Short answer
Autoflowers start flowering about 3–5 weeks after sprouting, with pistils at the main tip around day 21–35, on any light schedule. Preflowers at the nodes often show in week 4. Photoperiod plants wait for long nights instead: pistil clusters appear about 10–14 days after the flip to 12/12. Grower practice for auto timing; Supported for photoperiods.
Why
An auto's switch runs on age. Plants with two copies of the recessive allele at the Autoflower1 locus flowered even under long days in mapped populations [1]. Lines differ by a week or two, which is why the window is wide.
All cannabis makes preflowers with age. Single flowers at the nodes form through an internal juvenile-to-adult change, not the light schedule; in one cultivar the first pair appeared at the 7th node [2]. On a photoperiod plant under 18/6 that's as far as it goes. On an auto, clustered flowers at the tips follow.
Photoperiod plants measure the night. After the switch to short days, visible pistil clusters at the top took about 8–14 days in controlled studies [3], around 13–14 days on average across many hemp cultivars and three weeks in slow ones [4].
The numbers
| Plant type | First preflowers | Flowering starts | Trigger |
|---|---|---|---|
| Autoflower | ~week 3–4 | ~day 21–35 from sprouting | Age |
| Photoperiod, 18/6 | ~week 4–6 from seed | Not until nights lengthen | — |
| Photoperiod, after flip | Already present | ~10–14 days after 12/12 | Long nights |
What to do
- Note the sprout date; it's day 1 for an auto.
- From week 3, check the main tip with a loupe every few days.
- Log the day pistils cluster at the tip as "day 1 of flower".
- Finish main LST bends by about day 35; stems stiffen in the stretch.
- No pistils by week 7 under 18/6? Treat it as a photoperiod and flip it.
Common mistakes
- Calling a single pair of preflowers "flowering".
- Switching autos to 12/12 to "make them flower". They don't need it.
- Training hard after day 40, when stems have stiffened.
- Counting a photoperiod's flip as day 1 of flower.
Related
- Full article: Photoperiod vs autoflower
- Answers: how long autos take · when plants show sex · when to flip
- Article: the flip isn't day 1 of flower
- Terms: pre-flower · autoflower
