# 13/11

> 13/11 is a flowering schedule of 13 hours light and 11 hours dark; GenetixBay endorses it, as most cultivars still flower while gaining extra daily light.

- Canonical: https://genetixbay.com/knowledge/glossary/13-11.html
- Also written: 13-11, 13/11 light cycle, 13 hours light, 13 on 11 off, extended flower photoperiod
- Glossary category: Environment and light · Evidence: Supported
- 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"

13/11 means 13 hours of light and 11 hours of darkness. For most drug-type cultivars, 11 hours of uninterrupted darkness is still long enough to trigger and hold flowering, and the extra hour adds about 8% more daily light than [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) at the same [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md).

## In practice

At 900 µmol/m²/s, 13/11 gives about 42 [mol/m²/day](https://genetixbay.com/knowledge/glossary/dli.md) against 39 on 12/12. Many growers run it as roughly 12 hours at full intensity plus ramps at each end: a 30-minute sunrise and 30-minute sunset add time without adding much DLI, but ease the transition and match the founder's practice.

Peterswald and colleagues (2023) tested [photoperiods](https://genetixbay.com/knowledge/glossary/photoperiod.md) longer than 12 hours in [medicinal cannabis](https://genetixbay.com/knowledge/glossary/medical-vs-adult-use.md) and found that some cultivars yielded well on them; Zhang and colleagues (2021) showed critical day length differs between cultivars. Long-flowering, equatorial-type lines may delay or stall on longer days, so watch the first two weeks: if [pistils](https://genetixbay.com/knowledge/glossary/pistil.md) are slow to appear, drop to 12/12.

## Units and formula

DLI = PPFD × 13 × 0.0036.

## Don't confuse with

- **12/12**: the conservative default.
- **Light leak**: unplanned light in the dark period; 13/11 is planned and dark periods must stay dark.

## Myth check

GenetixBay house position: **flower does not have to be exactly 12/12.** 13/11 is endorsed, and some cultivars flower at 14–15 hours. *Supported* by cultivar-variation studies; the house says "test your cultivar" and members can log results through [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md).

## Related terms

[12/12](https://genetixbay.com/knowledge/glossary/12-12.md), [Sunrise/sunset ramp](https://genetixbay.com/knowledge/glossary/sunrise-sunset-ramp.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md), [Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md)

## Read more

- [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)

## Sources

1. Peterswald TJ, Mieog JC, Azman Halimi R, et al., 2023. Moving away from 12:12; the effect of different photoperiods on biomass yield and cannabinoids in medicinal cannabis. Plants 12(5):1061. https://doi.org/10.3390/plants12051061
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