Choosing Cannabis Genetics for Rooms Above 30 °C and 85% Humidity: Why No Thai Shortlist Exists Yet and How to Test Your Own (Part 2 of 2)

🧬 Genetics & Seeds Strain Reviews
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No reliable list of heat-tolerant cannabis strains for Thailand exists yet. This article explains why, gives a six-step method for building your own shortlist from your own room, and looks at what common forum recommendations such as Thai landrace, G13 Haze and White Widow can still tell you.

The starting point comes from a Dutch trial at Wageningen University. Two cultivars (named plant varieties; growers usually say “strains”) were flowered at 31 °C day / 27 °C night and at 25 °C day / 21 °C night. In the heat, both lost cannabinoid content. Only one of them, ‘Harmony CBD’, also lost flower weight; ‘Original Blitz’ did not. So “heat tolerant” is at least two traits: keeping yield, and keeping potency (cannabinoid content, measured by a lab).

This is Part 2. Part 1: Choosing Genetics for Rooms Above 30 °C and 85% Humidity: What the Heat Studies Say, and Why Any Thai Shortlist Is Still Provisional

4. Why no reliable Thai shortlist exists

There are four reasons, and each one makes the others worse.

1. No published heat-tolerance trial of cannabis cultivars in Thailand could be found. Thai researchers have compared Thai lines. One study in a Thai journal measured leaf cannabinoids in Hang Kra Rog Phu Phan, Hang Suea Sakon Nakhon and two Tanao Si lines. A hemp-seed trial at Maejo University in Chiang Mai compared four research lines (RPF1 to RPF4) across seasons. Neither study tested heat tolerance, and the Maejo lines cannot be matched to anything sold commercially.

2. The published heat, light and humidity studies used a handful of foreign cultivars:

  • ‘Original Blitz’ and ‘Harmony CBD’ in the Wageningen heat trial (Holweg and colleagues, Netherlands)
  • ‘Northern Lights’, ‘Hindu Kush’ and ‘Cannatonic’ in a 2025 far-red light study (Peterswald and colleagues, published in Scientific Reports)
  • ‘Cherry Berry’ in a 2025 humidity study (Corredor-Perilla and colleagues, published in Frontiers in Plant Science)

Some of these names are sold in Thailand, but by many unrelated sellers. Seed sold here under one of these names is not necessarily the same genotype that was tested. Genotype means the plant’s genetic make-up (genotype, phenotype and seed-label terms).

3. Strain names in this market do not reliably identify genetics. Sellers widely reuse well-known foreign names, and strain names rarely have legal protection. A shortlist of names only helps if the names identify the genetics. Here they often do not.

4. Grower reports mix many factors together. A plant that did well in Phuket did well at that temperature, that humidity and that day length. It also had that feed and that grower’s skill. Without a canopy temperature log and a lab potency test, you cannot tell what “handled the heat well” was due to.

Translator’s note: The same name problem applies to clones. A clone bought from another farm may not be the cultivar on its label. It can also bring pests or hop latent viroid (a tiny infectious particle that spreads through sap, seed and pollen) into your room. Keep new clones apart from your main room until you have checked them. The guide to vetting a Thai seed vendor covers copied strain names and old stock.

Treat every strain name on a list as a candidate to test, not as a promise. Buy test seed only from a seller you have checked.

5. The method: six steps to build your own shortlist

These six steps let you build a shortlist from your own results instead of borrowing someone else’s.

Step 1. Decide what “tolerance” means before you test. Pick one target:

These are different traits and probably need different genetics. Lumping them into one “heat tolerant” score is why the existing reports are so hard to use.

Step 2. Grow at least four cultivars at the same time. The viroid, far-red and off-season results in Part 1 only appeared because each study grew several cultivars. A trial with one cultivar cannot show differences between cultivars, and those differences are exactly what matters here. Four is a workable minimum. It is also the number of lines the Maejo hemp study used.

Step 3. Log temperature and VPD at the canopy, not room readings. VPD (vapour pressure deficit) combines temperature and humidity into one number that shows how strongly the air pulls water out of the leaves (VPD, temperature and night humidity). The published figures describe the conditions the plants actually experienced. A result without an environment log is not a result. Missing logs are the most common reason grower comparisons cannot be put together.

Step 4. Measure potency, not just weight. In the Wageningen trial, the cultivar that kept its weight in the heat still lost cannabinoid content. Selecting on weight alone can therefore select for the wrong trait.

Step 5. Record intersex plants, and record your own light leaks and stress events next to them. Here, the effects of environment and genetics are mixed up. The only way to separate them is to log the environment. An intersex plant in a tent with a 34 °C canopy or a light leak tells you about the tent, not about the seed.

Step 6. Record the days from flip to harvest. “Flip” is the day you switch the lights to the flowering schedule (when to switch from veg to flower). In a monsoon climate, finishing before the rains matters as much as yield. In one study of a single cultivar, high relative humidity delayed flowering by about three weeks (Corredor-Perilla and colleagues, 2025; see also high humidity and cannabinoid loss). No strain description will warn you about that.

Translator’s note: In a Thai room, a notebook or a spreadsheet with one row per day is enough. Record the date, the highest and lowest canopy temperature, humidity, power cuts, air-conditioner failures, light leaks, any plant showing male flowers, and the flip date. Write the cultivar name and where the seed came from at the top. This record is what lets other growers compare your result with theirs.

Translator’s note: If you grow outdoors or in a greenhouse, count back from the usual start of heavy rain in your area. Use the flip-to-harvest days you recorded to set the latest date a cultivar can start flowering and still finish before the rain.

6. What the forum recommendations can still tell you

The recommendations deserve to be taken seriously, but not read as more than they are.

Thai landrace material starts with a real advantage. A landrace is a local population kept and replanted by farmers in one area over many generations. These populations survived at this latitude, in this humidity and with this day length, without any climate control. That is a real history of selection by the local environment, and it is the strongest argument on the list before any testing. The downsides are long flowering, large plants, and lower cannabinoid content than modern hybrids. Southeast Asian Landrace Cannabis: What the Records Show for Thailand, Laos, Vietnam and Cambodia covers these trade-offs.

Haze-heavy lines share part of that advantage, because Haze genetics trace partly to tropical, low-latitude material. G13 Haze appearing in a southern Thailand recommendation fits with that. The downsides are very long flowering and large plants that are hard to keep at a manageable height.

White Widow is harder to judge. It is a widely distributed, long-established hybrid, so the name is attached to a lot of unrelated material. When it appears in a recommendation, the result may reflect the particular line that grower had, not anything about “White Widow” as a whole.

Autoflowers need a separate note. The thread about heat-stunted autoflowers points to a plausible cause. An autoflower has a short, largely fixed life cycle. That leaves little vegetative time (the leafy growth phase before flowering) to recover from a stress event. Little peer-reviewed autoflower research from 2024 to 2026 could be found, and most available material is seed-company content. So heat tolerance in autoflowers is essentially unstudied. See Autoflowering Cannabis: Ruderalis Genetics, Seed Choice, Pot Size, Light and Training and Autoflowering Cannabis: Where the Trait Comes From and Why Autos Need Different Care.

To test the forum’s names properly, put one Thai landrace, one Haze-type line and one or two modern hybrids into the same four-cultivar trial from section 5. That turns the recommendations into something you can measure.

7. Why this article offers no list, and what would change that

This article does not offer a Thai heat-tolerance shortlist. Such a list would rest on four forum recommendations with no environment logs and no potency data. People would read it as guidance, and it would not deserve that.

For now, the strongest evidence-based step is not to pick a name from a list. It is to grow four cultivars at once and log the environment.

These would change the picture:

  • Four cultivars in one room, with canopy temperature and VPD logged, potency tested, intersex plants recorded and flip-to-harvest days counted. If three or four growers in different provinces did this, the forum would have a useful tropical dataset.
  • Thai landrace grown next to a modern hybrid in the same room, with numbers. The landrace advantage is the most plausible hypothesis available, and nobody has tested it.
  • The 31 °C finding repeated on your own genetics. Use the two-cycle check from Part 1: log canopy temperature hourly at four points, change only the temperature between two cycles, test buds from the same positions, and dry and cure both harvests the same way.

Share your results here

If you have grown more than two cultivars side by side in a Thai room and measured potency, not just weight, please post the result below. The same applies if you have tested one cultivar from a hot cycle and a cooled cycle with identical drying and curing. Include the canopy temperature log with the numbers. Without the log, nobody can interpret the result. The template in How to Write a Strain Review Other Growers Can Use helps you include the facts other growers need.

Related reading:

References:

This article is technical horticultural information for readers operating legally where they live. Cannabis cultivation is regulated differently in every country. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. It may be sold only against a PT 33 prescription, and it may not be advertised or sold online. Selling flower requires a licence. The draft Cannabis Control Act is not law yet: the Cabinet approved the draft on 22 September 2026 and sent it to the House. Strain names mentioned are reported from growers’ published accounts and are not endorsements. Nothing here is legal advice.

Asiannabis Library: Growing and Genetics