Autoflowering cannabis (“autoflowers” or “autos”) starts to flower once the plant reaches a certain age. It does not wait for the nights to get longer. This category covers three things: where the trait comes from, what it costs you as a grower, and why many photoperiod growing habits give a poor result with autos.
Two kinds of plant: day length or age
Most cannabis is photoperiod cannabis. These plants grow leaves and stems while the days are long. They switch to flowers when the nights get long enough.
The growth phase before flowering is called the vegetative stage, or “veg”. Indoors, the grower decides how long veg lasts by keeping the lights on for many hours a day. A longer veg gives a bigger plant before flowering starts.
An autoflower does not respond to day length. It flowers on its own schedule, counted from the day the seed sprouts. The hours of light do not change that schedule.
Where the trait comes from
Autoflowers get this trait from ruderalis, a group of cannabis populations that flower by age, not by day length. Autoflower cultivars carry the ruderalis trait together with the traits of ordinary cannabis.
Why the trait helps near the equator
Near the equator, day length stays close to 12 hours all year. Twelve hours of light is short enough to make a photoperiod plant flower. As a result, photoperiod plants grown outdoors in Thailand often start flowering while still small. The grower cannot make veg longer without adding light.
An autoflower does not depend on day length. Its schedule from sprouting to flowering stays about the same in every month of the year. That predictable timing is its real advantage in the tropics.
What the trait costs: you cannot extend veg
With a photoperiod plant, you can fix an early problem by keeping the plant in veg for a few extra weeks. The plant recovers and then grows to full size before you switch it to flower.
An autoflower gives you no such option. Its flowering date comes on schedule whether the plant is ready or not. Say an auto loses two weeks to transplant shock, the slowdown after its roots are disturbed when it is moved to a new pot. Or say it loses two weeks to a nutrient deficiency. Either way, it never gets those two weeks back, so the harvest is smaller.
This leads to two rules that are the opposite of normal photoperiod practice.
Rule 1: Sow into the final pot and do not transplant
Every time the roots are disturbed, the plant stops growing for a while. With an auto, that lost growth shows directly as a smaller plant at harvest. So choose the final pot and growing medium before you sow, and sow the seed straight into that pot.
Translator’s note: In Thai heat, the final pot also has to protect the roots. A black plastic pot in direct sun can heat the root zone well above the air temperature. Shade the side of the pot, or use a light-coloured pot or a fabric grow bag. Fabric bags dry out faster at 30–35 °C, so check the moisture more often. The goal is to keep roots cool and evenly moist, so the plant does not lose days it cannot make up.
Translator’s note: Outdoors in the rainy season, raise pots off the ground on bricks or a pallet. This lets water drain freely. Roots that sit in water can develop root rot, and with an auto there is no time for recovery.
Rule 2: Avoid heavy training and defoliation
“Training” means bending or cutting stems to shape the plant. Examples include bending and supercropping and cutting the main tip (topping). Defoliation means removing leaves to open up the plant.
Heavy versions of these techniques are usually a mistake with autos. Each cut or bend needs recovery time. A photoperiod plant gets that time from a longer veg, but an auto starts flowering on schedule no matter what.
If you want to know whether a technique works in your setup, test it on one plant only. Grow a second, untouched plant from the same seed pack next to it, in the same conditions. Compare the dried harvest weight of both plants. This tells you whether the technique gained or lost yield for you.
What works better: light feeding, stable conditions, fewer interventions
Autoflower threads keep coming back to three habits instead of intervention: feeding lightly early on, keeping the environment stable and leaving the plant alone.
Feed lightly when the plant is young. Start below the full strength on the fertiliser feed chart. Raise the strength only when the leaves show the plant needs more. Young autos have small root systems, and a strong feed can cause nutrient burn, and an auto cannot make up the days it loses to that setback. Measure pH and EC (EC shows how much dissolved fertiliser is in the water) every time you mix a feed. Your notes will then show which strength the plant handled well.
Keep conditions stable. Sudden changes in temperature, humidity or watering slow growth for a few days. A photoperiod plant can absorb that delay, but an auto cannot.
Translator’s note: Indoors in Thailand, the usual threats are heat above 30 °C, high humidity and power cuts. See grow room climate control for target ranges. Before you sow, write down what you will do if the air conditioner or fans stop for several hours. You want a plan that keeps the room from overheating during the plant’s short life.
Translator’s note: Outdoors, you can plan the calendar because an auto’s cycle runs a roughly fixed number of weeks from sowing. Take the breeder’s stated seed-to-harvest time, add a margin and count forward from your sowing date. Try to keep the flowering weeks out of the wettest part of the rainy season. Dense buds in high humidity raise the risk of bud rot (botrytis).
Leave the plant alone. Each extra intervention, such as moving the pot, pruning or changing the feed often, carries a small risk of a setback. Do only what the plant clearly needs.
How to start with autos in Thailand
- What you gain: flowering time that does not depend on the season or on extra veg lighting. This makes planning easier both outdoors and indoors.
- How to apply it here: sow into the final pot, keep the root zone cool and drained, feed lightly and plan around the rainy season as described above.
- Where to start: run a small trial of a few plants from one seed pack. Record the sprouting date, every setback and how many days it cost, and the dry harvest weight. This tells you the real seed-to-harvest time for that cultivar in your conditions before you plant more.
Before buying seed, check the vendor with the seed vendor checklist. Also read what is legal when buying seeds in Thailand.
Translator’s note: Cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. It was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025. Flower may be sold only with a PT 33 (ภ.ท.33) prescription, for a maximum of 30 days. Advertising and selling flower online are prohibited. So this category is for growing knowledge only. Do not post offers to sell flower here.
Further reading
- The full guide on ruderalis genetics and growing autos (linked in the “Where the trait comes from” section): Autoflowering Cannabis: Ruderalis Genetics, Light Independence, and How to Grow Autos Correctly.
- Elsewhere on the forum, in Genetics and Seeds: Seeds: Regular vs Feminized vs Autoflower.
Ask your question under this topic
If your auto has a problem, post it in this category and include these details: the plant’s age in days from sprouting, pot size, growing medium, indoor or outdoor, light hours, feed EC and pH, and a clear photo of the whole plant and of the affected leaves. These details show other growers whether the problem is a setback the plant can still recover from. To document a full run, start a thread in Grow Diaries.