Flowering not starting or very slow after the flip
No visible flowers 3+ weeks after switching to 12/12 or 13/11 (normal onset 8-14 days, slow cultivars up to 3 weeks)..
- Also called
- not flowering
- still vegging after flip
- no pistils after 3 weeks
- won't flower
Likely causes, most likely first
Ranked for this sign on its own. Your stage, medium and readings change the order, so run the checker for your plant.
LikelyLight leak or interrupted dark period
Fix: Seal the dark period: pinholes, zips, vents and equipment LEDs (tape them), and check the timer. Even dim night light can delay flowering in sensitive cultivars and reveal herm-prone genetics. Use a green headlamp only for brief checks.
Recovery: Flowering resumes within 1-2 weeks; no new single-blade leaves.
How to confirm it
- Check for light leaks in the dark period. Sit inside the tent or room for 5 minutes after lights-off until your eyes adjust. Look for pinholes, zips, vents, indicator LEDs on equipment and light under doors. Also confirm the timer actually switches off and stays off.
Less likelyLong-flowering or light-sensitive cultivar
Fix: After the flip there is a 10-14 day transition before flowering really starts (up to 3 weeks in slow cultivars). Count from the flip and add the transition; flip earlier or train flatter for big-stretch cultivars.
Recovery: Pistils at the top of the main stem by about 3 weeks after the flip.
How to confirm it
- Check for light leaks in the dark period. Sit inside the tent or room for 5 minutes after lights-off until your eyes adjust. Look for pinholes, zips, vents, indicator LEDs on equipment and light under doors. Also confirm the timer actually switches off and stays off.
- Compare with siblings or the same cut. Put the affected plant beside siblings or clones of the same cut in the same conditions. A trait shared by all of a cultivar (purple stems, foxtailing, pale colour) is genetic; a problem on one plant points to its roots, a virus or its genetics; a problem on every cultivar points to the environment or feed.
Look for
Top of the main stem 3 weeks after flip; a photo of the tent in the dark with the lights off..
The full guide: Photoperiod and light leaks: 18/6, 12/12 and the dark period
Guidance, not a guarantee. The checker weighs common causes against what you tell it; confirm with the tests before you treat, change one thing at a time and watch the new growth.
