Photoperiod
Category: Cultivation · Level: Intermediate
Photoperiod refers to the duration of light and dark periods in a 24-hour cycle that triggers cannabis plants to transition from vegetative growth to flowering.
Photoperiod is the daily balance of light and darkness a plant receives, and photoperiod cannabis is cannabis whose flowering is triggered by it. Cannabis is a short day plant: it stays in vegetative growth while nights are short and starts to flower once it gets an uninterrupted dark period of roughly 12 hours or more. Outdoors that happens in late summer as days shorten; indoors the grower decides.
How growers use it: a typical indoor schedule is 18 hours of light and 6 of darkness (18/6) for vegetative growth, then a switch to 12/12 to start flowering. Because the plant only flowers when told, the grower controls size: a short vegetative phase gives small plants, a long one gives large plants and bigger yields. Flowering then usually takes 8 to 10 weeks for most hybrids and longer for many sativas. Mother plants for cuttings can be kept vegetative indefinitely under long days.
What can go wrong and how it compares: light leaks during the dark period, even a lamp switched on briefly, can stress plants, delay flowering or cause hermaphrodite flowers that make seeds. Autoflowering cannabis, bred from ruderalis genetics, ignores the light cycle and flowers by age, finishing faster but with less control over size. Photoperiod plants are grown from regular seeds (male or female) or feminized seeds; regular in that sense refers to sex, not to light. Outdoor growers in the northern hemisphere usually see photoperiod plants start flowering from August.
Also known as
Light cycle, Photoperiod strain, Photoperiodic cannabis
Used in
- Cultivation
Related terms
- Autoflowering: Autoflowering cannabis plants begin to flower based on age rather than changes in light cycle, derived from Cannabis ruderalis genetics.