Light Schedule
Definition: A light schedule is the daily ratio of light to darkness a cannabis plant receives, written as hours on and off, such as 18/6 or 12/12. Photoperiod plants stay vegetative under long days and are switched to flowering indoors with 12 hours of uninterrupted darkness, while autoflowers bloom based on age instead.
How Light Schedule works
For photoperiod cannabis, the light schedule is the main control switch. The plant measures the length of each night, and once nights get long enough, it shifts from growing leaves to building flowers.
Common schedules
| Schedule | Used for |
|---|---|
| 18/6 | Vegetative growth for photoperiods; a full-cycle option for autoflowers |
| 20/4 | Popular for autoflowers to raise total daily light |
| 12/12 | Triggering and maintaining flower in photoperiods |
Darkness has to be real darkness
During 12/12, the dark period needs to be uninterrupted. A light leak or repeated door openings during lights-off can confuse the plant, slow flowering, or contribute to stress traits.
Autoflowers are different
Autoflowers inherit ruderalis genetics that start flowering after a set number of weeks, usually 3 to 5 from sprout, regardless of day length. Most growers keep them on 18/6 or 20/4 from seed to harvest.
Outdoors
Outside, the sun sets the schedule. Days get shorter after the summer solstice around June 21, and most photoperiod strains begin flowering between late July and late August depending on latitude and genetics. Northern growers often choose fast-finishing strains so harvest lands before cold, wet fall weather.
Indoors, use a reliable digital or mechanical timer and run the light period during the coolest part of the day if heat is a concern.