Cannabis is a “short day/long night” plant, meaning it needs those conditions to bloom. The system cannabis uses to detect light changes is a group of receptors in the pigments of their leaves. There are 2 different receptors, each with a different task: Phytochrome Red and Phytochrome Far Red. They absorb light in different wavelengths and transmit that information to the plant.
Far-Red can be manipulated through the amount of light given to the plant. This receptor keeps the plant in the vegetative stage. Light in the far-red spectrum will signal this receptor to pass the chemical signal to veg. During times of light, both receptors are balanced in number, but in darkness, Far-Red receptors slowly change into Red receptors.
With longer dark periods, the number of Far-Red receptors reduces until there aren’t enough to counter the signal from the Red receptors, which tell the plant to flower.
Want to speed up your switch from veg to flower? Give plants an uninterrupted 24-36 hours of darkness before going to a 12-12 cycle. It will make more of the Far-Reds change into Reds, giving a more powerful signal to your plant that it is time to bloom. Normally, the transition can take a couple of weeks to be seen on your plants. By giving them a really long night, you can speed this up dramatically.