Photoperiod of feminized and regular cannabis plants

The photoperiod, or light cycle, is the main factor determining the transition from the growth phase to the flowering phase. When growing outdoors, nature will take care of this change, but when growing cannabis indoors, it is up to the grower. Over the last two decades there has been a great boom in autoflower varieties of cannabis that ignore the photoperiod, leading to the recent use of the term ‘photoperiodic’ for feminised and regular strains.

Photoperiod of cannabis plants grown outdoor in open space

Photoperiodic cannabis varieties grown outdoors are completely dependent on the sun. However, it is not only the intensity of the sunlight that matters, but also its length, which, when shortened, signals photoperiodic cannabis plants to start flowering. The phase change usually occurs towards the end of summer, usually during the month of August. The switch may be accelerated by bad weather for a week in early August.

However, beware of sexual maturity, if the cannabis plant is not sexually mature, which is indicated by presence of so-called preflowers, the phase transition may take a little longer. There are, however, varieties that have recommendations for SoG or Scrog cultivation. One of the prerequisites for these cannabis varieties, besides their uniformity and homogeneity, is that they do not have to be sexually mature for a smooth transition of the flowering phase.

Photoperiod of cannabis plants grown in greenhouse

When growing cannabis outdoors, sunshine is the most important factor that we cannot control. When growing in a greenhouse, the photoperiod can be influenced to a certain extent, as we can use artificial lighting to force the cannabis plants into a longer growth phase, or alternatively cover the greenhouse to accelerate the change to the flowering phase. However, these techniques are mainly used on an industrial scale and are mostly unfeasible for hobby cannabis growers due to discretion, cost, know how etc. In an ordinary greenhouse without these mechanisms, growing cannabis in relation to the photoperiod is identical to growing in the open outdoor.

Changing the light cycle in indoor cannabis cultivation

When growing marijuana indoors, we have the light cycle and the intensity of the world completely under our control. Based on the cultivation plan and information about the cannabis variety, the grower determines when it is appropriate to shorten the photoperiod. For the growing phase, the recommended light duration is 18 hours straight followed by 6 hours of darkness. If you want the cannabis plant to start flowering, it is necessary to shorten the light cycle to 12/12.

Keep in mind that when growing indoors, you must keep the cannabis plants in complete darkness for the duration of the night period. Even a tiny source of light can be a big problem. For example, light coming in through the passive air supply in growboxes (an opening in the bottom of the growbox, usually fitted with insect netting). Plants stressed by light will not flower at all, or will often hermaphroditise.

In the picture on the right you can see the growbox in the kitchen, you might say that this location is unsuitable because of the brightness of the surroundings. However, in this case the builder has chosen the location of the box very well so that no light can penetrate through the ventilation holes. In this case, the only problem could be direct sunlight from the window coming through the growbox.

Summary of photoperiod in cannabis cultivation

Most photoperiodic cannabis varieties usually need a sun cycle of less than 15 hours to start flower. Different places on Earth have different lengths of sunlight cycles, so it is best to find out from nearby cannabis growers what day and month their cannabis usually starts flowering and adjust fertilisation and training techniques accordingly. When growing photoperiodic cannabis varieties indoors, the light is controlled and it is up to the grower and his cultivation plan.