Cannabis is an annual plant, it reaches the end of its lifecycle in the fall, sheds seeds, then dies. During its life, it goes through two distinct phases of growth: vegetative and flowering. The first phase is vegetative when the plant grows branches and leaves. The second stage is flowering when the plant grows buds.
Flowering and vegetative growth phases are regulated by a hormone called florigen. Florigen accumulates in cannabis plants only when they aren’t exposed to long periods of light everyday. When daily light exposure is low, florigen accumulates . After surpassing a threshold florigan causes plants to flower.
The amount of light a cannabis plant receives each day determines its phase of growth. More than 12 hours of light exposure each day and it will remain in a state of vegetative growth. Less than 12 hours light exposure each day and it switches to its flowering growth phase.
Outdoors, growth phases are determined by the number of hours the sun is up each day. Vegetative growth happens in the spring when the days are longer. As the days grow shorter in the fall, daily light exposure drops, florin accumulates, and cannabis plants shift to their flowering phase.
Vegetative growth is typically maintained indoors by attaching lights to a timer device and exposing plants to 16 – 24 hours of artificial light per day.
Growers can control when their plants flower indoors. By maintaining daily light exposure above 12 hours plants will stay in vegetative growth, theoretically forever. Or they can be forced to switch to flowering growth, even only after a short period of vegetative growth, by setting their daily light exposure to 12 hours a day or less.
Indoors, setting the light schedule to 12 hours of darkness per day causes cannabis plants to enter their flowering growth phase. To sustain flowering, plants must be keep in total, uninterrupted darkness. Light leaks can cause a reversal back to vegetative growth.
Darkness is typically achieved inside a tent that can be zipped up, shielding plants from light.