Autoflowering cannabis starts to flower at a fixed age. It does not wait for the nights to get longer. Modern autoflower varieties are much better than their old reputation for small, weak plants. According to the original article, current varieties from specialist breeders often give 100–200 grams per plant outdoors and reach 20–25% THC. They go from seed to harvest in 70–85 days. This part explains where the autoflowering trait comes from and how to choose seed. It also covers how pot size, light and training differ from photoperiod plants.
Most poor results with autos come from treating them like photoperiod plants. Common mistakes are transplanting, topping too late, giving too little light and feeding too strong. Each section below ends with what to do instead. Part 2 covers feeding, climate, harvest timing, planning several harvests and growing outdoors in Thailand.
Translator’s note: The yield and THC figures above come from the original article. Seed listings usually quote results from good conditions, so treat them as the top of the range (how to read a yield claim). A plant at 20–25% THC is high-THC cannabis, not hemp (where Thai law draws the line). 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. Part 2, section 12, sets out the Thai rules that matter to growers.
Autoflower or photoperiod: the difference, and what it means near the equator
A photoperiod plant keeps growing leaves and stems while the days are long. This is the vegetative stage. The plant starts to flower only when the nights become long enough. Indoors, growers trigger flowering by switching the lights to 12 hours on and 12 hours off (12/12).
An autoflower ignores day length. It starts to flower at about 3–5 weeks from seed under any light schedule. Its genes set most of its timing, from seed to harvest.
Translator’s note: In Thailand and the rest of Southeast Asia, day length changes little through the year and stays close to 12 hours. In Chiang Mai it runs from about 11 hours in late December to about 13¼ hours in late June. In Hat Yai it runs from about 11¾ to about 12½ hours. Outdoors, the nights can therefore be long enough to make photoperiod plants flower while they are still small (why this happens and how growers stop it). Autoflowers avoid that problem because their timing depends on age, not on the season.
Translator’s note: What you get from an auto is a short, predictable cycle in one room under one light schedule. A photoperiod plant is still the better choice if you want large plants, a mother plant to take clones from, or control over how long the plant grows before it flowers. To compare the two, grow a few autos and a few photoperiod plants side by side in one small trial. Record the days to harvest and the dry weight of each plant. Then you can compare them on your own site and under your own climate.
1. Where the autoflowering trait comes from
1.1 Cannabis ruderalis: plants from a short, cold summer
Cannabis ruderalis is a subspecies of Cannabis sativa. It grows wild in Central Asia and Eastern Europe. Its range covers the far north of Russia, Kazakhstan, Mongolia and nearby areas. There, the growing season runs only from May to August, and summer daylight lasts almost the whole day.
Plants in that climate cannot wait for shorter days before they flower. If they waited, the early autumn cold would kill them before they set seed. Over time, the plants that survived were the ones that flowered at a set age.
That age-based timing is what ruderalis gave to modern autos. A ruderalis plant starts to flower about 3–4 weeks after it sprouts, whatever the day length. The genes for this trait sit at different places on the chromosomes (loci) from the genes that control flowering time in indica and sativa.
Translator’s note: Ruderalis adapted to short, cold summers, not to tropical heat. Keep this in mind when you choose seed for a hot climate (Part 2, section 12).
1.2 The first crosses, and how the trait is inherited
The first commercial autoflowering hybrids appeared in the early 2000s. Breeders made them by crossing ruderalis with indica or sativa varieties.
The autoflowering trait is recessive. A plant needs two copies of the autoflowering gene to show it. The first-generation cross (F1) between a photoperiod plant and ruderalis is therefore usually a photoperiod plant. If you pollinate an F1 plant with itself or a sibling, about 25% of the next generation (F2) carries two copies and autoflowers.
Translator’s note: The original article says that crossing the F1 back to ruderalis also gives about 25% autos. That figure is wrong for a backcross. The article treats the trait as a single recessive gene. Under that model, crossing an F1 back to the autoflowering parent gives about 50% autoflowering plants in that first backcross generation.
Early commercial autos were crossed back to ruderalis again and again to fix the trait. This also brought in a large share of ruderalis genes. Those genes explain why first- and second-generation autos were small and weak, with loose, airy buds.
1.3 How modern autos are bred
Modern breeders work the other way round. They start with a strong photoperiod variety. Then they bring in the autoflowering genes through repeated backcrossing. In each generation they keep only the plants with the best non-ruderalis traits: vigour, cannabinoid profile, bud structure and terpene aroma.
The result is a plant with ruderalis timing. In every other way, it looks and performs like its photoperiod parent. According to the original article, seed companies such as 420 Fast Buds, Dutch Passion and Dinafem have bred autos this way for more than 15 years.
What to do: before you buy, check that the listing names the photoperiod parent and links to grow reports. This tells you whether the line comes from years of selection or from a quick early cross.
1.4 Turning a photoperiod variety into an auto
Some growers try to breed their own autos by crossing an autoflower with a favourite photoperiod variety. The trait is recessive, so only about 25% of the F2 from such a cross will autoflower. The rest need 12/12 light to flower. Fixing the trait takes several backcross generations and strict selection. Keeping the photoperiod parent’s quality at the same time takes even longer.
Commercial autos are developed this way. That is why the best auto breeders spend 3–5 years or more on steady selection, and quick F2 releases are much less reliable. If plants from one pack start flowering at very different ages, the line is probably not stable yet. Note it in your grow log and report it in Breeders Lab.
2. Choosing autoflower seed
2.1 Signs of well-bred genetics
Autoflower seed ranges from well-stabilised lines to poorly bred ones with very mixed plants. These signs point to good genetics:
- Even germination: seed from stable lines germinates at 90–95% within 72 hours under proper conditions.
- Uniform plants: plants from one batch look alike at the same age. Large differences in leaf shape, growth speed or flowering time mean the line is not stable. These visible traits make up the plant’s phenotype.
- Breeder information: detailed phenotype descriptions, grow reports, and independent grow logs from different environments.
- Reliable service: reputable breeders replace seed that fails to germinate.
What to do: when you open a new pack, count how many seeds have sprouted at 72 hours. The count shows whether the batch meets the 90–95% mark. It also gives you evidence if you ask for replacement seed.
2.2 Seed types: first-generation, selfed, regular and feminised
- F1 autos: a first-generation cross. They grow with extra vigour (hybrid vigour, or heterosis). Seed taken from F1 plants gives unpredictable offspring.
- S1 autos (selfed): a female plant is made to produce pollen and then pollinates itself. The offspring are very similar to the mother. Repeated selfing risks inbreeding depression, meaning weaker plants from too much inbreeding.
- Regular autos: give both male and female plants, usually about 50/50. Breeders need them as a source of true males for breeding projects.
- Feminised autos: made by treating a female plant with colloidal silver or STS (silver thiosulfate) so that it produces pollen (the two methods step by step). More than 99% of the plants are female. This is the standard commercial product.
For a production grow, feminised F1 autos from reputable breeders give the most uniform plants and the best value.
2.3 Buying seed in Thailand
Translator’s note: The Thai rules on importing, selling and buying cannabis seed are not simple, and online claims that seed sales are “fully legal” are often out of date. The forum topics on what is legal when buying seed and how to check a seed vendor cover the main risks: copied strain names, old stock and false feminisation claims. To find a source, ask the brand’s official distributor in Thailand. You can also ask other growers in Seeds and Genetics Trading.
Translator’s note: Autos are grown from seed, not from clones. This avoids the pests, diseases and wrong-strain risk of buying clones from another farm. Seed is not automatically clean, though, because hop latent viroid can pass through seed. Grow a new seed line as a small batch, away from your main crop, until you have seen how it grows.
3. How autos differ from photoperiod plants in practice
3.1 You cannot control the vegetative stage
With photoperiod plants, you decide when flowering starts by changing the light schedule. That lets you:
- keep the plant in vegetative growth until it reaches the size you want;
- return a flowering plant to vegetative growth by going back to 18 hours of light and 6 of dark (18/6) if something goes wrong;
- take clones from a mother plant so that every plant has the same genes.
You cannot do any of these with autos. The countdown starts at germination. The plant starts to flower at 21–35 days, whatever you do. Its genes set the time from seed to harvest, give or take about 10 days depending on conditions.
3.2 Little time to recover from stress
Imagine a photoperiod plant that suffers transplant shock, overwatering, or two weeks of feed at the wrong pH. You can keep it under vegetative light until it recovers. An auto in the same two weeks starts flowering anyway. It enters flower with damaged roots and fewer leaves and stems, so the final yield is smaller.
So the main rule for autos is to avoid stress:
- Do not transplant. Sow the seed straight into the final pot.
- Do not train hard. Topping is possible, but only at the right age (section 5.2).
- Do not overwater or underfeed in the first weeks.
- Keep the pH stable from day one.
Translator’s note: A small seedling in a large, wet pot can sit in soaked growing medium for days in humid weather. Water only a small circle around the seedling at first. Widen the circle as the roots spread.
Translator’s note: In Thai conditions, the common stresses are heat, pots drying out on hot afternoons, and power cuts. A power cut does not change when an auto flowers, because autos do not depend on the light schedule. The heat that builds when fans and air-conditioning stop is still a stress. Keep a minimum–maximum thermometer in the room. After an outage, it shows how hot the room got, so you can tell heat damage from a feeding problem.
3.3 Root volume and final pot size
An auto cannot stay in vegetative growth longer to build more roots before it flowers. The roots it has when flowering starts set the upper limit for yield. Pot size directly limits how many roots the plant can grow.
Pot sizes for competitive auto yields:
| Strain type | Minimum | Best | Notes |
|---|---|---|---|
| Dwarf / compact autos | 5 L | 7 L | final height under 70 cm |
| Standard autos | 7 L | 11 L | usually 70–100 cm |
| XXL / long-flowering autos | 11 L | 15–20 L | varieties of 100 days or more |
Fabric pots are strongly preferred. When root tips reach the fabric wall, they dry and stop growing (air pruning). The plant then grows more side roots inside the pot. More roots means more surface to take up water and nutrients.
What to do: choose the pot size from the table before you sow, based on the expected height of your variety. In hot weather, check fabric pots in the morning and the afternoon. The fabric lets them dry out faster than plastic pots.
4. Light schedule and light intensity
4.1 How many hours of light
Autos flower whatever the schedule. Your only choice is how many hours of light to give the plant for photosynthesis:
- 18/6 (18 hours light, 6 hours dark): the most common recommendation. It gives enough photosynthesis time, a rest period and a moderate power bill.
- 20/4: a little more photosynthesis time. Some evidence suggests a slightly higher yield, mainly in varieties that keep responding to very strong light (a high light saturation point).
- 24/0 (light all the time): disputed. Some growers report good results. Others see abnormal growth or signs of stress in some varieties. Ruderalis ancestry may make some autos more tolerant of 24/0 than photoperiod varieties.
The original article found no peer-reviewed trial comparing 18/6 with 20/4 for autos. Advice either way comes from growers and seed-company tests. Treat it as a starting point, not a rule.
Translator’s note: Each extra hour of light adds heat as well as power cost. In a Thai room, that means more cooling. Autos do not need darkness at any particular time. You can run the lights at night and put the dark period in the hottest afternoon hours. Try this for one week, then compare your room temperature log with the week before.
4.2 Light intensity
PPFD (photosynthetic photon flux density) measures how much usable light reaches the canopy each second, in µmol/m²/s (light and DLI explained). Modern autos respond strongly to high PPFD. Some older auto genetics showed stress under strong light. Current commercial autos from reputable breeders use 600–800 µmol/m²/s well. Some can use up to 1,000 µmol/m²/s if they get enough CO₂ and nutrients.
A practical target for most setups:
- vegetative growth: 400–600 µmol/m²/s;
- flowering: rising to 600–800 µmol/m²/s.
To reach these levels, you need enough lamp power from LED or HPS (high-pressure sodium) lamps (lamp types compared). A common mistake is to give autos too little light because growers assume they need less than photoperiod plants.
What to do: measure the light at canopy height with a PAR meter, a light meter that reads PPFD. Measure at the centre and at each corner. Raise or lower the lamp until every point is within the target range.
5. Training autos
5.1 Bending the plant (low-stress training)
Low-stress training (LST) means bending branches and tying them down to make a flatter, wider canopy. It suits autos best because nothing is cut, so the plant does not need time to heal.
Start at 14–21 days from seed, when 3–4 nodes are visible. Nodes are the points where leaves join the stem. Use soft plant ties to pull the main stem down toward horizontal, fixing it to the pot rim or to stakes. As side branches grow upward, tie them down too. The goal is many main buds (colas) at about the same height.
Finish training by day 30–35. By then the plant is flowering and its stems harden quickly. Bending them after that risks breaking them.
5.2 Topping
Topping means cutting off the main growing tip (the apical meristem). You can top autos, but only at the right time. Top between the 4th and 5th node, usually at day 18–24. This leaves the most recovery time before flowering. Done right, topping gives two roughly equal main colas instead of one. It may also fill more of the canopy.
Done too late, after day 25–28, topping cuts yield badly. The plant cannot recover before it flowers. Many experienced growers use LST on autos instead of topping. They only top a variety they know responds well.
5.3 Removing leaves
Keep defoliation (removing leaves) to a minimum. It makes sense to remove large fan leaves that shade the lower bud sites. Stripping most of the fan leaves hurts autos far more than photoperiod plants, because autos have so little time to recover. Remove no more than 20–30% of the leaves in any one session.
What to do: write the sowing date on each pot. Mark day 14, days 18–24 and days 30–35 on a calendar. That way you train within the windows above and do not have to guess.
Part 1 checklist
- Choose autos when you want short, predictable cycles under one light schedule.
- Check that the seed listing names the photoperiod parent, and count germination at 72 hours.
- Sow straight into a final pot sized from the table in section 3.3.
- Measure PPFD at the canopy. Aim for 400–600 µmol/m²/s in vegetative growth and 600–800 in flower.
- Start LST at day 14–21. Top only between day 18 and 24, or not at all.
If you have questions, or results with autos in Thai or Southeast Asian conditions, post them as replies below this topic.
References
- Small, E. (1972). Interfertility and chromosomal uniformity in Cannabis. Canadian Journal of Botany, 50(9), 1947–1949.
- Russo, E. B. (2007). History of Cannabis and Its Preparations in Saga, Science, and Sobriquet. Chemistry & Biodiversity, 4(8), 1614–1648. https://doi.org/10.1002/cbdv.200790144
- ElSohly, M. A., et al. (2017). Phytochemistry of Cannabis sativa L. In Phytocannabinoids. Springer.
Continued in Part 2: Autoflowering Cannabis: Feeding, Climate, Harvest Timing, Planning Harvests and Growing Outdoors in Thailand (Part 2 of 2)