The local cannabis populations of Thailand, Laos, Vietnam and Cambodia are disappearing. Most are being lost by crossing with imported plants, not by eradication. This part explains what traits they are reported to carry, how they are lost, and what real preservation requires.
A landrace is a crop population that farmers keep and resow in one area, generation after generation. Its plants vary from each other, and nobody kept formal breeding records. It is a population, not a single strain. Each claim below carries one of three labels. Documented means traceable to a paper, book or official record. Commonly reported means repeated by growers but with no first-hand record. Seedbank claim means it comes from a seed listing and cannot be checked.
Growing cannabis is a criminal offence in Vietnam, Laos and Cambodia, and Thai cannabis flower is tightly controlled. This article is not a growing guide and names no seller. Preservation is discussed only as work for institutions, for licensed operators, and for places where that work is legal.
This is Part 2. Part 1: Southeast Asian Landrace Cannabis: Thailand, Laos, Vietnam and Cambodia — History, Traits and What Is Left to Preserve
7. Traits reported for Southeast Asian sativa populations
Everything in this section is what these sativa populations are reported to do. Almost none of it has been measured on documented plants growing where they come from.
7.1 Flowering time and day length
Many cannabis plants start flowering only when the day becomes shorter than a set length. That length is called the plant’s critical photoperiod. Near the equator, day length barely changes during the year. A plant that waits for clearly short days can therefore wait a long time.
Zhang and colleagues documented this in Frontiers in Plant Science in 2021. In the hemp cultivars they tested, critical photoperiods ranged from about 13 hours 45 minutes to more than 15 hours 30 minutes, and a few fibre cultivars showed no clear limit. Fibre and grain cultivars from further south flowered later than those from the north. In sensitive types, a change of just fifteen minutes in day length changed when flowering started.
For Southeast Asian populations, flowering periods of fourteen to twenty weeks are commonly reported. These figures come from growers, not from measurement.
Translator’s note: In Thailand, day length changes only a little through the year: from about 10 hours 55 minutes to 13 hours 20 minutes in Chiang Rai, and from about 11 hours 40 minutes to 12 hours 30 minutes in Hat Yai. Fifteen minutes can change flowering in sensitive plants. That means a security lamp or street light near a licensed outdoor or greenhouse crop can make the plants’ day longer. A flowering period of 14–20 weeks also means flowering runs through much of the rainy season. For how temperate-bred plants behave at Thai latitudes, see Why Photoperiod Plants Flower at Six Weeks in Southern Thailand.
What you can do: If you run a licensed trial of a long-flowering local type, record three dates for each plant: the start of flowering, the first visible pistils and the harvest. This turns grower estimates into measured data.
7.2 Plant shape, stretch and flower density
These traits are commonly reported across independent accounts:
- very strong stretch after flowering starts: the internodes (the pieces of stem between two leaf joints) become very long;
- a tall, open frame;
- loose flowers in long, airy clusters, not dense buds;
- thin leaflets;
- late and uneven ripening, with different parts of one plant ready at different times.
What you can do: In a licensed trial, measure plant height at the start of flowering and again when stretch stops. Use the numbers to check whether your room or greenhouse is tall enough before the next run.
7.3 Humidity and mould
In the monsoon, humidity stays high for months while plants are flowering. That is exactly when Botrytis cinerea does its damage. It is the fungus behind grey mould, or bud rot.
Plant-disease research on greenhouse cannabis, especially by the group of plant pathologist Zamir Punja at Simon Fraser University in Canada, points to two main causes: high humidity and dense flower structure. A loose, airy flower dries quickly after rain, but a dense one holds water.
Mould tolerance in regional populations is therefore plausible. But true genetic resistance in named populations is only commonly reported. Avoiding mould because of flower shape (“structural escape”) is also a different trait from resistance. With escape, the flower simply dries before the fungus can grow. With resistance, the plant fights the fungus itself.
Translator’s note: An airy plant loses this advantage in a crowded room with weak air movement. In Thai rainy-season conditions, good airflow and spacing still matter for any cultivar. Mould on flower can also leave toxins behind: see Mycotoxins on Cannabis Flower. If you need a fungicide, use only products registered with the Department of Agriculture (กรมวิชาการเกษตร), and follow the label.
What you can do: In a licensed trial, check every plant for bud rot after each wet week and record the result. Compare airy and dense plants from the same seed lot. This shows whether low mould comes from flower shape or from something else.
7.4 Aroma chemistry
The Thai leaf profile from Watanakul and colleagues (2024) was led by ocimene, limonene and terpinolene, with caryophyllene also present. This differs from the myrcene-led profiles typical of commercial cultivars. For background on these aroma compounds, see Terpenes: What They Do and What Is Overstated.
But it is one measurement, taken on leaf, from one Thai type. It is not a fingerprint for the whole region. Linalool, pinene and humulene appear in descriptions of regional plants, but no published measurement exists for them.
What you can do: When you read a lab certificate for material sold as Thai, check whether the terpenes were measured on flower or on leaf. Do not accept one aroma profile as proof of origin.
7.5 Why these plants struggle far from the tropics
Taken to northern latitudes, these plants run into five problems:
- Long summer days delay flowering or stop it altogether.
- Autumn comes before the flowers are ripe, so frost and rain arrive in the middle of flowering.
- A sixteen-week flowering period does not fit into a short frost-free season.
- The stretch is too tall for most indoor rooms.
- Cool, humid nights slow ripening, in exactly the conditions that most favour mould.
None of this means the plants are weak. They are adapted to a year of roughly twelve-hour days and to a monsoon. A plant like this is behaving correctly when it fails to finish in Poland in October.
Translator’s note: Thai growers face the same mismatch in reverse. Cultivars bred for temperate countries often flower early in Thailand, and their dense buds are exposed to rainy-season humidity. Neither group of plants is “bad”. Each fits a different climate.
What you can do: Before choosing any cultivar, licensed growers should compare where it was bred with where it will grow, including day length, humidity and season. Then start with a small trial lot, so that a mismatch costs you a few plants rather than a whole crop.
8. Why these genetics matter to modern breeding
| Trait carried | Why it is rare elsewhere | Who needs it |
|---|---|---|
| Tolerance of almost constant twelve-hour days | temperate breeding never had to select for it | licensed growers in the tropics |
| Avoiding bud rot in the wet season | dense-flowered cultivars were bred in rooms with dehumidifiers | humid greenhouse and outdoor crops |
| Aroma led by terpinolene and ocimene | the commercial gene pool narrowed to myrcene and limonene | breeders who want something different from the standard profile |
Cannabis is dioecious, meaning male and female flowers grow on separate plants (see Cannabis Sex Determination). Its pollen is carried by the wind. A landrace therefore crosses with any cannabis flowering nearby. No certification system or variety register protects it. A single season of stray pollen is enough to cross a population.
What you can do: If you are a licensed breeder and want these traits, look first for material with a written origin record (Part 1 explains how to check). Selection takes years, so verify the starting material before you begin.
9. Genetic erosion: how a landrace disappears
Most landraces are lost by dilution, not eradication. Imported cultivars arrive in a village, and they flower quickly and form dense buds. Many descend from OG Kush, Blue Dream, Girl Scout Cookies and Gorilla Glue. Their pollen reaches any local plants flowering nearby.
The hybrid plants finish sooner and give denser flowers, so those are the seeds growers keep. Within a few cycles the local population becomes a hybrid swarm: almost every plant is a mix of local and imported parents, in different proportions. The original population no longer exists as a separate group.
The second cause is the end of seed saving. Growing becomes a crime, and younger people leave farming. Bought seed is cheaper than keeping your own. Spoken knowledge dies with the people who held it.
In 2024, Halpin-McCormick and colleagues found wider genetic diversity in modern cultivars than in the landrace samples available to them. That result says as much about how few landraces sit in collections as it does about the plants.
Welling and colleagues explained why in Frontiers in Plant Science in 2016. International narcotics treaties have restricted collections of cannabis genetic material. As a result, East Asian material is a priority but is poorly represented. Prohibition stopped not only growing but also the collecting and storing of samples.
Translator’s note: Since 2022, imported cultivars have spread widely in Thailand. A licensed farm keeping a local population can get stray pollen from any flowering plant within pollen range. That includes plants on neighbouring farms, and any male or hermaphrodite plant among its own crops.
What you can do: If you keep a local population under licence, list every other cannabis crop flowering near it, including your own. Keep the local plants in a closed structure, or arrange for them to flower at a different time. This keeps outside pollen from crossing into the seed you save.
10. What preservation actually requires
The method is the same as for any outcrossing crop, meaning a crop whose plants are fertilised by pollen from other plants. It is set out in the Genebank Standards for Plant Genetic Resources of the Food and Agriculture Organization (FAO) of the United Nations. A genebank is a facility that stores seed or plant material so that it can be grown again later.
Isolated seed increase. Cannabis is wind-pollinated and dioecious. If you grow out a seed lot to make more seed without isolating it, you are making a cross. Isolation means distance, an enclosed structure, or pollinating branches inside bags.
A large enough population. The value of a landrace is the variation between its plants. If you make new seed from only two or three plants, you get a bottlenecked line: most of the variation is lost, and the seed no longer represents the source. The FAO standards require a population large enough to keep its genetic integrity, but they set no number for cannabis. Any exact figure quoted as “official” was made up.
Records of origin. Record the location (with coordinates if possible), the altitude, the date and the collector’s name. Add the local name and the local use, the number of parent plants, and the number of generations since collection. The Đà Lạt material described in Part 1 has none of these records.
Legally, this work belongs to four groups:
- regional research institutes and universities, which are the only ones able to take samples from the field;
- licensed growers, in places where growing is legal;
- public genebanks;
- herbaria (libraries of pressed, dried plant specimens) holding lawfully collected specimens.
Community members can contribute spoken history and photographs, but nothing involving seed. Informal seed movement across borders is not on that list. It is a crime in most of the region. It also destroys provenance, the record of where the seed came from.
In short: how a landrace is kept from being crossed away
- Let it flower only where no other cannabis pollen can reach it.
- Make each new seed generation from many parent plants, and record how many.
- Write down where it came from, who collected it and how many generations it has been grown since.
- Leave the plant work to legally authorised institutions and licensed operators. Everyone else can record history, names and photographs.
Translator’s note: In Thailand, research, export, sale or processing of a controlled herb without a licence breaks Section 46 of the Thai Traditional Medicine Protection Act B.E. 2542. Under Section 78, the penalty is up to 1 year in prison, a fine of up to 20,000 baht, or both. A licensed farm planning seed-keeping work should first confirm which licence covers it: see Licensing and Compliance in Thailand. Heat is a practical problem for stored seed. At 30–35 °C room temperature, or in a fridge that loses power in an outage, seed may lose viability sooner. See Storing Seeds and Pollen for what determines seed life.
What you can do: If you are an institution or licensed operator, write your origin record sheet before you sow anything. Use the fields listed above as its columns. A seed lot without these records cannot later be shown to be a landrace.
10.1 Ethics and the legal reality
- The community where the plants come from holds both the knowledge and the material. Under national law and the Convention on Biological Diversity (a United Nations treaty), benefits must be shared with that community. This is not optional.
- A local name is not a brand. Putting a place name on a product that has no collection record misleads buyers. It is the biggest source of false information on this subject.
- Collect from many plants and record how many. Never ask anyone to break their own country’s law for you.
| Country | Growing | Moving seed |
|---|---|---|
| Thailand | flower is a controlled herb; sale only with a PT 33 prescription since June 2025 | licensing and plant import rules apply |
| Laos | illegal; severe drug penalties | criminal offence |
| Vietnam | illegal | criminal offence |
| Cambodia | illegal; enforcement comes and goes | criminal offence |
Tolerated does not mean legal. A law that is not enforced is still a law, and enforcement priorities can change without warning. In this region, carrying cannabis seed across a border is a drug offence whatever your intention. Saying you are preserving a landrace is no defence.
Translator’s note: For the current Thai rules on seed, see Buying Cannabis Seeds in Thailand in 2026.
What you can do: Before any seed-related step, check the law of every country the seed would pass through. If any one of them prohibits it, do not move the seed.
Share what you know
Families who grew cannabis for fibre, for cooking or for a grandmother’s remedy hold primary evidence, and very little of it has been written down. That includes families in Isan, the northern Thai hills, Lào Cai or Lai Châu, the Lao uplands or a Khmer village. If this is your family, post below this topic and include:
- the local name, and what it means;
- what it was used for, and which part of the plant;
- how tall it grew and how long it took to finish;
- who kept the seed.
If you have old photographs, post them with a rough location and date. You can also post photographs of plants growing near you today, but only photographs; do not collect or move anything. Corrections are especially welcome. If something in this article is wrong, reply and explain how you know.
References
- Zeven (1998). Review of landrace definitions. Euphytica.
- Camacho Villa and colleagues (2005). Plant Genetic Resources.
- Zhang and colleagues (2021). Critical photoperiod in cannabis. Frontiers in Plant Science.
- Plant-disease work on greenhouse cannabis by Punja’s research group, Simon Fraser University (no single paper named in the original article).
- Watanakul and colleagues (2024). Leaf terpenoids of the Thai ‘Hang Krarok’ type. Journal of Pharmacy.
- Halpin-McCormick and colleagues (2024). Comparison of nine cannabis collections. Genetic Resources and Crop Evolution.
- Welling and colleagues (2016). Cannabis genetic resource collections. Frontiers in Plant Science.
- Food and Agriculture Organization of the United Nations. Genebank Standards for Plant Genetic Resources.
- Convention on Biological Diversity.
- Thai Traditional Medicine Protection Act B.E. 2542, Sections 46 and 78.
This article is part of the Asiannabis Community educational series on cannabis genetics and heritage. It is for education only, in places where growing cannabis is legally permitted.