Light Cycles: Vegetative and Flowering

The growth of Cannabis sativa, or commonly known as cannabis, is a process requiring meticulous attention to several key factors including temperature, humidity, nutrient intake, and notably, light cycles. This article details the significance of light cycles during the two primary growth phases of cannabis: the vegetative stage and the flowering stage.

Overview of Light Cycles

In the context of plant cultivation, the term ‘light cycle’ refers to the schedule of light and darkness that a plant is exposed to, which directly influences its growth, health, and overall development. Cannabis plants, like many others, align their biological rhythms with the patterns of day and night. The variation in these light cycles primarily determines the transition from the vegetative phase to the flowering stage.

Photoperiodism, the response of an organism to the length of the day or night, plays a vital role in the life cycle of cannabis. The plant is classified as a short-day plant or long-night plant, meaning it initiates flowering when the length of darkness exceeds a certain threshold.

The Vegetative Stage and Light Cycle

The vegetative phase is the first stage in the life cycle of a cannabis plant, post the seedling phase. It is during this period that the plant develops its structure – the roots, stems, and leaves. A robust structure during the vegetative phase ensures an optimal yield during the flowering stage.

In the vegetative stage, cannabis plants require a substantial amount of light, approximately 18 to 24 hours per day. This extended light exposure promotes photosynthesis, leading to faster growth and development. The ’18 hours light, 6 hours darkness’ regimen is most commonly adopted as it allows the plant to rest, although some growers prefer a continuous 24-hour light cycle.

Using a timer to automate the light and darkness periods is highly recommended to ensure consistency. Any significant disruption in the light schedule during this phase may stress the plant and possibly trigger premature flowering.

It is equally important to consider the quality and type of light during this phase. Blue light spectrum, available from metal halide lights or specific LED grow lights, is particularly beneficial as it encourages the growth of foliage and keeps the plants compact, which is desirable in indoor growing conditions.

Transitioning from Vegetative to Flowering Stage

The transition from the vegetative stage to the flowering stage is a crucial moment in a cannabis plant’s life cycle. This change is triggered when the periods of darkness are lengthened. As in nature, this is a response to the approaching autumn when the days become shorter.

Indoor growers have the advantage of controlling the light cycle to manipulate the plant’s development. Typically, switching the light schedule to 12 hours of light and 12 hours of uninterrupted darkness per day initiates the flowering stage. It is worth noting that during this transitional phase, the plants may exhibit a growth spurt, often doubling or even tripling in size.

Any light exposure during the dark period can disrupt the plant’s transition and flowering process. Thus, growers must ensure their indoor space is light-proof during the dark hours.

The Flowering Stage and Light Cycle

In the flowering stage, the cannabis plants produce buds, which are the smokable parts rich in cannabinoids. The light cycle during this stage is significantly different from the vegetative phase.

The 12/12 light-dark cycle is maintained throughout the flowering phase. This balance ensures the plants receive sufficient light to produce energy through photosynthesis, while the uninterrupted dark periods allow them to flower.

The type of light also changes during the flowering stage. Cannabis plants benefit from the red and far-red spectrum of light at this phase, which can be supplied by high-pressure sodium lights or specific LED grow lights. The red light spectrum aids in promoting the production of flowers or buds.

It’s important to remember that the length of the flowering stage varies among different cannabis strains and can range from 8 to 12 weeks or more. Therefore, patience and careful observation are required to determine the optimal harvest time.


Light cycles play a pivotal role in the cultivation of cannabis plants, directly influencing their growth, development, and yield. By understanding and accurately manipulating these light cycles, growers can optimize the plant’s vegetative growth and flowering, ensuring a successful cultivation cycle. Just as crucial is the quality and spectrum of light used during each stage, which aligns with the plant’s specific growth needs at different phases of its life cycle.