When a plant is photoperiod-dependent, it means it reacts to the length of the dark period. Depending on the amount of light hours it receives, the plant will know when to enter a different phase in its life cycle. When growing outdoors, once the nights get longer and the days get shorter, a cannabis plant will enter the flowering phase. Indoors, the grower is in full control of the light schedule and can decide when to flip the plant into the flowering phase.