# Why do photoperiod plants need long nights to flower?

> Photoperiod cannabis is a short-day plant: it measures the uninterrupted night with phytochrome, a light-sensing pigment, and flowers once the night passes a cultivar-specific critical length, around 10–11 hours of darkness in drug-type cultivars tested. 12/12 works for essentially all cultivars; 13/11 also works for many. Light during the night, even very dim, resets the count and can delay flowering in sensitive cultivars.

- Canonical: https://genetixbay.com/knowledge/answers/why-flowering-needs-darkness.html
- Topic: Basics · Type: Definition
- Evidence: Established · Status: Draft: awaiting expert review
- Full article: [Photoperiod and light leaks: 18/6, 12/12 and the dark period](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- Also asked as: Why 12/12? | Phytochrome and flowering | Night length and flowering | Does cannabis measure day or night?
- 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

[Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis is a [short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md), which really means a long-night plant: it flowers once the uninterrupted dark passes a cultivar-specific length, around 10–11 hours in the drug-type cultivars tested. [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) triggers essentially all cultivars; [13/11](https://genetixbay.com/knowledge/glossary/13-11.md) works for many and is house-endorsed. *Established* plant physiology.

## Why

The plant times the night, not the day. [Phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md) in the leaf flips between active and inactive forms under red and [far-red light](https://genetixbay.com/knowledge/glossary/far-red.md). Through a long night the active form decays, and past the critical night length, flowering signals switch on at the [shoot tips](https://genetixbay.com/knowledge/glossary/apical-meristem.md).

The threshold varies by cultivar. One drug-type [genotype](https://genetixbay.com/knowledge/glossary/genotype.md) needed at least about 10.8 h of uninterrupted dark [2]. In [hemp](https://genetixbay.com/knowledge/glossary/hemp.md), some essential-oil cultivars had a critical day length between 13 h 45 min and 14 h [1]. At 13 h, two THC-dominant cultivars flowered within about 1.5 days of their 12 h timing [5].

The nights must keep coming. Inflorescence onset needed at least three consecutive short days, and returning plants to long days pulled the forming inflorescence apart [3].

Night light resets the count. Stray light as dim as about 2 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) delayed flowering in hemp [1], and a conference report saw delays in the most sensitive cannabis cultivars at 0.01 µmol/m²/s [4].

## The numbers

| Schedule | Night | Effect on photoperiod cannabis |
|---|---|---|
| [24/0](https://genetixbay.com/knowledge/glossary/24-0.md), 20/4, [18/6](https://genetixbay.com/knowledge/glossary/18-6.md) | 0–6 h | Stays vegetative |
| 14/10 | 10 h | Flowered in all three lines in one study; test yours |
| 13/11 | 11 h | Flowers; endorsed |
| 12/12 | 12 h | Flowers in essentially all cultivars |
| Stray night light | — | Delays seen from 0.01–2 µmol/m²/s |

Sunrise and sunset ramps count as day; the plant measures the dark after them.

## What to do

1. Give flowering plants an uninterrupted night of at least 11–12 hours.
2. Sit in the dark tent for 5 minutes; any light you can see is a leak.
3. Tape over device [LEDs](https://genetixbay.com/knowledge/glossary/led.md) and seal zip ends and duct ports.
4. Use a timer you trust and check it after power cuts.
5. Trying 13/11 or 14/10? Test one cultivar first.

## Common mistakes

- Thinking plants need "rest" in veg.
- Opening the tent during the dark period.
- Assuming a minute of light can't matter.

## Related

- Full article: [Photoperiod and light leaks](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- Answers: [12/12 from seed](https://genetixbay.com/knowledge/answers/12-12-from-seed.md) · [lights stayed on in flower](https://genetixbay.com/knowledge/answers/lights-stayed-on-during-flower.md) · [when outdoor plants flower](https://genetixbay.com/knowledge/answers/when-do-outdoor-plants-flower.md)
- Article: [The flip isn't day 1 of flower](https://genetixbay.com/knowledge/environment/flip-to-flower-transition.md)
- Terms: [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md) · [short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md)

## Follow-up questions

**How does phytochrome control flowering?** Phytochrome is a pigment in the leaf that switches between an active and inactive form under red and far-red light. During a long night the active form decays; once the night is longer than the cultivar's critical length, flowering genes switch on at the shoot tips. A pulse of light in the night resets this clock, which is why a light leak can stall flowering.

**Does cannabis need a dark period in veg?** No, cannabis doesn't need a dark period in veg; GenetixBay endorses 24/0. Photoperiod cannabis measures night length only to decide when to flower, so continuous light simply keeps it vegetative. Short nights, or none, are what stop flowering. The trade-off of 24/0 is more electricity per day than 18/6, not a measured yield penalty.

**Why do outdoor plants flower in late summer?** Outdoor photoperiod cannabis flowers in late summer because nights lengthen after the June solstice in the northern hemisphere and eventually pass the plant's critical night length. The exact date depends on latitude and cultivar: some hemp cultivars responded to days as long as 13 h 45 min to 14 h, while others need shorter days.

## 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. Moher M, Jones M, Zheng Y. 2021. Photoperiodic response of in vitro Cannabis sativa plants. HortScience 56(1):108-113. https://doi.org/10.21273/HORTSCI15452-20
3. Alter H, Sade Y, Sood A, et al. 2024. Inflorescence development in female cannabis plants is mediated by photoperiod and gibberellin. Horticulture Research 11(11):uhae245. https://doi.org/10.1093/hr/uhae245
4. Eckels M, Bugbee B, 2024. Night light pollution delays flowering in soybean and cannabis (ASHS 2024 conference abstract). https://ashs2024.sched.com/event/1dtbo/ce-4-night-light-pollution-delays-flowering-in-soybean-and-cannabis
5. Ahrens A, Llewellyn D, Zheng Y. 2024. Longer photoperiod substantially increases indoor-grown cannabis' yield and quality. Plants 13(3):433. https://doi.org/10.3390/plants13030433