# When should I flip my plants to flower?

> Flip a photoperiod plant to 12/12 (or 13/11) when it is about half the final height your space allows, because most plants grow 50–100% taller, sometimes up to 3×, in the first two to three weeks after the flip. The plant must also be mature, showing preflowers at the nodes. The flip is not day 1 of flower: real flowering starts about 10–14 days later.

- Canonical: https://genetixbay.com/knowledge/answers/when-to-flip-to-flower.html
- Topic: Basics · Type: How-to
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [The flip isn't day 1 of flower: the 10–14 day transition and how to count it](https://genetixbay.com/knowledge/environment/flip-to-flower-transition.md)
- Also asked as: When should I switch to 12/12? | When should I flip? | How big should my plant be before flowering? | When to flip photoperiod plants | How tall should plants be at the flip?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Flip a [photoperiod plant](https://genetixbay.com/knowledge/glossary/photoperiod.md) to [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) when it is about half the final height you can fit under the light, and once it shows [preflowers](https://genetixbay.com/knowledge/glossary/pre-flower.md). 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](https://genetixbay.com/knowledge/glossary/stretch.md) timing; the half-height rule is *Grower practice*.

## Why

The stretch belongs to the flip. Under short days, stems and [internodes](https://genetixbay.com/knowledge/glossary/internode.md) 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](https://genetixbay.com/knowledge/glossary/pistil.md) 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

1. Measure from the top of the pot to the lowest height your light can safely sit above the canopy.
2. Flip when the plant reaches half of that, or a third for a known stretchy cultivar.
3. Check for preflowers with a [loupe](https://genetixbay.com/knowledge/glossary/jewelers-loupe.md) before flipping.
4. Make the dark period truly dark from the first night.
5. 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](https://genetixbay.com/knowledge/glossary/light-leak.md) in the first two weeks, while induction is still being set up.

## Related

- Full article: [The flip isn't day 1 of flower](https://genetixbay.com/knowledge/environment/flip-to-flower-transition.md)
- Answers: [how long flowering takes](https://genetixbay.com/knowledge/answers/how-long-does-flowering-take.md) · [how tall plants get indoors](https://genetixbay.com/knowledge/answers/how-tall-do-plants-get-indoors.md) · [plant too tall for the tent](https://genetixbay.com/knowledge/answers/plant-too-tall-for-tent.md)
- Terms: [the flip](https://genetixbay.com/knowledge/glossary/flip.md) · [stretch](https://genetixbay.com/knowledge/glossary/stretch.md) · [13/11](https://genetixbay.com/knowledge/glossary/13-11.md)

## Follow-up questions

**Is the flip day 1 of flower?** No, the flip to 12/12 is not day 1 of flower. Switching to long nights starts floral induction, and the shoot tip spends roughly 10–14 days rebuilding itself before the first pistil clusters show at the top of the main shoot. Breeder flowering times often count from that point, so a stated 8 weeks usually means harvest about 9.5–10 weeks after the flip.

**Should I wait for preflowers before flipping?** Waiting for preflowers before flipping is sensible. Preflowers are single flowers at the nodes that appear with age, not with the light schedule, and they show the plant is mature enough to flower and what sex it is. In research they first formed around the 7th node. A plant flipped before then still flowers, but only after it reaches maturity.

**Can I flip on 13/11 instead of 12/12?** Yes, you can flip on 13/11; GenetixBay endorses it. In one study of two THC-dominant cultivars, 13 hours of light delayed flowering by about 1.5 days in one and not at all in the other, while yield rose 1.35× and 1.5×. Sensitive cultivars may take a day or two longer to set flower, so test your own cultivar.

## Sources

1. Alter H, Sade Y, Sood A, et al. 2024. Inflorescence development in female cannabis plants is mediated by photoperiod and gibberellin. Horticulture Research 11(11):uhae245. https://doi.org/10.1093/hr/uhae245
2. US Plant Patent PP30,434 (cannabis plant 'LW-BB1'), botanical description of post-induction stretch. https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/PP30434
3. Spitzer-Rimon B, Duchin S, Bernstein N, Kamenetsky R. 2019. Architecture and florogenesis in female Cannabis sativa plants. Frontiers in Plant Science 10:350. https://doi.org/10.3389/fpls.2019.00350
4. Zhang M, Anderson SL, Brym ZT, Pearson BJ. 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153
5. Ahrens A, Llewellyn D, Zheng Y. 2024. Longer photoperiod substantially increases indoor-grown cannabis' yield and quality. Plants 13(3):433. https://doi.org/10.3390/plants13030433