The cannabis flowering stage light cycle is 12 hours of light and 12 hours of uninterrupted darkness. Flip the timer to 12/12 when you want a photoperiod plant to stop stretching and start building buds. Most indoor growers switch after 3 to 6 weeks of vegetative growth, or when the canopy fills about 70 percent of the tent footprint.
Prerequisites
- Photoperiod seeds or clones. Autoflowering plants ignore the light schedule and are not covered here.
- A digital timer rated for your fixture's wattage.
- A grow space you can seal against outside light for the dark period.
- A thermometer and hygrometer, since temperature and humidity shift once the lights go off for half the day.
How to set the flowering light cycle
- Pick a 12-hour window you can hold for 8 to 11 weeks. Many growers run lights at night to beat summer heat.
- Set the timer so the light turns on and off at the same clock times every day.
- Verify the dark period is dark. Sit inside the closed tent for two minutes after lights out and look for glow from vents, zippers, or power strips.
- Move watering, pruning, and inspection into the light period.
- Hold the schedule until harvest. Do not shorten the dark period for a heat wave or a late visit.
What happens week by week
Flowering runs in phases. Expect this sequence after the flip:
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- Week 1 to 2: Stretch. Stems lengthen and the plant may double in height. Pre-flowers reveal sex.
- Week 3 to 4: Bud sites form along the branches and the first white pistils cluster.
- Week 5 to 7: Buds swell and gain weight. Trichomes appear and terpene output peaks.
- Week 8 to 11: Pistils darken and curl inward. Check trichomes for ripeness.
Light intensity during 12/12
Fewer light hours mean less total light. Raise output or move the fixture closer to keep daily light integral in the flowering range, roughly 30 to 45 mol per square meter per day for cannabis. Watch leaf temperature. Leaves that curl, bleach, or crisp at the top mean the fixture sits too close or the dimmer runs too high.
Common problems
Plants that will not flower
A single light leak, a glowing LED on a power strip, or a timer that fires during the dark period keeps a plant in vegetative mode. Fix the leak, then wait 7 to 10 days.
Cannabis Flower Stretch Stage Guide
Hermaphrodites
Stress from light interruptions, heat, or a failed timer can push a female plant to grow male flowers. Remove affected plants to protect the crop.
Stalled bud growth
Cool night temperatures slow development. Keep the dark-period temperature within about 10 degrees Fahrenheit of the light-period temperature.
Exceptions to 12/12
- Autoflowering cultivars flower by age, so 18/6 or 20/4 works from seed to harvest.
- Long-flowering sativas from equatorial regions finish under 11/13 or 10/14 late in the cycle.
- Breeding and reversal projects sometimes use 12/12 from seed to keep plants small.