# Photoperiod vs autoflower, and how autoflowering is inherited

> Photoperiod cannabis flowers when nights get long; autoflowering (day-neutral) cannabis flowers by age, even under 18-24 h light. In the populations studied so far, autoflowering behaves as a single recessive gene (Autoflower1, on chromosome 1): a photoperiod x auto F1 is all photoperiod, and about 1 in 4 of its F2 plants are autos.

- Canonical: https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.html
- Category: Basics · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Any stage · Systems: Any system
- 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)"

## Questions and answers

**What is the difference between autoflower and photoperiod?** Photoperiod cannabis flowers only when nights get long enough, usually 12 hours of darkness indoors, so the grower decides how long it vegetates. Autoflowering cannabis is day-neutral and starts flowering at a fixed age, usually 3-5 weeks from sprouting, whatever the light schedule. Photoperiods can be cloned and kept as mothers; autos finish seed to harvest in about 9-14 weeks but are impractical to clone.

**Is the autoflower gene dominant or [recessive](https://genetixbay.com/knowledge/glossary/recessive.md)?** The autoflower trait is recessive in every population mapped so far. Research by Toth and colleagues in 2022 located a major locus, Autoflower1, on chromosome 1, and a plant needs two copies of the auto allele to autoflower. [Heterozygous](https://genetixbay.com/knowledge/glossary/heterozygous.md) plants still behave as photoperiods, though in field trials they flowered about two weeks earlier. Whether all commercial autos share this locus is still contested.

**If I cross an auto with a photoperiod, what do I get?** Crossing an autoflower with a photoperiod gives an F1 that is 100% photoperiod, because the auto trait is recessive and every F1 plant carries just one copy. Crossing those F1 plants together gives an F2 where about 1 in 4 plants autoflower. Backcrossing the F1 to the auto parent gives roughly half autos, so recovering autos from such a cross takes at least two generations.

**Can I grow autos on [12/12](https://genetixbay.com/knowledge/glossary/12-12.md)?** Yes, autoflowers will still flower on 12/12, because they flower by age rather than night length. But it wastes their potential: an auto on 12/12 gets much less daily light than on its usual 18-20 hours, which gives about 1.5-1.7 times the daily light at the same intensity. With a fixed countdown, the plant cannot make up the lost growth later.

**What light schedule for autoflowers?** Autoflowers are usually grown on 18/6 to 20/4 from seed to harvest, with no switch to 12/12 needed. Some growers use [24/0](https://genetixbay.com/knowledge/glossary/24-0.md), though that is debated. At the same intensity, 18-20 hours gives an auto about 1.5-1.7 times the daily light of a 12/12 photoperiod, which also means using that much more lighting power per day. Per kilowatt-hour, autos are a closer race.

**Why did my autoflower not flower?** An 'autoflower' that never flowers under long days is almost always a photoperiod plant in disguise. Autoflowering is recessive, so a seed must carry two copies of the auto allele, and poorly [stabilised lines](https://genetixbay.com/knowledge/glossary/ibl.md) throw photoperiod plants. That is a quality-control failure by the breeder, not bad luck. A true auto normally starts flowering about 3-5 weeks from sprouting, whatever the light schedule.

**Are autos better than photoperiods?** Whether autos are better than photoperiods depends on what 'better' means. Autoflowers are very hard to beat on yield per square foot per year, because they finish in about 9-14 weeks. Photoperiods give more yield per plant, are more forgiving, can be cloned and kept as mothers, and usually test slightly higher. A dialled-in [sea of green](https://genetixbay.com/knowledge/glossary/sog.md) or [SCROG](https://genetixbay.com/knowledge/glossary/scrog.md) with a dedicated nursery can out-yield autos per area.

**Do autos yield more per year?** Autoflowers usually yield more per square foot per year than photoperiods vegged in the same space. A 12-week auto cycle fits about 4.3 harvests a year versus about 3.7 for a 14-week photo cycle, so an auto needs only about 86% of the photo's per-run yield to match it. Against a sea of green with a dedicated nursery, about 5.2 cycles a year, the auto must out-yield by about 20%.

**Are autoflowers less potent?** Autoflowers usually test slightly below their photoperiod sisters, in the experience of the GenetixBay founder, but the best elite autos match them; he has had one lab-test at 30%, a rare feat for any flower. No controlled head-to-head study of matched auto and photoperiod lines has been found, so treat this as experienced grower observation rather than research. Breeding has steadily narrowed the gap.

**Are autoflowers good for beginners?** Autoflowers are good for beginners, but not because they are easy. They are short, quick and need no light-schedule change, so a beginner gets a harvest in about 9-14 weeks. Yet autos run on a strict countdown and punish overwatering, [transplanting](https://genetixbay.com/knowledge/glossary/transplanting.md), topping and hunger with smaller yields. GenetixBay labels 'autos are easy' a myth: they quickly show who has a green thumb.

**Short answer.** A photoperiod plant waits for long nights (12 h dark indoors) to flower; you choose how long it vegetates. An autoflower starts flowering at a fixed age, usually 3-5 weeks from sprouting, whatever the light schedule. In mapped populations, autoflowering is a **single recessive trait**: a plant must carry two copies of the auto allele to autoflower [1].

## What's happening

Photoperiod cannabis measures night length. Day-neutral (autoflowering) plants lack that brake: their flowering switch runs on age. Autoflowering germplasm is usually traced to northern or Central Asian populations popularly called "ruderalis" (*Supported* as a history claim; the taxonomy itself is contested) [4].

## How autoflowering is inherited

Toth et al. (2022) mapped a major locus, **Autoflower1**, to chromosome 1 using bulked segregant analysis. [Segregating](https://genetixbay.com/knowledge/glossary/segregation.md) populations fit a single recessive gene: about 25% of F2 plants flowered under long days (observed 31.8% and 27.3%, not significantly different from 25%) [1]. Call the photoperiod allele **A** and the auto allele **a**:

| Cross | Offspring genotypes | What you see |
|---|---|---|
| Photo (AA) x Auto (aa) | all Aa | 100% photoperiod (F1) |
| F1 (Aa) x F1 (Aa) | 1 AA : 2 Aa : 1 aa | ~75% photo, ~25% auto (F2) |
| F1 (Aa) x Auto (aa) (backcross) | 1 Aa : 1 aa | ~50% photo, ~50% auto |
| Auto (aa) x Auto (aa) | all aa | 100% auto |

Two useful details from the same study [1]:

- **Heterozygotes are not identical to pure photoperiods.** In the field, Aa plants flowered about two weeks earlier than AA plants and were intermediate in height and biomass, but they still did not flower under continuous light. So "recessive" describes autoflowering itself, not every effect of the gene.
- **Not every auto source has been shown to share this locus.** Some day-neutral [hemp cultivars](https://genetixbay.com/knowledge/glossary/hemp.md) did not match the markers, and a complementation cross was inconclusive. The authors say it is not clear the mechanism is the same in all cultivars (*Contested* for the whole [gene pool](https://genetixbay.com/knowledge/glossary/gene-pool.md)).

A 2023 preprint mapping day-neutral flowering in an F2 of 318 plants also found clean two-class segregation with no intermediate plants (*Supported*, not yet peer reviewed) [5]. An industry claim names a pseudo-response regulator (PRR37-like) gene as the candidate; treat that as unconfirmed until peer-reviewed.

## Numbers that matter

| Parameter | Photoperiod | Autoflower |
|---|---|---|
| Flowering trigger | Night length past a critical value (often ~10-11 h dark; cultivar specific) [2] | Plant age |
| Indoor veg schedule | 18/6 typical | Not needed |
| Indoor flower schedule | 12/12 | 18/6 to 20/4 common; 24/0 used, debated |
| Seed to harvest | ~3-5 months indoors | ~9-14 weeks |
| Clone and re-veg | Yes, keep a mother indefinitely | Clones flower on the parent's clock; impractical |
| Recovery from stress | Can veg longer to recover | Little time to recover; avoid heavy stress early |

> **House position: autoflowers.** An auto runs on a strict countdown from germination, so any setback costs it time it can't get back. **Plant it straight into its final pot**: transplanting is almost a no-no, because of how the auto's taproot reacts (the founder's observation, compared with photoperiods). **Train with LST, don't top**: topping will never produce what proper low-stress training does on an auto. *Grower practice*, house-endorsed.

## Are autos better? Better how?

Ask "are autos better?" and the honest answer is: how long is a piece of string? Better at what?

**Per square foot per year: very hard to beat.** An auto goes from seed to jar in about 9–14 weeks with no veg in the flower space, so you fit more harvests into a year. The only photoperiod setup that reliably beats it on space is a dialled-in sea of green or SCROG fed by a dedicated nursery, where the flowering space only ever holds flowering plants (see [SOG](https://genetixbay.com/knowledge/training/sog.md) and [SCROG](https://genetixbay.com/knowledge/training/scrog.md)).

**The arithmetic.** Yield per square foot (or m²) per year = yield per cycle × cycles per year. The cycles are where autos win:

| Setup | Weeks the space is tied up per cycle | Cycles a year | Per-cycle yield an auto needs to tie |
|---|---|---|---|
| Photoperiod, vegged in the same space (4 wk veg + 9 wk flower + 1 wk reset) | 14 | ~3.7 | ~86 % of the photo's |
| Photoperiod SOG/SCROG with a dedicated nursery (9 wk flower + 1 wk reset) | 10 | ~5.2 | ~120 % of the photo's |
| Autoflower sown in its final pot (11 wk seed to harvest + 1 wk reset) | 12 | ~4.3 | — |

Divide 52 weeks by the weeks per cycle; the last column is photo cycles ÷ auto cycles. The week counts are typical, not fixed: a 9-week auto or a 12-week photo moves the numbers, so plug in your own. What the table shows is the shape of the argument. Against a photo that vegs in the flowering space, an auto only has to yield about 86 % as much per run to match it per year, and a good auto does. Against a dialled-in sea of green with its own nursery, the auto would need to out-yield the photo by about a fifth per run, which is a big ask. (The nursery's own space and power aren't counted in that row, which flatters it.)

For scale, one controlled study of a photoperiod cultivar flowered for 81 days at 12/12 measured 116 g/m² of dry flower at the lowest light level and 519 g/m² at the highest (1,800 µmol/m²/s at the canopy), rising in a straight line in between [6]. We haven't found a comparable peer-reviewed g/m² figure for autoflowers. One more honest note: autos usually run 18–20 h of light, so at the same intensity they get 1.5–1.7 times the daily light (and use that much more lighting power per day) as a 12/12 photo. Per square foot they're hard to beat; per kilowatt-hour it's a closer race.

**[Cannabinoids](https://genetixbay.com/knowledge/glossary/cannabinoid.md): usually a little behind, sometimes level.** In the founder's experience, autos usually test slightly under their photoperiod sisters, but the best of them give photos a real run. We haven't found a controlled head-to-head study of matched auto and photo lines, so treat this as experienced grower observation. [Breeding autoflowers](https://genetixbay.com/knowledge/breeding/breeding-autoflowers.md) explains why autos lagged and how breeders closed the gap.

> **Founder's records.** The founder has had an elite autoflower lab-test at **30 %** and has pulled **3/4 lb (about 340 g) in under 80 days**. That's one grower's results with one elite line, not a typical number: 30 % is a rare feat for any flower (see [buyer beware](https://genetixbay.com/knowledge/basics/buyer-beware-label-claims.md)). It shows what the ceiling looks like when everything goes right.

**Finicky: any stress costs yield.** An auto can't be vegged longer to recover. Overwater it, transplant it, top it, let it get root-bound or hungry in week 3, and the clock keeps running: it flowers on schedule, smaller. That's why the house rules above are so strict.

**"Good for beginners": true, but not because they're easy.** Autos are short, quick and need no light-schedule change, so a beginner gets a harvest fast. But they're the opposite of forgiving. They show very quickly who has a green thumb and who doesn't. An elite auto grower naturally makes an even better photoperiod grower, because a photo forgives what an auto punishes. It doesn't work the other way round.

| Better how? | Autoflower | Photoperiod |
|---|---|---|
| Yield per area per year | Very hard to beat | Beats autos with a dialled-in SOG/SCROG and dedicated nursery |
| Yield per plant | Lower | Higher; veg as long as you like |
| Cannabinoids | Usually slightly under photo sisters; the best match them | The benchmark |
| Forgiveness | Very little: stress costs yield | Can veg longer to recover |
| Light power per day | 18–20 h | 12 h in flower |
| Clones, mothers, selection | Impractical | Yes |
| Learning curve | Quick feedback, harsh lessons | More forgiving |

## Decision guide

1. Want the shortest first grow, the most harvests per square foot without a nursery, or outdoor crops at high latitude? Autoflower, and be strict with it.
2. Want to keep a mother, clone, select and breed? Photoperiod.
3. Breeding autos? Read [breeding autoflowers](https://genetixbay.com/knowledge/breeding/breeding-autoflowers.md) first: it takes at least two generations to recover autos from a photo cross.

## Common mistakes

- Expecting F1 "auto x photo" seed to autoflower. It won't if the trait is recessive.
- Transplanting autos at all, or topping them. They don't pause the clock while recovering: sow into the final pot and use [LST](https://genetixbay.com/knowledge/training/lst.md).
- Treating autos as the easy option. They punish mistakes harder than photos do.
- Assuming every seed in an "auto" pack is homozygous. Poorly stabilised lines throw photoperiod plants; this is a quality-control failure, not bad luck.

## Evidence

- Day-length control of photoperiod flowering and cultivar-specific thresholds: *Established* [2][3].
- Single recessive locus Autoflower1 in the mapped populations: *Supported* (peer reviewed, limited germplasm) [1].
- That all commercial autos share the same locus: *Contested* [1].
- "Ruderalis origin" of the trait: *Supported* as history, label as a claim.
- Autos winning on yield per area per year (except against a dialled-in SOG/SCROG with a nursery): *Supported* by cycle arithmetic and the founder's experience; per-cycle auto yields in g/m² need peer-reviewed data. Photo yield response to light: *Supported* [6].
- Autos testing slightly under photo sisters: founder's observation; no controlled comparison found.
- "Autos are easy": *Myth* (house position).

## 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. 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
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. Hillig KW. 2005. Genetic evidence for speciation in Cannabis (Cannabaceae). Genetic Resources and Crop Evolution 52:161-180. https://doi.org/10.1007/s10722-003-4452-y
5. Genetic mapping of SNP markers and candidate genes associated with day-neutral flowering in Cannabis sativa L. (preprint). bioRxiv 2023.04.17.537043. https://www.biorxiv.org/content/10.1101/2023.04.17.537043
6. Rodriguez-Morrison V, Llewellyn D, Zheng Y. 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020