# Photoperiod

> The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.

- Canonical: https://genetixbay.com/knowledge/glossary/photoperiod.html
- Also written: photoperiodism, photoperiodic, day length, photoperiod plant, photoperiod strain, photo, photos
- Glossary category: Plant biology · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Photoperiod is the number of hours of light (and, crucially, uninterrupted dark) in a 24-hour cycle. Photoperiodism is a plant's flowering response to it. Most cannabis is photoperiodic: a [short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md) that stays vegetative under long days and switches to flowering once the dark period passes a cultivar-specific threshold. The plant measures the night through [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md), so the length of uninterrupted darkness is what counts.

## In practice

"Photoperiod" or "photo" also names the plant type, as opposed to an [autoflower](https://genetixbay.com/knowledge/glossary/autoflower.md). Indoors, growers hold photos in veg with [18/6](https://genetixbay.com/knowledge/glossary/18-6.md) or [24/0](https://genetixbay.com/knowledge/glossary/24-0.md), then flip to a long-night schedule to flower.

House positions:
- **24/0 in veg is fine**: cannabis doesn't need a dark sleep period in veg.
- **Flower doesn't have to be exactly 12/12**: 13/11 works for many cultivars, and some flower at 14-15 h of light. In practice that is about 12 h of high intensity with sunrise and sunset ramps adding the rest.
- **Test your cultivar**: critical day length varies, which is exactly what [GenetixBay trials](https://genetixbay.com/knowledge/glossary/bay-trials.md) are for.

[Light leaks](https://genetixbay.com/knowledge/glossary/light-leak.md) during the dark period can delay flowering or contribute to stress in sensitive cultivars.

## Don't confuse with

- **Autoflower**: flowers by age, largely ignoring day length.
- **DLI**: total light dose per day, separate from day-length signalling.
- **Light schedule (18/6, 12/12)**: the setting; photoperiod is the biology.

## Related terms

[Short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md), [Autoflower](https://genetixbay.com/knowledge/glossary/autoflower.md), [Phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md), [Flowering stage](https://genetixbay.com/knowledge/glossary/flowering-stage.md), [12/12](https://genetixbay.com/knowledge/glossary/12-12.md)

## Read more

- [Photoperiod vs autoflower, and how autoflowering is inherited](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md)
- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)

## Sources

1. 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
2. Petit J, Salentijn EMJ, Paulo MJ, Denneboom C, Trindade LM. 2020. Genetic architecture of flowering time and sex determination in hemp (Cannabis sativa L.): a genome-wide association study. Frontiers in Plant Science 11:569958. https://doi.org/10.3389/fpls.2020.569958