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 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 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).
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flowering in an F2 of 318 plants also found clean two-class segregation with no intermediate plants (Supported, not yet peer reviewed) [5].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-reviewedbeing 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.
Decision guide
- Want the shortest, simplest first grow, or outdoor crops at high latitude? Autoflower.
- Want to keep a mother, clone, select and breed? Photoperiod.
- Breeding autos? Read breeding autoflowers 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.
- 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.
