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 — and the decisions you make in the next two weeks set the trajectory for everything that follows.
Before you flip: the checklist
- Height check. Plants stretch 50–100% after the flip (some sativa-dominant hybrids stretch even more). If your plant is at 60 cm and the light hangs at 120 cm, subtract pot height, light clearance, and double the plant height — do the math now, not in week 3
- Canopy even? The stretch amplifies unevenness. A cola that's 10 cm taller than its neighbors in veg will be 20 cm taller in flower, blocking light from everything below it. Level the table with LST first
- Healthy? Fix deficiencies and pests before the flip — flowering plants have less capacity to recover, and treatments like neem oil can't be used on buds
- Training done? All topping and heavy training should be at least one week behind you. The plant needs time to recover before the hormonal shift
- 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 (fans, power strips, humidity controllers — even small indicator lights count)
- Check for light leaks: sit inside the closed tent during lights-off — after 5 minutes your eyes should see nothing
- Zip the tent fully. A 1-inch gap at the intake port is enough to interrupt the signal
What light leaks cause: the plant receives a contradictory photoperiod signal. Mild leaks delay or stall flower development. Severe or repeated leaks trigger hermaphroditism — the plant produces pollen sacs alongside pistils, seeding itself and your entire tent. See the hermaphrodite prevention guide for the full list of stressors.
What changes at the flip
| Factor | Veg | Flower |
|---|---|---|
| Light cycle | 18/6 | 12/12 |
| Humidity | 50–65% | 40–50%, dropping to 35–45% late flower |
| Nutrients | High N (3-1-2) | Transition → bloom (1-3-2) |
| Temperature | 24–28 °C | 22–26 °C; cooler nights help color |
| Training | Active topping/LST | Maintain ties only — no new cuts |
Nutrient transition timing: don't switch to bloom feed on day one of 12/12. The plant still wants nitrogen through the stretch — it's building new vegetative growth at its fastest rate of the entire grow. Start transitioning to bloom feed around week 2–3 of 12/12, when vertical growth stops and bud sites start stacking pistils. For detailed bloom feeding, see the flower feeding strategy.
The stretch (weeks 1–3 of flower)
This is the final growth spurt. The plant can gain 50–100% of its pre-flip height in just two weeks. What to do:
- Keep training ties maintained. Growth is fast enough to pop loose ties in 48 hours
- Raise the light as needed. The canopy is closing the gap daily — check distance every 2–3 days
- Spread tops apart as they elongate. The stretch is your last chance to create space between bud sites for airflow and light penetration
- Don't panic at the speed. The stretch is normal and expected — it stops by week 3 in most cultivars
By the end of the stretch, pistils cluster into real bud sites and the plant's energy pivots from height to flower mass. You'll know it's over when the daily height gains stop and the canopy "freezes."
Managing a stretch that's too much
If the plant is approaching the light despite your planning:
- Supercrop the tallest colas — bend them at 90° to bring them to canopy level. See supercropping
- Raise the light as high as possible, then dim slightly if needed (less heat = less stretch)
- Increase the day/night temperature difference — cooler nights slow stretch
- As a last resort, lower light hours to 11/13 (this slows growth slightly but controls height)
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) — two thin threads emerging from the calyx at each node
- Males show small round pollen sacs like tiny bunches of grapes, on short stems at the nodes
One open male sac seeds your whole tent. Remove males the day you spot them — carefully, in a garbage bag, without shaking. For a visual reference and more detail, see the sexing guide.
The first two weeks of flower: a timeline
| Day | What's happening |
|---|---|
| 1–3 | No visible change. Plant is receiving the signal |
| 4–7 | Stretch begins. Rapid vertical growth |
| 7–10 | Pre-flowers appear at nodes (pistils or pollen sacs) |
| 10–14 | Stretch at full speed. Pistil clusters forming |
| 14–21 | Stretch slows and stops. Bud sites defined |
Common flip mistakes
Flipping too early. A small plant in a big pot produces a small harvest — more veg time (within reason) always means more yield. If you flip at week 3 of veg, expect one main cola and not much else.
Flipping too late. The plant doubles in flower. If you veg until the canopy fills 80% of vertical space, the stretch pushes it into the light. Always flip at 50% of usable height or less.
Changing everything at once. The flip is stressful enough — don't also transplant, defoliate, and switch nutrients on the same day. Change the timer, and adjust everything else gradually over the next two weeks.
Ignoring the stretch. "I'll deal with it later" turns into bleached top colas two weeks later. Stay on top of light height daily during the stretch.
For a deep dive into what happens after the stretch ends, see flowering week by week. For pre-flip canopy work, see vegetative stage 101.