Short answer. Plan a photoperiod grow in five blocks: about 2 weeks of seedling, 3–6 weeks of veg at 18/6 (24/0 also works), the flip to 12/12 (13/11 also works), a 10–14 day transition before flowering really starts, and then the breeder's flowering time counted from that point. A breeder's "8 weeks" usually means harvest about 9.5–10 weeks after the flip. Log both counters: 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 is a transition of about 10–14 days before flowering really begins, and most growers don't account for it. Breeder flowering times often count from the first visible pistils or "flower set", not from the flip, which is why runs go "longer" by about the same amount. Count from the flip and add the transition. GenetixBay journals and Jardin track both "days since flip" and "days of flower". The evidence is in the flip-to-flower transition.
What do you need to run a photoperiod calendar?
A photoperiod calendar needs a reliable timer, a truly dark tent and a way to log dates. The rest of the kit is in the master guide.
- [ ] A timer you trust: a digital timer or controller with a battery backup, or a mechanical timer you check weekly
- [ ] A light-tight tent with no light leaks (check from inside, lights off, after 5 minutes for your eyes to adjust)
- [ ] A dim green or low-lumen headlamp for any unavoidable dark-period checks, used for seconds, not minutes
- [ ] A wall or phone calendar, plus a grow journal with two columns: days since flip and days of flower
- [ ] Plant ties or a trellis net, set up before the flip (training basics)
- [ ] A 60x loupe for trichome checks from about week 7 of flower (when to harvest)
- [ ] The breeder's stated flowering time, written down with a note of how they count it if they say
What does a photoperiod grow look like on a calendar?
A photoperiod grow takes about 16–20 weeks from seed to harvest, plus drying and curing. The worked example below germinates on Monday 11 January 2027, vegs 4 weeks, flips on Monday 22 February and uses a cultivar the breeder lists as "8 weeks". Swap in your own dates.
| Date (example) | Days since flip | Days of flower | Stage | Light | What to do |
|---|---|---|---|---|---|
| 4–10 Jan | – | – | Setup | Test run | Run the empty room 48 h; log temp and RH |
| 11 Jan | – | – | Germinate | 18/6 | Seeds in, 22–26 °C (72–79 °F) |
| 11–25 Jan | – | – | Seedling (2 weeks) | 18/6, 150–300 µmol/m²/s | Small waterings, gentle light |
| 25 Jan–22 Feb | – | – | Veg (4 weeks) | 18/6 or 24/0, 300–600 | Pot up, start LST, top if you want |
| 22 Feb | 0 | – | Flip | 12/12 or 13/11 | First long night tonight. Log it |
| 23 Feb–8 Mar | 1–14 | – | Transition and stretch | 12/12, ramp to 600–900 | Finish training, check sex, keep dark dark |
| ≈ 6 Mar (range 4–8 Mar) | ≈ 12 (10–14) | 1 | Flower set | 12/12 | ≥ 3 pistil pairs at the main top. Start the second counter |
| ≈ 15 Mar | 21 | ≈ 9 | Stretch ends | 12/12 | Last tucking; RH to 45–55 % |
| 15 Mar–26 Apr | 21–63 | ≈ 9–51 | Bulking flower | 12/12, 600–900 | Feed through; daily 5-minute check |
| 26 Apr | 63 | ≈ 51 | Start trichome checks | 12/12 | Loupe every 2–3 days; RH 40–50 % |
| ≈ 1 May (window 26 Apr–10 May) | ≈ 68 (63–77) | ≈ 56 | Harvest window | 12/12 | Cut at a few clear, mostly cloudy, some amber |
| ≈ 1–13 May | – | – | Dry 7–14 days | Dark | 15–21 °C (60–70 °F), 55–62 % RH |
| ≈ 13 May–10 Jun+ | – | – | Cure 2–8 weeks | Dark | Slow moisture decline in jars |
The "8 weeks" here became 68 days, about 9.7 weeks, after the flip: 12 days of transition plus 56 days of flower. Slow cultivars, especially some long-flowering tropical lines, can take three weeks or more to show flower set [2].
How do you plan a photoperiod grow, step by step?
Plan a photoperiod grow by fixing the flip date first and counting outwards from it.
- Pick your germination date. Count back from when you want to harvest: seed to harvest is roughly 2 (seedling) + 3–6 (veg) + 1.5–2 (transition) + breeder weeks.
- Run seedlings for about 2 weeks at 18/6 and 150–300 µmol/m²/s, until 3–4 sets of true leaves. See germinating seeds.
- Veg for 3–6 weeks at 18/6, or 24/0 (house-endorsed: cannabis doesn't need a sleep period in veg), at 300–600 µmol/m²/s. Pot up and start training. See training basics.
- Choose the flip by height, not by date. Flip when plants are about half the final height you can fit between pot rim and the light's safe distance, because plants commonly stretch 50–150 % in the 2–3 weeks after the flip (stretching). In a SCROG, flip at 70–80 % net fill (SCROG).
- Flip to 12/12 or 13/11 and log day 0. The first long, unbroken night is day 0 of "days since flip". Some cultivars flower at 14–15 h of light, usually as about 12 h at high intensity plus sunrise and sunset ramps; test your cultivar.
- Keep the dark period completely dark. Light leaks can delay or weaken flowering in sensitive cultivars and reveal herm-prone genetics [6]. See photoperiod and light leaks.
- Watch for flower set, days 8–14. When you can see at least three pairs of pistils at the top of the main shoot [1], log day 1 of "days of flower".
- Check sex during the transition. Any male flowers or herm sacs show in this window. See telling sex and spotting herms.
- Calculate the harvest window. Harvest estimate = flip date + transition (your logged days, or 10–14) + breeder flowering time. Start loupe checks a week before the earliest date.
- Harvest by trichomes, not the calendar. The calendar tells you when to start looking. The plant decides. See when to harvest.
- Record the real numbers. Write down your actual transition days and days of flower. Next run with the same cultivar, plan with your numbers, not the breeder's.
How do you calculate a harvest date from the breeder's flowering time?
Calculate a photoperiod harvest date as flip date + 10–14 days of transition + breeder flowering time, then confirm with trichomes. This works because breeder times usually count from flower set, and controlled studies put flower set about 8–14 days after short days begin, averaging 13–14 days across 27 hemp cultivars [1][2].
| Breeder says | Transition | Days after flip | Weeks after flip |
|---|---|---|---|
| 7 weeks (49 d) | 10–14 d | 59–63 | 8.4–9.0 |
| 8 weeks (56 d) | 10–14 d | 66–70 | 9.4–10.0 |
| 9 weeks (63 d) | 10–14 d | 73–77 | 10.4–11.0 |
| 10 weeks (70 d) | 10–14 d | 80–84 | 11.4–12.0 |
| 12 weeks (84 d) | 14–21 d | 98–105 | 14.0–15.0 |
If a breeder clearly states they count from the flip, skip the transition and use their figure as is.
Does 13/11 or a longer day change the calendar?
A 13/11 schedule changes the calendar very little for most cultivars and can add light. In a controlled study of two THC-dominant cultivars, 13 h days delayed flowering onset by about 1.5 days in one and not at all in the other, and the longer day raised yield [4]. Another study found that some lines yield more under 14 h, with a possible cost in THC concentration [5]. The house position: flower doesn't have to be exactly 12/12. Test your cultivar, one change at a time, and log it.
How will you know it's working?
You'll know the calendar is working when the plant hits each milestone close to plan and your two counters line up with what you see:
- Days 1–7 after the flip: stems lengthen faster; internodes stretch; new growth looks lighter green.
- Days 8–14: white pistil clusters form at the top of the main shoot and branch tips. This is day 1 of flower.
- Days 14–21 (stretch ending): height gain slows; the canopy is set. The PP30,434 plant description is a good picture of a typical post-flip stretch [7].
- Flower weeks 2–6: buds swell week on week; leaves stay green.
- Final 2–3 weeks: pistils darken and recede, lower leaves fade slowly, trichomes go cloudy.
- Logs: dark period always 11–12 h (or your chosen night) with no breaks, and the timer never skipped.
Inflorescence onset needs at least three consecutive short days, and interrupting short days pulls the developing flower apart [3]. If flower set is late past day 21, check the dark period first.
What mistakes do beginners make with the photoperiod calendar?
Beginners mostly lose time by counting the wrong day 1 or by breaking the night.
- Counting the flip as day 1 of flower and harvesting 10–14 days early, with mostly clear trichomes.
- Flipping too late in a small tent, then fighting the light with plants that doubled in height.
- Light leaks from zips, a power LED on a fan controller, or opening the tent "just for a second".
- Timer failures: a power cut on a cheap digital timer that resets, or a mechanical timer with a pin out.
- Switching back and forth between 18/6 and 12/12. Returning to long days pulls the inflorescence apart and can trigger re-veg [3].
- Trusting pistil colour as the harvest signal instead of trichomes.
- Not writing down the real transition for your cultivar, so every run is a guess.
Evidence
- Flower set about 8–14 days after short days, averaging 13–14 days: Supported by controlled studies [1][2].
- At least three consecutive short days needed; returning to long days disrupts the inflorescence: Supported [3].
- 13 h days with small delay and higher yield: Supported by one cannabis trial [4]; cultivar-dependent responses to 14 h: Supported [5].
- Night light delaying flowering: Supported (conference data) [6].
- Breeder counting from flower set, and counting a 10–14 day transition: Grower practice, house position.
- 24/0 in veg and 13/11 in flower: house-endorsed, with the supporting data above.
