on vents and doors is not optional here. Thrips are smaller than whitefly and aphids and need a finer mesh, and finer mesh restricts airflow, so make vent openings larger to compensate.
- Entry discipline. Double doors or an anteroom, sticky traps near entrances, and clean footwear and tools.
- Match the predator to the pest. Amblyseius swirskii is used mainly against thrips and whitefly, not spider mites; for two-spotted spider mite, Phytoseiulus persimilis is the usual choice, and Aphidius colemani parasitises aphids. Release early, before pests reach damaging levels, and check whether the supplier has data for the temperatures your house actually reaches, because some predators work poorly in sustained heat.
Greenhouse, Indoor or Both: A Decision Framework
There is no single right answer; these questions decide it:
- Electricity cost and reliability. High cost or an unreliable supply pushes towards a greenhouse.
- Land. Cheap land favours a greenhouse; expensive urban space favours indoor rooms.
- What your buyer requires. A pharmaceutical supply contract with tight specifications is easier to meet indoors; biomass for extraction is not.
- Which season you flower in. Flowering in the cooler, drier months is far easier than flowering through the monsoon.
- Capital. A greenhouse gets you into production for less; a sealed room gets you consistent production for more.
- Your licence. Premises are specified in the licence; check before you build.
A combined model often suits this region best: a greenhouse for mothers and vegetative growth, with night-break lighting so the plants stay vegetative, and a smaller sealed, conditioned room for flowering, where humidity control, a reliable light cycle and consistent quality matter most. The expensive controlled space is then only as large as the flowering area needs to be.