# Photoperiod and light leaks: 18/6, 12/12 and the dark period

> Photoperiod cannabis stays vegetative under long days (18/6 up to 24/0; no sleep period is needed in veg) and flowers when the uninterrupted night is long enough: 12/12 or 13/11, and some cultivars flower at 14–15 h, usually as ~12 h at high intensity plus sunrise and sunset ramps. Light during the dark period, even very dim, can delay or weaken flowering in sensitive cultivars; leaks can also expose herm-prone genetics, triggering male flowers on susceptible plants. Some cultivars yield more at 13–14 h days, at a possible cost in THC concentration.

- Canonical: https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.html
- Category: Environment · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Transition, Flower · 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

**Why is my plant not flowering on 12/12?** A [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis plant that is not flowering on 12/12 usually has light reaching it during the dark period or a faulty timer. Research shows stray light as dim as 0.01–2 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) can delay flowering in sensitive cultivars. Remember the first pistil clusters normally take about 10–14 days after the flip, so worry only if stretching and leaf growth continue 3–4 weeks in.

**How dark does the grow room need to be at night?** A cannabis flowering room needs to be as close to completely dark as you can measure during the night period, and certainly below about 0.01–2 µmol/m²/s of stray light. A hemp study found delays from roughly 2 µmol/m²/s, and conference work saw delays in the most sensitive cultivars at 0.01 µmol/m²/s. If your adapted eyes can see light after five minutes, it is a leak.

**Do light leaks cause [hermaphrodites](https://genetixbay.com/knowledge/glossary/hermaphrodite.md)?** Light leaks are a commonly reported trigger for hermaphrodite flowers in cannabis, but in GenetixBay's breeder position a leak reveals herm-prone genetics rather than creating them: a stable plant beside the same leak stays female. Controlled evidence is limited, so this is grower practice. Fix the leak, and cull any plant that produced [pollen sacs](https://genetixbay.com/knowledge/glossary/anther.md) rather than breeding from it.

**Can I flower on 13/11 or 14/10?** Yes, many cannabis cultivars flower on 13/11, which GenetixBay endorses, and some flower at 14–15 hours of light. In one study every line flowered at 14 hours and some yielded more, but two high-THC lines lost [THC](https://genetixbay.com/knowledge/glossary/thc.md) concentration. In practice the house approach is about 12 hours at high intensity plus sunrise and sunset ramps. Trial your own cultivar before committing.

**Can I run 24/0 in veg?** Yes, you can run 24/0 lighting in veg; GenetixBay endorses it, because cannabis does not need a sleep period in vegetative growth. As a [short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md), cannabis measures night length only to decide when to flower, so continuous light simply keeps it vegetative. The trade-off is more electricity per day than 18/6 or 20/4, which is grower practice rather than a measured yield finding.

**Do sunrise and sunset ramps count as daylight?** Yes, sunrise and sunset ramps count as daylight for a cannabis plant's flowering clock, even though they add little to the [daily light integral](https://genetixbay.com/knowledge/glossary/dli.md). The plant measures the uninterrupted dark that follows, so a 12-hour high-intensity day plus ramps makes a 13–15 hour photoperiod. When calculating DLI, count the ramp periods at their real, lower PPFD.

**What photoperiod for [autoflowers](https://genetixbay.com/knowledge/glossary/autoflower.md)?** Autoflowering cannabis is usually grown on 18/6 to 20/4 from seed to harvest. Autoflowers are day-neutral, meaning the photoperiod does not trigger their flowering, so there is no need to switch to 12/12. Long days simply give them more total light during their short, fixed life cycle. This schedule is grower practice rather than a result from a controlled trial.

**Short answer.** Photoperiod cannabis measures the **night**, not the day. Keep nights short (18/6 is standard veg) to stay vegetative; give a long, **uninterrupted** night to flower. **House position:** 24/0 is fine in veg (cannabis doesn't need a sleep period in veg), and flower doesn't have to be exactly 12/12: **13/11 is endorsed**, and some cultivars flower at 14–15 h of light. Light in the dark period, even very dim, can delay or weaken flowering: a hemp study reports delays from stray light at about **2 µmol/m²/s** [1], and a 2024 conference report saw delays in the most sensitive cannabis cultivars at **0.01 µmol/m²/s (10 nmol)** of continuous white [LED](https://genetixbay.com/knowledge/glossary/led.md) light [2]. Make the room truly dark.

## What's happening

Phytochrome in the leaf tracks red and [far-red light](https://genetixbay.com/knowledge/glossary/far-red.md). During a long night the "active" form decays; once the night is longer than the cultivar's critical night length, flowering genes switch on. A pulse of light in the middle of the night resets the clock (that's the "night break" commercial growers use deliberately to keep [mothers](https://genetixbay.com/knowledge/glossary/mother-plant.md) vegetative with less energy [4]).

Critical night length varies by cultivar; many drug-type cultivars flower once nights exceed roughly 10–11 h, which is why 12/12 is the safe default ([1] tested hemp).

**Sunrise and sunset.** In practice a GenetixBay flowering day is about **12 h at high intensity**, with dimmed sunrise and sunset ramps adding the rest of a 13–15 h photoperiod. The ramps are low-intensity light, so they add little DLI, but they count as day: the uninterrupted dark that follows is what the plant measures.

**The flip isn't day 1 of flower.** Expect roughly 10–14 days of transition before pistil clusters show at the tops; see [the flip-to-flower transition](https://genetixbay.com/knowledge/environment/flip-to-flower-transition.md).

## Numbers that matter

| Situation | Schedule | Notes |
|---|---|---|
| Veg, photoperiod | 18/6, 20/4 or 24/0 | 24/0 endorsed (house position): cannabis doesn't need a sleep period in veg. It costs more energy per day (*Grower practice*) |
| Mothers | 18/6, or 12/12 + 1 h night break | Night break can cut veg lighting energy by nearly a third [4] |
| Flower, default | 12/12 or 13/11 | 13/11 endorsed (house position); in practice ~12 h at high intensity plus sunrise/sunset ramps |
| Flower, cultivar trial | 14/10 or 15/9 | Some cultivars flower at 14–15 h. Some cultivars yielded more at 14 h; two high-THC lines lost THC concentration [3] |
| Autoflowers | 18/6 to 20/4 throughout | Day-neutral; photoperiod doesn't trigger flowering |
| Dark-period stray light | As close to 0 as you can measure; certainly < 0.01–2 µmol/m²/s | Thresholds from [1][2]; cultivar dependent |

## How to find and fix leaks

1. Sit in the room with the lights off for 5 minutes until your eyes adapt. Anything you can see is a leak.
2. Usual culprits: tent zipper ends, duct ports, power-strip LEDs, controller displays, smoke detector LEDs, phone screens, door gaps, light from a neighbouring veg room.
3. Fix with black tape, foil tape, light-tight duct boots and a black-out curtain on doors.
4. If you must work in the dark period, use a dim green headlamp briefly (*Grower practice*; green is less active on phytochrome than red, but not inactive).
5. Use a timer you trust; check it after power cuts. A drifting mechanical timer is a slow leak.

## How to recognise a problem

- Plants keep stretching and making leaves 3–4 weeks into 12/12.
- Flowers start, then "re-veg" (rounded single-finger leaves on buds).
- Uneven flowering between plants nearest a leak and the rest.
- Hermaphrodite flowers ([bananas](https://genetixbay.com/knowledge/glossary/nanners.md)/pollen sacs) late in flower. Light leaks are a commonly reported trigger, along with heat and other stress. In GenetixBay's breeder position the leak *reveals* susceptible genetics: a stable plant beside it stays female. Fix the leak, and cull the plant that hermed (*Grower practice*; controlled evidence is limited). See [hermaphrodites](https://genetixbay.com/knowledge/diagnosis/hermaphrodites.md).

## Common mistakes

- "One minute of light won't matter." For sensitive cultivars it can.
- Opening the tent to check on plants at night.
- Assuming outdoor street lighting can't reach a greenhouse.

## Evidence

The long-night requirement is *Established*. Low-intensity stray light delaying flowering is *Supported* by [1] for hemp and by conference work for cannabis [2] (not yet peer reviewed at time of writing). Longer flowering photoperiods increasing yield in some cultivars is *Supported* by one study [3], which also found lower THC concentration in two high-THC lines, so test your cultivar. 24/0 veg and 13/11 flower are house positions (*Grower practice*, founder-endorsed); no sleep period in veg is consistent with cannabis being a short-day plant that measures night length only to decide when to flower. Leaks triggering hermaphrodite flowers in susceptible plants is *Grower practice*; the genetic basis of susceptibility is the GenetixBay house position.

## 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. 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
3. 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
4. Examining the night break method in Cannabis sativa horticulture: vegetative daily light integral affects yield of extractable biomass, 2025. Plants 14(19):3095. https://pmc.ncbi.nlm.nih.gov/articles/PMC12525983/