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Flowering · 8 minute read · Updated October 20, 2026

When should I switch my plants to 12/12?

Most growers think the flip is about the calendar. It is really about the ceiling: the plant you flip today is not the plant you will be fitting under the light three weeks from now.

Why the flip is a space decision, not a date

Cannabis grown from seed or clone as a photoperiod plant stays in vegetative growth while the nights are short. Researchers describe it as a short-day plant: it starts flowering once the night is long enough. Indoors, growers hold plants in veg under 18 hours of light and switch to 12 hours of light and 12 of dark to start flowering.

Here is the part that trips people up. The switch does not stop growth. In a 2024 study at Israel's Volcani Institute, plants moved from 18 hours of light to 12 grew taller faster than plants left on 18 hours during the first 14 days, with the fastest stem growth around days 5 to 10. After that, their height leveled off.

So ask yourself one question before you flip: if these plants keep climbing for two more weeks, where will the tops end up? Answer that and the date picks itself.

How big should plants be when you flip?

A long standing grower rule is to flip when the plants have filled about half the space they are meant to fill. That is a rule of thumb, not a measured number, and it works because the stretch fills the rest.

Check three things:

• Floor coverage. Look down from the light. If the branches cover about half the tent floor and you plan to keep spreading them, you are close. A ScrOG net that is half to two thirds full is the same signal.
• Height. Measure from the top of the pot to the tallest top.
• Health. Flip a plant that is green and growing well. Flowering stacks onto whatever problem the plant already has. The problem library helps you name what you see first.

When did you last measure your plants with a tape instead of by eye? A number written down today is what lets you plan next week.

How much will my plants stretch after 12/12?

It depends on the strain, and we found no controlled study that gives one stretch number that holds across strains. The Volcani study shows the shape of it: the internodes already on the plant at the switch got longer, new internodes stayed short, and height stabilized after about two weeks. Breeder notes often say whether a strain stretches a little or a lot.

Instead of trusting one number, plan for a range. This table shows where a plant ends up if it grows by half, doubles, or more than doubles.

Now add the pot height, the flowering hanging distance your light maker recommends (see LED light height by stage) and the fixture itself. If the total runs past the tent height in the doubling column, flip sooner or train the tops down harder now.

How long should you veg before flipping?

There is no right number of weeks, only a right size. Veg time is how you trade plant count against time:

• More plants, short veg. Research rooms fill space with many small plants. In the University of Guelph light study, rooted cuttings got about 2 weeks under 18 hours of light before flowering. In an Australian photoperiod study, plants spent 18 days under 18 hours before the treatments began.
• Fewer plants, long veg. A home tent with a few plants needs more weeks of veg and training to fill the same floor. Each plant gets bigger, and each one carries more of the harvest.

Your plant count is also a legal limit in most places that allow home growing, so check home grow laws by state first. Then see how many plants fit in a 4x4 tent for the space math.

How do you switch to 12/12?

1. Change the timer once. Set 12 hours on and 12 hours off. Many growers simply cut the light period in one step. Pick lights-off hours you can keep the tent dark and the room cool.
2. Make the dark truly dark. Cannabis needs an uninterrupted dark period to flower. A 2026 Utah State dissertation found that light during the dark period disrupts the plant's timekeeping and weakens flowering, sometimes entirely. Check for leaks with the room lights on and the tent closed. Our guide to light leaks and light schedules walks through it.
3. Stay on 12/12. In the Volcani study, plants given only 1 to 3 days of short days and then moved back to long days stopped forming flowers and went back to veg. Even after 5 to 21 days, flowers did not hold together once long days returned: stems grew and leaves appeared on top of the flower clusters. Do not switch back and forth.
4. Finish topping first. Do your last cut a week or two before the flip so the branches have hardened by the stretch.

Is exactly 12 hours the only option?

For a home grow, 12/12 is the safe choice. Researchers are testing small changes, and the results are mixed:

• A University of Guelph study of ten cultivars found flowering started in all of them at up to 14 hours of light, with delays from none to about 4 days compared with 12 hours. Yield in the early weeks peaked at different day lengths for different cultivars, all under 13 hours, and the authors said each cultivar should be tested.
• An Australian study that started flowering at 14 hours of light raised flower weight in all three lines tested, but a steady 14 hours lowered THC concentration in both THC lines.

Those are tests on a few strains in research rooms. Without your own trial on your own strain, longer days risk slow or stalled flowering. Keep it at 12.

What happens in the first two weeks after the flip?

The day numbers come from the Volcani study of one cultivar. Your strain may run a few days earlier or later. Male flowers or pollen sacs ("nanners") are worth catching early; see hermaphrodite cannabis. After the stretch, the season changes; flowering week by week picks up from here.

What about autoflowers?

Autoflowers are photoperiod insensitive, often called day neutral. They flower on their own as they age, not because the nights get longer. In a 2022 genetics study, most high-cannabinoid cultivars did not flower under continuous light, except plants carrying two copies of the gene the authors named Autoflower1.

So there is no flip to time. Most growers keep autos on one long light schedule from seed to harvest. More on that in autoflower vs photoperiod.

Whichever you grow, write down the date of the flip and the height on that day. Next grow, the same strain will tell you exactly how much it stretched. A grow journal, paper or an app like Canna GLAP, keeps that note with the plant.

Questions growers ask

Read next

• How to train cannabis plants to fill a grow tent
• Light leaks and light schedules: protect the dark period
• Autoflower vs photoperiod: which should you grow?
• How high should an LED grow light be at each stage?

Education only, for adults 21+ where home cultivation is legal. Updated October 20, 2026.