Light schedules
| Schedule | Used for | Notes |
|---|---|---|
| 18/6 | Photoperiod veg · autos full-cycle | The default. More light = more growth while roots keep up. |
| 12/12 | Photoperiod flowering | THE flip — 12h uninterrupted dark triggers and holds flowering. Light leaks cause revegging/herms. |
| 20/4 | Autoflowers | Autos flower on age regardless; 20/4 trades electricity for a bit more growth. 24/0 works but most give a dark rest. |
| 6/2 ×3 | Veg (heat management) | Splits the day into 3 cycles to spread heat; niche but legitimate. |
Intensity matters as much as hours: seedlings ~200–400 PPFD, veg ~400–600, flower ~600–1000. If leaves taco upward or bleach under an LED, raise the light before blaming nutrients.
Common questions
What light schedule should I use?
18/6 through veg, 12/12 to flower a photoperiod plant. Autoflowers do not respond to photoperiod and are commonly run 18/6 or 20/4 for their whole life.
How much light does cannabis actually want?
Roughly 200–400 PPFD for seedlings, 400–600 in veg and 600–1000 in flower. Research has pushed yield up linearly to 1800 µmol/m²/s under enriched conditions, but that requires CO2, temperature and feeding to match.
Does more light mean more potency?
No — and this surprises people. A controlled trial found dry flower yield rose 4.5x from 120 to 1800 PPFD with no change in cannabinoid concentration. You get more grams, not stronger grams.
What is the critical photoperiod for flowering?
It varies by cultivar. Research across nine cultivars found critical day lengths as short as 13 h 45 min, so 12/12 is a safe default rather than a biological threshold.
Sources: Rodriguez-Morrison et al. 2021, Frontiers in Plant Science — yield rose 4.5× from 120 to 1800 PPFD · Chandra et al. 2008 — photosynthesis peaks near 1500 µmol at 30 °C · Llewellyn et al. 2022, Frontiers in Plant Science — 1000 vs 600 µmol · Zhang et al. 2021 — critical photoperiod by cultivar
- Digital timer or smart plug — light-schedule drift causes more stress than most growers realise
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Keep reading
- Autoflowers run on age, not light
- Training around the stretch
- Lights-on vs lights-off environment targets
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