Switch a photoperiod cannabis plant to flower by setting the timer to 12 hours of light and 12 hours of dark. Most growers flip between week 4 and week 6 of veg, or when the canopy fills about half the height and floor space they can give it. Autoflowers make the call themselves: they start flowering around day 21 to day 35 from sprout, whatever the timer says.
cannabis flowering stage nutrients
When should you flip photoperiod cannabis to 12/12?
Flip when the plant has the size you want, not when a calendar page says so. Each extra week of veg adds branches and yield, and each extra week adds height you may not have room for.
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- Space: the canopy covers about 50% of the floor and half the vertical room under the light.
- Age: 4 to 6 weeks from seed, 2 to 4 weeks from a rooted clone.
- Structure: 4 to 6 sets of true nodes with side branches forming at each one.
- Sex: pre-flowers visible at the nodes, which confirms a female before you spend 8 more weeks on it.
- Calendar: count back from your target harvest. Add 8 to 10 weeks of flower plus 2 weeks to dry and cure.
Why the stretch decides your timing
Cannabis stretches in the first 2 to 3 weeks of flower. Most plants finish at 1.5 to 2 times their flip height, and sativa-leaning cuts can go higher than that.
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Measure the gap between the top of the canopy and the light. A plant that finishes 10 inches from a hot LED will bleach and stress. Tight headroom means you flip sooner, even if the plant looks small.
How to read the plant before you flip
Node count beats plant height
Nodes are the points where leaves and branches meet the stem. A plant with 5 or 6 node sets has enough growing points to fill a small tent. Height on its own misleads you: a stretched seedling under a weak light can stand tall with 2 nodes and no branching.
Pre-flowers show the plant is mature
Pre-flowers sit at the nodes as a single calyx with two white hairs (female) or a small ball on a short stalk (male). A plant showing pre-flowers is ready to flower. A plant that shows nothing yet will still flower under 12/12, it just needs a few extra days to switch on.
Pot size and roots
Roots fill the pot around the same time the canopy fills the space. A plant in a 5-gallon pot inside a 2x4 tent reaches that mark near week 5 of veg. Roots circling the drainage holes mean the plant has run out of room to grow down and should grow up instead.
How long to veg by grow style
- 2x2 or 3x3 tent, one or two plants: 4 to 5 weeks of veg.
- 4x4 tent, four plants: 5 to 6 weeks of veg.
- Sea of green, many small clones: 1 to 2 weeks, then flip.
- Screen of green: 6 to 8 weeks, with the screen about 70% full at the flip.
- Outdoor in a legal climate: the sun sets the date. Plants flip as daylight drops below about 12 hours in late summer.
When do autoflowers switch to flower?
Autoflowers flip on their own, most between day 21 and day 35 from sprout. Nothing you do with the timer changes that. Run them on 18/6 or 20/4 for the whole cycle, since dropping to 12/12 cuts light hours and yield.
A stalled autoflower at day 40 with no flower sites points to stress: cold root zone, overwatering, or a pot that is too small. Fix the cause and the plant starts on its own.
What changes the day you flip
- Light: 12/12 for photoperiod plants. Check the timer with your own eyes at lights-off.
- Feed: move to a bloom ratio with more phosphorus and potassium and less nitrogen.
- Air: flower rooms want 40 to 50% humidity, not the 60 to 70% of veg.
- Canopy: strip the bottom third and any growth that gets no light. Energy goes to the tops.
- Records: write down the flip date and count forward 8 to 10 weeks.
Signs you flipped too early
- Bare soil around the pot because the canopy never spread.
- Flower sites forming on a 10-inch plant with 3 nodes.
- A final yield that fills one jar for 8 weeks of watching.
None of these ruin the grow. They cost weight. Next run, add a week of veg or start with a bigger pot.
Signs you waited too long
- Leaves pressed flat against the light or the tent wall.
- Plants taller than the space you can hang the light above them.
- Light hitting the floor between plants, which means you lost usable space.
If you are already there, flip today. Bend tall branches outward under a screen and raise the light as far as the tent allows.
Flip checklist
- Water the day before the flip so the plant enters flower without drought stress.
- Set the timer to 12/12 and confirm it at lights-off.
- Seal light leaks in the tent. A pinhole during the dark period can trigger male flowers.
- Prune the lower third and any shaded growth.
- Switch to bloom nutrients at the next feeding.
- Note the flip date and set a reminder for week 8.
How long does flowering take after the switch?
- Indica-leaning: 7 to 9 weeks.
- Hybrids: 8 to 10 weeks.
- Sativa-leaning: 10 to 14 weeks.
Breeder numbers count from the first flower clusters, not the flip. That adds about a week of transition to the total. Judge harvest by trichome color on the buds, not by the calendar.
FAQ
Can I switch to 12/12 before week 4?
Yes. Small plants flower fine, you just get less weight. A 2-week flip suits a sea of green with many small plants packed in tight.
Do I have to wait for pre-flowers before I flip?
No. Pre-flowers confirm sex and maturity, but 12/12 triggers flowering on its own. Plants that show no pre-flowers take a few extra days to start.
What happens if I flip back to veg?
The plant revegetates. You get single-blade leaves, slow growth, and stress that can produce male flowers. Pick a flip date and stay with it.
Does the plant flower overnight?
No. Expect 5 to 14 days of transition. Stretch starts first, flower clusters follow.