Short answer
Flip a photoperiod plant to 12/12 when it is about half the final height you can fit under the light, and once it shows preflowers. Most cannabis grows 50–100% taller after the flip, and stretchy cultivars up to 3×. Then count on 10–14 days of transition before flowering really starts. Supported for the stretch timing; the half-height rule is Grower practice.
Why
The stretch belongs to the flip. Under short days, stems and internodes lengthened fastest from about day 5 to day 10, before the compact flower cluster took over [1]. A plant-patent description puts the height gain at 50–100% over the first two to three weeks [2].
The flip starts induction, not flowering. Visible pistil clusters at the top of the plant appeared about 8–14 days after short days began in controlled studies [3][4]. That gap is why runs go "long" against breeder times.
A plant has to be mature to respond. Preflowers at the nodes form with age, not with the light schedule [3], so they are a good sign the plant is ready.
You don't have to run exactly 12/12. At 13 hours, onset moved by about 1.5 days or not at all, with more yield [5].
The numbers
| Space for the plant (top of pot to lowest safe light height) | Flip at about |
|---|---|
| 60 cm (2 ft) | 30 cm (12 in) |
| 90 cm (3 ft) | 45 cm (18 in) |
| 120 cm (4 ft) | 60 cm (24 in) |
| Very stretchy cultivar (up to 3×) | One third of the space |
What to do
- Measure from the top of the pot to the lowest height your light can safely sit above the canopy.
- Flip when the plant reaches half of that, or a third for a known stretchy cultivar.
- Check for preflowers with a loupe before flipping.
- Make the dark period truly dark from the first night.
- Log "days since flip" and, when 3+ pairs of pistils cluster at the top, "day 1 of flower".
Common mistakes
- Flipping by the calendar ("four weeks of veg") instead of by height.
- Forgetting the stretch and running out of headroom in week 3.
- Counting the flip as day 1 of flower and harvesting two weeks early.
- Light leaks in the first two weeks, while induction is still being set up.
Related
- Full article: The flip isn't day 1 of flower
- Answers: how long flowering takes · how tall plants get indoors · plant too tall for the tent
- Terms: the flip · stretch · 13/11
