Marijuana Flowering Stage Light Schedule: 12/12 Guide

Quick answer

Photoperiod cannabis plants flower under 12 hours of light and 12 hours of dark per day. The dark period must be continuous. Autoflowering plants ignore the light cycle and flower on their own under 18 hours of light and 6 hours of dark.

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Why 12/12 starts flowering

Cannabis measures night length with phytochrome pigments. When the dark period passes about 10 to 12 hours, the plant stops vegetative growth and builds flower sites. A timer holds that ratio. Most plants show the first pistils 7 to 14 days after the flip.

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The dark period rules

Twelve hours of dark is a minimum. Light during that window delays flower formation and can push female plants to grow male flowers. Check these sources of stray light:

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• Timer faults and power outages that shift the cycle
• Indicator LEDs on fans, dehumidifiers, and controllers
• Zippers, vents, and duct openings that pass room light
• Room lamps when the dark cycle runs during the day

Green headlamps are not safe. Plants respond to green wavelengths at low intensity.

Light intensity in flower

Flowering plants use more light than vegetative plants. Target a PPFD of 600 to 900 µmol/m²/s at the canopy. Over 12 hours that equals a daily light integral of 26 to 39 mol/m²/day. Hang LEDs 12 to 24 inches above the top colas and raise them as stems stretch. Leaves that cup upward, bleach white, or show brown patches sit under too much light.

Autoflower light schedules

Autoflowering plants carry ruderalis genetics. They start flowering 3 to 5 weeks from seed no matter the photoperiod. Run 18/6 or 20/4 for the whole cycle. A 24/0 schedule also works. It raises electricity cost and shows no proven yield gain.

Week by week

• Week 1 to 2: transition and stretch. Hold 12/12. Raise lights as stems lengthen.
• Week 3 to 6: bud sites form and stack. Keep PPFD at 600 to 900. Hold humidity at 40 to 50 percent.
• Week 7 to 9: buds swell. Drop humidity to 40 percent. Watch for bud rot.
• Week 10 to 12: sativa-leaning plants finish. Check trichomes under a 60x loupe.

Temperature

Keep the canopy at 68 to 78°F during lights on. A drop of 10 to 15°F during lights off limits stretch and supports terpene retention. Roots do best at 65 to 75°F.

Timeline to harvest

Indica-dominant plants finish 8 to 9 weeks after the flip. Sativa-dominant plants need 10 to 12 weeks. Add 1 to 2 weeks for the transition from veg. Seed to harvest runs 4 to 5 months for photoperiod plants and 10 to 12 weeks for autoflowers.

Common mistakes

• Flipping before the plant fills the grow space
• Running 13/11 or 14/10 in flower
• Opening the tent during the dark period
• Ignoring a timer that drifts by 30 minutes or more