GenetixBayEST. 2009
Environment · Beginner · 4 min read

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.

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Draft: awaiting expert review Updated
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Being reviewed. Our editors are double-checking a few figures in this article against primary sources. Marked passages may change.

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 light [2]. Make the room truly dark.

What's happening

Phytochrome in the leaf tracks red and far-red light. 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 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)being reviewed.

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.

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/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.

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.

Questions this article answers

Why is my plant not flowering on 12/12?

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.

How dark does the grow room need to be at night?

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.

Do light leaks cause hermaphrodites?

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.

Can I flower on 13/11 or 14/10?

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.

Can I run 24/0 in veg?

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.

Do sunrise and sunset ramps count as daylight?

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.

Is 24 hours of light OK in veg?

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.

What photoperiod for autoflowers?

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.

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. 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). ashs2024.sched.com/event/1dtbo/ce-4-night-light-pollution-de
  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. 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. pmc.ncbi.nlm.nih.gov/articles/PMC12525983/

Related

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PPFD and DLI by stage, with the evidence

In the best indoor trial, cannabis flower yield rose linearly with canopy PPFD from 120 to 1,800 µmol/m²/s at ambient CO₂ (116 to 519 g/m²) with no change in cannabinoid potency. Practical targets: 100–300 µmol/m²/s for seedlings and clones, 300–700 (up to ~900) in veg, and 800–1,200 in flower, or more with CO₂, as long as the plant shows no stress and the electricity is worth it.

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Light spectrum: blue, red, far-red and UV, what's real

Photon quantity (PPFD/DLI) and fixture efficacy matter far more than spectrum. Less blue slightly increased cannabis yield (about 12 % from 4 % to 20 % blue). Far-red stretches plants; its yield effect is contested and seems to trade off against cannabinoid concentration. UV has repeatedly failed to raise cannabinoids in controlled trials and can cut yield at high doses.

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Heat and light stress: taco leaves, bleaching and foxtails

Heat and light stress show at the top of the plant, closest to the light: leaves curl up or taco, bracts bleach white or yellow, buds may foxtail. Measure leaf temperature and canopy PPFD before blaming nutrients. Typical fixes: dim or raise the light 10–20 %, bring leaf temperature under about 30 °C (86 °F), restore airflow and root-zone water uptake.

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Hermaphrodite cannabis (bananas and nanners): what to do

Hermaphroditic flowers are anthers (often banana-shaped) or pollen sacs forming on a female plant. Cannabis is a dioecious species, and a well-bred female should stay female; herm-prone plants are mainly the product of bad breeding (repeated selfing, careless feminising, untested 'pollen chucker' seed). Stress such as light leaks or heat reveals that weakness rather than causing it in sound genetics. Fix the environment, remove the plant if it sheds pollen, and never keep it as a mother or breed from it.

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