Photoperiod cannabis flowers when nights get long: 12 hours of uninterrupted darkness tells the plant winter is coming. Indoors, you are the season. The day you set the timer to 12/12 is the day flowering begins.
Before you flip: the checklist
- Height check. Plants stretch 50–100 % after the flip. If your plant is at 60 cm and the light hangs at 120 cm, do the math now, not in week 3.
- Canopy even? The stretch amplifies unevenness. Level the table first.
- Healthy? Fix deficiencies and pests before the flip — flowering plants have less capacity to recover.
- Timer tested? A light leak or a timer glitch during flower causes hermaphroditism and re-veg. Verify the off-period is truly off.
Darkness is non-negotiable
The flowering signal is unbroken darkness. During the 12 off-hours:
- No tent peeks with the room light on
- Tape over power LEDs inside the tent
- Check for light leaks: sit inside the closed tent during lights-off — after 5 minutes your eyes should see nothing
What changes at the flip
| Factor | Veg | Flower |
|---|---|---|
| Light cycle | 18/6 | 12/12 |
| Humidity | 50–65 % | 40–50 %, dropping late |
| Nutrients | High N (3-1-2) | Shift to bloom (1-3-2) |
| Temperature | 24–28 °C | 22–26 °C, cooler nights |
Shift nutrients gradually — plants still want nitrogen through the stretch. Start transitioning to bloom feed around week 2 of 12/12, not day one.
The stretch (weeks 1–3 of flower)
Growth goes vertical and fast. Keep training ties maintained, raise the light as needed, and use the stretch to your advantage: gently spread tops apart so the canopy stays open. By week 3 the stretch stops, pistils cluster into real bud sites, and the plant's energy pivots from height to flower mass.
Sexing: catch males in week 1–2
If you're growing from regular (non-feminized) seed, check every plant at the nodes from day 7 after the flip. Females show white hairs (pistils); males show small round pollen sacs like tiny bunches of grapes. One open male sac seeds your whole tent. Remove males the day you spot them.