Photoperiod is the number of hours of light (and, crucially, uninterrupted dark) in a 24-hour cycle. Photoperiodism is a plant's flowering response to it. Most cannabis is photoperiodic: a short-day plant that stays vegetative under long days and switches to flowering once the dark period passes a cultivar-specific threshold. The plant measures the night through phytochrome, so the length of uninterrupted darkness is what counts.
In practice
"Photoperiod" or "photo" also names the plant type, as opposed to an autoflower. Indoors, growers hold photos in veg with 18/6 or 24/0, then flip to a long-night schedule to flower.
House positions: - 24/0 in veg is fine: cannabis doesn't need a dark sleep period in veg. - Flower doesn't have to be exactly 12/12: 13/11 works for many cultivars, and some flower at 14-15 h of light. In practice that is about 12 h of high intensity with sunrise and sunset ramps adding the rest. - Test your cultivar: critical day length varies, which is exactly what GenetixBay trials are for.
Light leaks during the dark period can delay flowering or contribute to stress in sensitive cultivars.
Don't confuse with
- Autoflower: flowers by age, largely ignoring day length.
- DLI: total light dose per day, separate from day-length signalling.
- Light schedule (18/6, 12/12): the setting; photoperiod is the biology.
