# When do autoflowers start flowering?

> Autoflowers usually start flowering about 3–5 weeks after sprouting, with white pistils at the main tip around day 21–35, whatever the light schedule. Preflowers at the nodes often appear a few days earlier, in week 4. Photoperiod plants are different: they flower only after long nights begin, with pistil clusters about 10–14 days after the switch to 12/12.

- Canonical: https://genetixbay.com/knowledge/answers/when-do-autoflowers-start-flowering.html
- Topic: Basics · Type: Numbers
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Photoperiod vs autoflower, and how autoflowering is inherited](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md)
- Also asked as: When do autos show pistils? | Autoflower preflower week | How many weeks until autos flower? | When does cannabis start flowering?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

[Autoflowers](https://genetixbay.com/knowledge/glossary/autoflower.md) start flowering about 3–5 weeks after [sprouting](https://genetixbay.com/knowledge/glossary/germination.md), with [pistils](https://genetixbay.com/knowledge/glossary/pistil.md) at the main tip around day 21–35, on any light schedule. [Preflowers](https://genetixbay.com/knowledge/glossary/pre-flower.md) at the nodes often show in week 4. [Photoperiod plants](https://genetixbay.com/knowledge/glossary/photoperiod.md) wait for long nights instead: pistil clusters appear about 10–14 days after the [flip to 12/12](https://genetixbay.com/knowledge/glossary/12-12.md). *Grower practice* for auto timing; *Supported* for photoperiods.

## Why

An auto's switch runs on age. Plants with two copies of the [recessive allele](https://genetixbay.com/knowledge/glossary/recessive.md) at the *Autoflower1* [locus](https://genetixbay.com/knowledge/glossary/locus.md) 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](https://genetixbay.com/knowledge/glossary/18-6.md) 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](https://genetixbay.com/knowledge/glossary/hemp.md) 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

1. Note the [sprout](https://genetixbay.com/knowledge/glossary/seedling.md) date; it's day 1 for an auto.
2. From week 3, check the main tip with a [loupe](https://genetixbay.com/knowledge/glossary/jewelers-loupe.md) every few days.
3. Log the day pistils cluster at the tip as "day 1 of flower".
4. Finish main [LST](https://genetixbay.com/knowledge/glossary/lst.md) bends by about day 35; stems stiffen in the [stretch](https://genetixbay.com/knowledge/glossary/stretch.md).
5. 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](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md)
- Answers: [how long autos take](https://genetixbay.com/knowledge/answers/how-long-do-autoflowers-take.md) · [when plants show sex](https://genetixbay.com/knowledge/answers/when-do-plants-show-sex.md) · [when to flip](https://genetixbay.com/knowledge/answers/when-to-flip-to-flower.md)
- Article: [the flip isn't day 1 of flower](https://genetixbay.com/knowledge/environment/flip-to-flower-transition.md)
- Terms: [pre-flower](https://genetixbay.com/knowledge/glossary/pre-flower.md) · [autoflower](https://genetixbay.com/knowledge/glossary/autoflower.md)

## Follow-up questions

**Why hasn't my autoflower started flowering?** An autoflower that shows no pistils by week 5–6 is usually either a slow line or a photoperiod plant in disguise. Autoflowering is recessive, so a seed needs two copies of the auto allele, and poorly stabilised lines throw photoperiods. If it is still vegging at week 7 under 18/6, treat it as a photoperiod and flip it to 12/12.

**When does a photoperiod plant start flowering?** A photoperiod cannabis plant starts flowering about 10–14 days after the switch to 12/12, when three or more pairs of pistils cluster at the top of the main shoot. Controlled studies found onset roughly 8–14 days after short days began, around 13–14 days on average in many hemp cultivars, and up to three weeks in slow ones. Preflowers in veg don't count.

**Are preflowers the start of flowering on an auto?** Preflowers on an autoflower are the first sign, not the start of real flowering. Single flowers at the nodes form with age in all cannabis, and research found the first pair around the 7th node. Flowering proper begins when pistils cluster at the main tip and branch tips; on an auto that usually follows within days, around weeks 3–5.

## Sources

1. Toth JA, Stack GM, Carlson CH, Smart LB. 2022. Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L. Frontiers in Plant Science 13:991680. https://doi.org/10.3389/fpls.2022.991680
2. Spitzer-Rimon B, Shafran-Tomer H, Gottlieb GH, et al. 2022. Non-photoperiodic transition of female cannabis seedlings from juvenile to adult reproductive stage. Plant Reproduction 35(4):265-277. https://doi.org/10.1007/s00497-022-00449-0
3. Spitzer-Rimon B, Duchin S, Bernstein N, Kamenetsky R. 2019. Architecture and florogenesis in female Cannabis sativa plants. Frontiers in Plant Science 10:350. https://doi.org/10.3389/fpls.2019.00350
4. Zhang M, Anderson SL, Brym ZT, Pearson BJ. 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153