Short answer
Outdoor photoperiod plants start flowering as days shorten past their threshold, often around 14–14.5 h of daylight. At 35–50° latitude that is usually late July to mid-August in the Northern Hemisphere, late January to mid-February in the Southern, with pistil clusters showing 10–14 days later. Supported; exact dates vary by cultivar.
Why
Cannabis measures night length. Once the night passes a cultivar-specific threshold, shoot tips switch from leaves to flower clusters [3].
Thresholds differ. Some hemp cultivars had a critical day length of 13 h 45 min to 14 h, and very dim twilight counted as light [1]. Many drug-type cultivars flower at 14 h days, and some at 14–15 h [4].
Latitude shifts the date. Days are longest at high latitude, so they fall through 14 h later: about 9 August at 40°, 23 August at 50° (sunrise to sunset, calculated).
Flowering is not instant. A transition of roughly 10–14 days follows before pistil clusters show at the tops.
Night light interferes. Stray light of about 2 µmol/m²/s delayed flowering in hemp [1], and as little as 0.01 µmol/m²/s in sensitive cannabis cultivars [2].
The numbers
| Latitude | Day falls below 14.5 h | Day falls below 14 h | Typical flower onset (N / S) |
|---|---|---|---|
| 30° | Never above it | ~7 July | Early–mid July / early–mid January |
| 40° | ~25 July | ~9 August | Late July–mid August / late January–mid February |
| 45° | ~6 August | ~17 August | Early–late August / early–late February |
| 50° | ~15 August | ~23 August | Mid–late August / mid–late February |
| 55° | ~22 August | ~28 August | Late August / late February |
Southern dates: add six months. Sunrise-to-sunset values; twilight adds a little.
What to do
- Note when your days drop below 14.5 h and 14 h; expect flowers from then.
- Count flowering weeks from first pistils, plus the 10–14 day transition.
- Keep plants away from streetlights and security lights.
- In short seasons, choose early cultivars or light deprivation.
Common mistakes
- Expecting outdoor plants to flower "on 12/12" in September.
- Planting under a security light.
- Assuming a breeder's 8 weeks starts the day you see the first pistil.
Related
- Full article: Photoperiod and light leaks: 18/6, 12/12 and the dark period
- Answers: When to plant outdoors · When to harvest outdoors · Why flowering needs long nights
- Article: The flip isn't day 1 of flower
- Terms: light deprivation · outdoor growing
