# The flip isn't day 1 of flower: the 10–14 day transition and how to count it

> Switching to 12/12 starts floral induction, not flowering. In controlled studies the first visible inflorescence (at least three pairs of pistils at the top of the main shoot) appeared roughly 8–14 days after short days began, around 13–14 days on average across many hemp cultivars, and up to three weeks in slow ones. Breeder flowering times often count from that point, so a stated 8 weeks usually means harvest about 9.5–10 weeks after the flip. Count both: days since flip and days of flower.

- Canonical: https://genetixbay.com/knowledge/environment/flip-to-flower-transition.html
- Category: Environment · Level: Beginner
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Veg, Transition, Flower · Systems: Any system
- 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)"

## Questions and answers

**Is the first day of [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) the first day of flower?** No, the first day of 12/12 is not the first day of flower; this is a myth. The flip starts floral induction, and the shoot tip then spends roughly 10–14 days rebuilding itself before real flowering begins. Counting the flip as day 1 of flower is why many growers harvest about two weeks early because the calendar says 'week 8'.

**How long after the flip do [pistils](https://genetixbay.com/knowledge/glossary/pistil.md) show?** Visible pistil clusters at the top of the main shoot usually appear about 8–14 days after the flip to 12/12 in controlled studies. Across 27 [hemp cultivars](https://genetixbay.com/knowledge/glossary/hemp.md) the average was 13–14 days, the fastest essential-oil types took about 13 days and slow cultivars around 21 days or more. Under the microscope, flower [primordia](https://genetixbay.com/knowledge/glossary/pre-flower.md) were already forming by about day 7–8.

**Why does my strain take longer than the breeder says?** A cannabis strain usually takes longer than the breeder's stated flowering time because many breeders count from flower set, the first visible pistil clusters, not from the flip. The 10–14 day transition gets added on top, so a stated 8 weeks typically means harvest about 9.5–10 weeks after switching to 12/12. This counting habit is grower practice and the GenetixBay house position, not a measured survey.

**When does flowering actually start after switching to 12/12?** Flowering actually starts when at least three pairs of pistils are visible at the top of the main shoot, the research definition of onset. After switching to 12/12 that typically takes about 10–14 days, though some northern hemp needed only 4–11 days and slow cultivars three weeks. Full flowering on main and side branches took until about day 22 in one study.

**Are preflowers the start of flowering?** No, preflowers are not the start of flowering. The single flowers that form in leaf axils during veg are triggered by plant age and internal signals, not by the [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md), so they appear under 18/6 or 24/0 too. Research found the first pair formed around the 7th node in one cultivar. They reveal sex and maturity; induced pistil clusters at the shoot tips mark real flowering.

**How do I count weeks of flower?** To count weeks of flower, keep two clocks. Days since flip starts at the first long night and is for planning the room, [stretch](https://genetixbay.com/knowledge/glossary/stretch.md) and comparing your own runs. Days of flower starts when three or more pairs of pistils cluster at the top of the main shoot, and is the clock to compare with breeder times and [harvest windows](https://genetixbay.com/knowledge/glossary/harvest-window.md). Record both in your journal.

**How long does the stretch last?** The cannabis stretch mostly happens in the first two to three weeks after the flip. Research found stem and [internode](https://genetixbay.com/knowledge/glossary/internode.md) elongation fastest from about day 5 to day 10 of short days. One plant-patent description gives a 50–100 % height gain over that window, and grower reports range from about 1.5× to 3×, so flip at roughly half the final height you can fit.

**How will I know my plant is really flowering?** A cannabis plant is really flowering when pistil clusters form at the top nodes and [main cola](https://genetixbay.com/knowledge/glossary/cola.md), not lone pairs in leaf axils. Supporting signs are the stretch slowing with short new internodes, new growth switching from [fan leaves](https://genetixbay.com/knowledge/glossary/fan-leaf.md) to stacked [bracts](https://genetixbay.com/knowledge/glossary/bract.md), and smaller top leaves with stronger aroma. Photographing the same top cola every 2–3 days from the flip makes the onset date obvious.

**Does 13/11 delay flowering?** A 13/11 photoperiod delays cannabis flowering very little: in one study of two THC-dominant cultivars, 13 hours delayed onset by about 1.5 days in one and not at all in the other, while yield rose 1.35× and 1.5×. GenetixBay endorses 13/11. Sensitive cultivars may need a day or two more, and [THC](https://genetixbay.com/knowledge/glossary/thc.md) fell in some lines at 14 hours, so test your cultivar.

**Short answer.** Switching to 12/12 starts **floral induction**. It doesn't start flowering. The shoot tip spends the next week or two rebuilding itself, and the first real pistil clusters at the top of the plant usually show **about 8–14 days after the flip** in controlled studies [1][4], with slow cultivars taking **three weeks or more** [4]. Breeders' flowering times often count from that point, not from the flip. So a breeder's **"8 weeks" usually means harvest about 9.5–10 weeks after the flip.** Count both: days since flip and days of flower.

> **House position: the flip isn't day 1 of flower.** The founder's ruling: after the switch to 12/12 there's a transition, generally about 10–14 days, before flowering really begins, and most growers don't account for it. That's why most runs go "longer" than the breeder's stated time by about the same amount: breeder times often count from first visible pistils or "flower set", not from the flip. Count from the flip and add the transition. Jardin and GenetixBay journals track both "days since flip" and "days of flower". The evidence for and against is below.

> **The founder, on "how will I know when she's actually flowering?":** "Talk to her." No, seriously: she'll tell you more than you bargained for.

## What's happening after the flip

Cannabis is a [short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md): it measures the uninterrupted night (see [photoperiod and light leaks](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)). Once the night is long enough, the shoot apex changes what it builds.

- **Under long days** the main shoot keeps making long internodes, big fan leaves and side shoots. Each node carries a pair of single flowers once the plant is old enough [1][2].
- **Under short days** the apex branches intensively, the internodes shrink and bracts stack into a compact inflorescence: the cola [1][3].

In one Israeli study, the apex of a cultivar showed bract and flower primordia under the microscope by about **day 7–8** of short days, but visible pistils at the top of the main shoot didn't appear until **day 12**; another cultivar showed them by **day 8** [1]. Full flowering, with inflorescences on main, second- and third-order branches, took until about **day 22** [1]. The plant is busy from day one. It just isn't visibly flowering yet.

The switch itself is fast and reversible. A follow-up study found inflorescence onset needed **at least three consecutive short days**, short days had to continue to keep the inflorescence compact, and returning plants to long days raised gibberellin and auxin at the apex and **pulled the inflorescence apart** [3]. That's the biology of re-veg, and why a leaky dark period in the first two weeks costs you time.

## Preflowers are not the start of flower

The single flowers that appear at the nodes in veg ("preflowers") are not induced by the flip. Spitzer-Rimon and colleagues concluded that "induction of solitary flowers is age-dependent and controlled by internal signals, but not by photoperiod" [1]. In seedlings the juvenile-to-adult shift happened autonomously; in the cultivar studied, the first pair of single flowers formed at the **7th node** [2]. Alter et al. confirm that solitary-flower initiation in seedlings is photoperiod-independent [3].

| | Solitary flowers ("preflowers") | Induced inflorescence (real flowering) |
|---|---|---|
| Trigger | Plant age, internal signals [1][2] | Long uninterrupted nights [1][3][4] |
| Appears under 18/6 or 24/0? | Yes | No (photoperiod types) |
| Where | One pair per node, in leaf axils | Clustered at the top of the main shoot and branch tips |
| What it tells you | Sex, and that the plant is mature | That flowering has started: day 1 of flower |

The research definition of flowering onset is the one to use: **at least three pairs of pistils visible at the top of the main shoot** [1][7]. Zhang et al. used the equivalent: forked stigmas protruding from bracts at the apical meristem [4].

## How long the transition takes: the data

| Study | Plants and schedule | Time from short days to flowering onset |
|---|---|---|
| Spitzer-Rimon et al. 2019 [1] | Three drug-type cultivars, 12/12 | Visible stigmata day 8 (one cultivar) and day 12 (two cultivars); sampling every 5–7 days, so coarse |
| Zhang et al. 2021 [4] | 27 hemp cultivars, 12 h days | Averaged **13–14 days**; fastest essential-oil types ~13 days, slowest ~21 days; fibre types from the far south took 21–25 days |
| Peterswald et al. 2023 [6] | Three medicinal lines at 10, 12, 14 h | One line had pistils on 60–73 % of plants by the first check; the other two had few or none then, and all three were at 100 % about 9 days later. |
| Ahrens et al. 2024 [7] | Two THC-dominant cultivars, 12 h vs 13 h | 13 h delayed onset by about **1.5 days** in one cultivar and not at all in the other (per-treatment day counts are in the paper's figure) |
| Lisson et al. 2000 [8] | Two fibre hemp cultivars, 8–16 h | Rapid flowering below about 14 h; flowering delayed more the longer the day, using [thermal time](https://genetixbay.com/knowledge/glossary/degree-day.md) from emergence |

**Reading the table.** Most cultivars studied showed onset between about one and two weeks after the switch, which matches the house figure of 10–14 days. It isn't universal: some northern hemp flowered in 4–11 days, and slow cultivars took three weeks [4]. Most of the data are hemp or a handful of drug-type cultivars, so treat 10–14 days as the typical case and **measure your own** (*Supported*).

## The stretch belongs to the transition

The stretch isn't random; it's part of the switch. Under short days, the main stem and internodes elongated fastest from about **day 5 to day 10**, and short-day plants outgrew long-day plants over the first 14 days, before the condensed inflorescence took over [3]. A US plant-patent description puts the general figure at **50–100 % height gain in the first two to three weeks** after floral induction, and 71 ± 13 % for the cultivar described [10]. That's a breeder's description, not a controlled multi-cultivar trial; grower reports run from about 1.5× to 3× (*Grower practice*). See [stretching](https://genetixbay.com/knowledge/diagnosis/stretching.md).

When the stretch slows and the new internodes stay short, the inflorescence is forming. That's a better sign of day 1 of flower than the calendar.

## Cultivar variation and the critical photoperiod

Day length decides how fast induction happens, and the threshold varies by cultivar.

- Explants of one drug-type genotype flowered best at 12.0 and 13.2 h; the authors concluded it needed **at least 10.8 h of uninterrupted dark** [5].
- In hemp, essential-oil cultivars flowered 1–2 days later above 13 h and 7–8 days later above 14 h; some had a critical photoperiod between **13 h 45 min and 14 h**, and as little as 15 minutes changed floral initiation in some [4].
- At 14 h every line in Peterswald et al. still flowered and made more flower mass, but two high-THC lines fell from 12 % to 7 % and from 10 % to 6 % THC [6].
- At 13 h, one cultivar flowered about 1.5 days later and the other not at all, with 1.35× and 1.5× the inflorescence yield and cannabinoid concentrations equal or higher. The study ended before full commercial maturity [7].
- Major-effect flowering-time loci (*Autoflower1*, *Early1*) have been mapped, so timing is partly simple genetics [9]. Autoflowers skip this question entirely; see [photoperiod vs autoflower](https://genetixbay.com/knowledge/basics/photoperiod-vs-autoflower.md).

> **House position: 13/11 is endorsed, and some cultivars flower at 14–15 h** (in practice about 12 h at high intensity, plus sunrise and sunset ramps). The data above support it: 13 h barely moved flowering onset in the cultivars tested [7], and every line flowered at 14 h [6]. Expect a longer photoperiod to add a day or two to the transition in sensitive cultivars [4][7], and watch THC concentration at 14 h [6]. Test your cultivar.

## What she's telling you

Signs that flowering has really started, in order of how much they can be trusted:

1. **Pistil clusters at the top nodes and the main cola, not lone preflowers.** Three or more pairs of pistils at the top of the main shoot is the research definition of onset [1][7]. A single pair in a leaf axil doesn't count; those form in veg too [1][2].
2. **The stretch slows and internodes shorten.** Short days condense the apex: elongation peaks around days 5–10, then the internodes stay short as bracts stack [3].
3. **New growth switches from fan leaves to bracts.** The apex branches into bract and flower primordia instead of making big leaves and long shoots [1][3].
4. **Leaf size, aroma and water use change.** New leaves at the tops come in smaller with fewer leaflets, the plant starts to smell, and water use shifts as growth moves from stems to flowers (*Grower practice*; the smaller leaves and fewer leaflets near the inflorescence are consistent with the architecture in [1], the rest is grower observation).

**Photo-log tip.** Shoot the same top cola from the same angle, at the same time of day, every 2–3 days from the flip. Side by side, the day the internodes stop growing and the pistil clusters appear is obvious. Note that date in your journal as day 1 of flower.

## How to count: days since flip vs days of flower

Keep two clocks.

| Clock | Starts | What it's for |
|---|---|---|
| **Days since flip** | The first long night | Planning the room, light schedules, stretch, comparing with your own past runs |
| **Days of flower** | First pistil clusters at the top of the main shoot (3+ pairs) | Comparing with breeder flowering times, harvest windows, [trichome](https://genetixbay.com/knowledge/glossary/trichome.md) checks |

Many breeders' stated flowering times appear to count from flower set, not the flip (*Grower practice*, house position). If you don't know which a breeder means, assume flower set and add the transition.

### Worked example: a breeder's "8 weeks"

- Stated flowering time: 8 weeks = 8 × 7 = **56 days** of flower.
- Transition from flip to flower set: **10–14 days** (house figure; consistent with [1][4]).
- 56 + 10 = **66 days**; 56 + 14 = **70 days** after the flip.
- 66 ÷ 7 ≈ 9.4 weeks; 70 ÷ 7 = 10 weeks.
- **Plan for harvest about 9.5–10 weeks after the flip**, then decide on the day by the trichomes, not the calendar (see [trichome ripeness](https://genetixbay.com/knowledge/harvest-cure/trichome-ripeness.md)).

If you flower at 13/11 or longer, allow another day or two for a sensitive cultivar [4][7].

## Common mistakes

- Counting the flip as day 1 of flower, then harvesting two weeks early because "it's week 8".
- Calling preflowers in veg "flowering". They form under long days [1][2].
- [Light leaks](https://genetixbay.com/knowledge/glossary/light-leak.md) or a broken timer in the first two weeks. Induction can stall or reverse while it's still being set up [3]. Make the room dark.
- Running out of headroom: most of the stretch comes in this window [3][10]. Flip at about half the final height you can fit.
- Comparing your run with a breeder's number without knowing which clock they used.

## Evidence

- Short-day induction and cultivar-specific critical photoperiods: *Established* [4][5][6][8].
- Visible inflorescence typically appearing about 1–2 weeks after short days begin: *Supported* [1][4][6]; mostly hemp and a few drug-type cultivars; some cultivars much faster or slower [4].
- Solitary flowers ("preflowers") being age-dependent and photoperiod-independent: *Supported* [1][2][3].
- Induction needing continuous short days, and long days reversing the inflorescence: *Supported* by one study [3].
- Stretch concentrated in the first two to three weeks: *Supported* for timing [3]; size from one patent description [10] and *Grower practice*.
- 13 h adding little or no delay: *Supported* by one study of two cultivars [7]; 14 h lowering THC in high-THC lines: *Supported* by one study [6].
- Breeder flowering times counting from flower set, and runs going long by about the transition: house position (founder's experience) and *Grower practice*; not measured in any study we've found.

## Quick answers

- [How long does the flowering stage take?](https://genetixbay.com/knowledge/answers/how-long-does-flowering-take.md): Most photoperiod cannabis flowers for about 7–10 weeks, counted from the first pistil clusters, but harvest comes about 9–12 weeks after the flip, because the flip isn't day 1 of flower: a 10–14 day transition comes firs
- [In which weeks of flower do buds grow the most?](https://genetixbay.com/knowledge/answers/when-do-buds-grow-the-most.md): Cannabis buds put on most of their mass from roughly week 3 to week 7 of flower, counted from the first pistil clusters, and keep gaining weight and resin right through the final weeks.
- [When should I flip my plants to flower?](https://genetixbay.com/knowledge/answers/when-to-flip-to-flower.md): 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.

## Sources

1. 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
2. Spitzer-Rimon B, Shafran-Tomer H, Gottlieb GH, et al. 2022. Non-photoperiodic transition of female cannabis seedlings from juvenile to adult reproductive stage. Plant Reproduction 35(4):265-277. https://doi.org/10.1007/s00497-022-00449-0
3. 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
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. Moher M, Jones M, Zheng Y. 2021. Photoperiodic response of in vitro Cannabis sativa plants. HortScience 56(1):108-113. https://doi.org/10.21273/HORTSCI15452-20
6. Peterswald TJ, Mieog JC, Azman Halimi R, et al. 2023. Moving away from 12:12; the effect of different photoperiods on biomass yield and cannabinoids in medicinal cannabis. Plants 12(5):1061. https://doi.org/10.3390/plants12051061
7. 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
8. Lisson SN, Mendham NJ, Carberry PS. 2000. Development of a hemp (Cannabis sativa L.) simulation model 2. The flowering response of two hemp cultivars to photoperiod. Australian Journal of Experimental Agriculture 40(3):413-417. https://doi.org/10.1071/EA99059
9. Toth JA, Stack GM, Carlson CH, Smart LB. 2022. Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L. Frontiers in Plant Science 13:991680. https://doi.org/10.3389/fpls.2022.991680
10. 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