Short answer
Photoperiod cannabis is a short-day plant, which really means a long-night plant: it flowers once the uninterrupted dark passes a cultivar-specific length, around 10–11 hours in the drug-type cultivars tested. 12/12 triggers essentially all cultivars; 13/11 works for many and is house-endorsed. Established plant physiology.
Why
The plant times the night, not the day. Phytochrome in the leaf flips between active and inactive forms under red and far-red light. Through a long night the active form decays, and past the critical night length, flowering signals switch on at the shoot tips.
The threshold varies by cultivar. One drug-type genotype needed at least about 10.8 h of uninterrupted dark [2]. In hemp, some essential-oil cultivars had a critical day length between 13 h 45 min and 14 h [1]. At 13 h, two THC-dominant cultivars flowered within about 1.5 days of their 12 h timing [5].
The nights must keep coming. Inflorescence onset needed at least three consecutive short days, and returning plants to long days pulled the forming inflorescence apart [3].
Night light resets the count. Stray light as dim as about 2 µmol/m²/s delayed flowering in hemp [1], and a conference report saw delays in the most sensitive cannabis cultivars at 0.01 µmol/m²/s [4].
The numbers
| Schedule | Night | Effect on photoperiod cannabis |
|---|---|---|
| 24/0, 20/4, 18/6 | 0–6 h | Stays vegetative |
| 14/10 | 10 h | Flowered in all three lines in one study; test yours |
| 13/11 | 11 h | Flowers; endorsed |
| 12/12 | 12 h | Flowers in essentially all cultivars |
| Stray night light | — | Delays seen from 0.01–2 µmol/m²/s |
Sunrise and sunset ramps count as day; the plant measures the dark after them.
What to do
- Give flowering plants an uninterrupted night of at least 11–12 hours.
- Sit in the dark tent for 5 minutes; any light you can see is a leak.
- Tape over device LEDs and seal zip ends and duct ports.
- Use a timer you trust and check it after power cuts.
- Trying 13/11 or 14/10? Test one cultivar first.
Common mistakes
- Thinking plants need "rest" in veg.
- Opening the tent during the dark period.
- Assuming a minute of light can't matter.
Related
- Full article: Photoperiod and light leaks
- Answers: 12/12 from seed · lights stayed on in flower · when outdoor plants flower
- Article: The flip isn't day 1 of flower
- Terms: phytochrome · short-day plant
