Photoperiod
Definition: A photoperiod cannabis plant starts flowering in response to day length, specifically when it receives an uninterrupted dark period of about 12 hours. Indoors, growers keep photoperiods vegetative under 18 hours of light, then switch to 12 hours on and 12 off to trigger buds. Outdoors, flowering begins naturally in late summer.
How Photoperiod works
Cannabis is a short-day plant. A pigment called phytochrome tracks how long the nights last, and once darkness stretches long enough the plant shifts its energy from leaves and stems to flowers. The dark period is what matters most. A single light leak, or a late check with a bright flashlight during flowering, can confuse the signal and, in sensitive plants, contribute to hermaphrodite flowers.
Indoor control
Because you choose when to flip the lights, you choose the plant’s size. A photoperiod can stay in the vegetative stage for 3 weeks or 3 months. Longer veg means bigger plants and more yield per plant, at the cost of time and electricity. Photoperiods also clone well and can be kept as mother plants indefinitely, something autoflowers cannot do.
Outdoor timing in the US
After the June solstice, days shorten. Most photoperiod strains begin flowering between late July and mid-August across the lower 48 states, and finish between mid-September and late October depending on genetics. Southern growers can sometimes run two crops, while northern growers should pick strains with 8 to 9 week flowering times to beat fall rain and frost.