Marijuana Flowering Stage Light Schedule: 12/12 Guide

The flowering light schedule for photoperiod cannabis is 12 hours of light and 12 hours of uninterrupted dark. Keep the dark period free of light leaks. Set a timer and leave it alone. Autoflowers can flower under more light, but 12/12 works for them too.

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Why 12/12 starts flowering

Cannabis measures night length. When the dark period reaches about 12 hours, the plant shifts from vegetative growth to flower production. A longer dark period or a shorter one both delay or disrupt that shift. The plant needs a stable signal, not a perfect one.

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Set the timer once

1. Choose a 12-hour window that matches your routine, such as 7 a.m. to 7 p.m. or 8 p.m. to 8 a.m.
2. Plug the light into a mechanical or digital timer rated for the load.
3. Set the timer to switch on and off at the same times each day.
4. Confirm the off period with a phone camera or a light meter after the room goes dark.

Protect the dark period

Any light during the dark cycle can stress a photoperiod plant. Cover indicator LEDs on equipment with tape. Seal gaps around doors and vents. Do not open the tent during lights-off unless the work is urgent. A green headlamp is not a free pass; use it for seconds, not minutes.

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Light distance and intensity in flower

Flowering plants use more light than vegetative plants. Raise intensity if the canopy is even and leaf edges stay flat. Move the fixture up or dim it if top leaves cup, bleach, or show burned tips. Follow the manufacturer's hang height for the flowering stage and adjust from there. Measure at the canopy, not at the wall.

What changes at the flip

The first two to three weeks of 12/12 bring stretch. Keep the light close enough to limit stretching but far enough to avoid burn. Defoliate lower growth that receives no light. Switch to bloom nutrients after stretch slows, not on day one.

Autoflowers and light cycles

Autoflowers do not depend on a dark trigger. They can flower under 18/6, 20/4, or 12/12. More light hours can mean more growth if the plant handles the intensity. Choose one schedule and keep it for the whole run.

Common schedule mistakes

• Changing the timer to "catch up" after an outage. Reset to the original window and accept the lost hours.
• Running 13 or 14 hours of light in flower. That can delay flowering or cause revegetation.
• Using a timer with a short power loss backup that resets to a default. Check the timer after any outage.
• Letting a porch light, TV, or phone screen hit the canopy during lights-off.

Week-by-week check

Week 1 to 3: hold 12/12, watch stretch, adjust height. Week 4 to 6: keep the schedule, raise intensity if leaves allow. Week 7 to harvest: hold 12/12 to the end. Do not shorten the dark period to "finish faster."