Marijuana plants use day and night light cycles to facilitate growth. Light is an important factor in marijuana growing, whether indoor or outdoors. The amount of light received by your plants will determine when they start flowering or forming buds. Basically, the light cycle is divided into two: The vegetative phase and the flowering phase. This is for marijuana plants that have controllable light systems (indoor plants). This article will cover the light cycles during both phases and give important tips involved with lighting so as to achieve the desired end result during harvest day.

Vegetative stage light cycle

In indoor growing, the light cycle at the vegetative phase focuses on creating a good root system, a strong stem and an attractive floral appearance of the marijuana plants. The plant develops many leaves that aim at converting light into sugars (food) with the help of carbon dioxide.

Vegetative stage light cycle should be between 18 and 24 hours of light daily. This is because the process of energy making by the leaves require much light. There exist two types of chlorophyll receptors in marijuana plants, A and B. We normally have the red and blue spectrums of light. For indoor growers, the lighting system should be able to produce the entire spectrum so that the plants are not left struggling for light. It would make them weak. Therefore, it is vital to invest in a good lighting system.

To keep the light cycle uniform, it is always advisable to have a timer. The vegetative phase requires at least 14 hours of light. More hours won’t do any harm. However, for the sake of saving energy and giving your plants some rest, you could adapt the 18/6 system. Here, the plants are exposed to 18 hours of light and 6 hours of darkness.

The Flowering phase light cycle

For most indoor growers, induced flowering vital to ensure that the plants mature faster. This is both for the large scale and small scale producers. The 12-12 hour lighting system is introduced. Some growers give this process a boost by first putting their plants into a period of 36 hours in total darkness. It tricks the marijuana into believing that the flowering season has commenced. The 12-12 system then follows. Put your crops under 12 hours of darkness and 12 hours of light.

During the flowering phase, light supply should be strong and efficient enough. It is also the stage at which to adjust your nutrients and separate males from females.

Sometimes you may find that the light is too much. This is signified by the temperature of the room. Poor positioning of the lights is the sole contributor of this. You could use a thermometer to measure the temperature of the room. You could also place your hand over the canopy. If the heat is too much, adjust the lights accordingly.

Light Challenges

Sometimes we may encounter challenges with the lights. Black outs are occurrences which cannot be overlooked. Marijuana plants are very sensitive to changes and long black outs could land you in trouble. During the 12-12 hour system, black outs won’t have much effect unless it goes on for days. However, during the 18/6-hour light cycle, black outs could retard your plants. Backups such as generators would be ideal especially for large scale producers. For small scale, you could simply use a lantern that distributes light evenly. It will save the situation.


The light cycle in marijuana growing is not complex and any indoor grower would comfortably implement it. It ensures that the plants flower early, are healthy and give a good harvest that has lots of THC. Just be sure to exercise the 18-6 hour during the vegetative stage and 12-12 hour to induce flowering. The latter should be maintained till the plants mature.