The Cannabis Light Cycle: How much light do my plants need?

Photoperiod Plants and the Cannabis Light Cycle

The Cannabis Light Cycle is the switch that controls what stage of the life cycle your cannabis plants are in. In general, the cannabis plant’s life cycle has three stages.

Seedling or Clone Stage
The Vegetive Stage
The Flowering Stage

As a cannabis grower you control how long your crop is in the vegetive. As long as you give your plants 18 hours of light per day, they will continue to grow in the vegetive stage. Because you determine when your plants start the flower stage, you control the overall size of the finished crop. As a cannabis grower you need to be aware that when you switch your plants to the flower stage they will double is size before it is time to harvest. Often the grow space will determine how long you can leave your plants in Veg.

So, the length of time a cannabis plant spends in the vegetive stage is variable and under your control. Plants are exposed to light in a daily cycle, defining the photoperiod. The grower can delay harvest and extend the flowering stage past the optimum flowering period. This will degrade the quality of the finished product. When you grow indoors, flowering will begin once you switch your lights to 12 hours of darkness. For most cannabis strains, the flowering period will last about 7-9 weeks, although some sativas require even longer for their buds to mature.

Cannabis’s Internal Circadian Rhythm

Photoperiod is defined as the time plants are exposed to light in a daily cycle. Plants can be classified based on photoperiodic response in day-neutral, short-day and long-day plants. Day-neutral plants flower at the same time irrespective of the photoperiod. In photoperiodism flowering and other developmental processes are regulated in response to the photoperiod, or day length. Short-day plants flower when day length is below a certain threshold, while long-day plants flower when day length is above a certain threshold. In many plants, photoperiodism is controlled by the overlap between the day length cue and the plant’s internal circadian rhythms.

A plant’s light cycle or photoperiod refers to the duration of light exposure it receives within a 24-hour period. In simpler terms, it’s the length of time a plant is exposed to light and darkness in a day. Photoperiod is a critical environmental factor that influences various physiological processes in cannabis plants, particularly in terms of their growth, development, and flowering.

Cannabis plants have evolved to sense changes in photoperiod as a means of adapting to different seasons and environments. Some plants are classified as “long-day plants,” which means they require a relatively longer period of light exposure to initiate flowering. Conversely, “short-day plants” initiate flowering when exposed to shorter periods of light. There are also “day-neutral plants” that are less sensitive to photoperiod and more influenced by other factors when it comes to flowering.

Photoreceptors

The mechanism through which plants sense photoperiod involves light-sensitive pigments known as photoreceptors. One of the most well-known photoreceptors involved in photoperiodic responses is phytochrome, which exists in two interconvertible forms: Pr (red light-absorbing) and Pfr (far-red light-absorbing). The ratio of these two forms can indicate the length of the dark period, which in turn informs the plant about the prevailing photoperiod.

Understanding your cannabis plant’s photoperiod requirements is crucial to successful indoor gardening. Growers must manipulate light exposure to control the flowering of plants, optimize growth, and even extend the vegetive growth cycle.

The choice of the best light cycle for vegetative growth is a subject of frequent discussion and debate. In this review, we will explore the fundamentals of light cycles and explain our preference for the 18/6 cycle during vegetive growth.

Cannabis Light Cycle Basics:

In the cannabis community, light cycles are expressed as a fraction with “Lights on” time over “Lights off” time.

24/0: Lights are on 24 hours and off 0 hours
18/6: Lights are on 18 hours and off 6 hours
12/12: Lights are on 12 hours and off 12 hours. 12/12 timing is used during the flowering period with Photoperiod plants.

The duration of the light cycle plays a crucial role in plant growth as it directly impacts their ability to photosynthesize. Adequate light is essential for optimal plant development.

The growing community often experiences confusion regarding the ideal light duration for vegetative growth in both photoperiod and auto-flowering plants, although photoperiod plants usually require 12 hours of light during the flowering stage.

Understanding Basic Plant Processes

Plants utilize light energy to fuel their growth through the process of photosynthesis. The process of photosynthesis comprises two main cycles: the light-dependent cycle, which utilizes light and water to produce energy in the form of ATP and NADPH, and the light-independent cycle, also known as the Calvin cycle, responsible for synthesizing carbohydrates. Despite being termed “light-independent,” the Calvin cycle depends on the energy generated in the light-dependent cycle to operate.

During the light-independent cycle, plants extract carbon dioxide from the air and convert it into carbohydrates, which are the end products of photosynthesis. This synthesis process occurs only when the plant is exposed to light, despite its label as “light-independent,” as it necessitates the energy produced in the light-dependent phase.

Cannabis plants have the ability to thrive under continuous 24/0 lighting during the vegetative stage. Unlike certain plants that solely absorb carbon dioxide at night and thus necessitate a dark period for complete photosynthesis, Cannabis is classified as a C3 plant. C3 plants can assimilate carbon dioxide throughout the day and do not require a dark cycle for the photosynthesis process to occur. However, some strains may potentially benefit from a dark cycle, although the advantages of employing a 24/0 lighting regimen are minimal and may not justify the additional electricity costs.

18/6 For Vegetative Growth

Just thinking about photosynthesis and the Calvin cycle has led many growers to the conclusion that 24/0 lighting is best for vegetative growth. However, there are additional factors to consider. Like other living organisms, plants are governed by a circadian rhythm and a hormonal clock system that functions in sync with a light on, light off cycle. Research has shown that there are important connections between a plant’s internal clock and genetic expressions, which can play an important role in the overall health and performance of a plant. A massive review on continuous lighting studies has shown that some plants perform negatively while others perform positively when grown under 24/0 lighting.

The potential benefits of 24/0 lighting are somewhat faster vegetative growth. However, it comes at the expense of additional electricity. Because of diminishing returns, the growth realized during the “additional” light time may be less efficient. You are using 25% more electricity, but you may not realize 25% faster growth. Because of potential negative responses and less efficient returns from electricity, we recommend vegetating all cannabis plants under 18/6 lighting. The vegetation stage generally last between two to four weeks and longer. It really depends on the finished size you are targeting. You can expect that your plants will double in height during the flowering stage. We generally veg for four weeks.

12/12 For Flowering

Although cannabis plants can thrive without any darkness during the vegetative stage, growers widely acknowledge that an uninterrupted dark period is required for most cannabis plants to initiate flower production.The need for a dark period during flowering is not related to the plant’s ability to undergo photosynthesis or utilize light energy for carbohydrate synthesis. Instead, it is a result of photoperiodism.

Photoperiodism is an evolutionary trait that allows plants to sense changes in day length or “photoperiod” and respond accordingly. One of the most common responses to photoperiodism is the induction of flowering. This mechanism enables plants to produce flowers during the optimal time of the year and before the onset of winter conditions.

The majority of cannabis plants exhibit photoperiodic behavior, meaning they will only start flowering when they receive a specific duration of uninterrupted darkness. The dark period plays a crucial role in triggering the photoperiodic response. For most cannabis plants, this dark period must be 12 hours, which constitutes half of the complete 24-hour cycle. This requirement limits the amount of energy that can be provided to the plants during the flowering phase and is one of the differentiating factors between photoperiodic and auto-flowering cannabis varieties.

Light Cycles for Auto-Flowering Plants

Auto-flowering plants exhibit day-length neutrality, meaning they are not reliant on specific light or dark intervals to initiate the flowering process. These plants are created through the breeding of cannabis with a day-neutral species called ruderalis, which naturally has a rapid flowering cycle. The advantage of auto-flowering plants is that they can tolerate longer light cycles during the flowering stage, allowing for increased total energy absorption throughout the flowering cycle.

Unlike photoperiodic plants that typically require a switch to a 12/12 light-dark cycle for flowering, auto-flowering plants can continue to receive 18 hours of light and 6 hours of darkness throughout their entire life cycle. This extended light period can potentially result in higher yields. However, it’s important to consider that maintaining longer light cycles will increase electricity consumption and the overall cost of the cultivation. While it may not be more efficient in terms of returns to electricity, auto-flowering plants offer the advantage of potentially achieving multiple harvests within a specific timeframe, which can be beneficial for certain growers.

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Disclaimer:

The information presented on 420GrowHelp.com should not be regarded as financial, legal, or medical advice. It is your responsibility to familiarize yourself with and abide by the local laws and regulations pertaining to the cultivation, possession, and use of cannabis. Decisions regarding cannabis cultivation should be made in consultation with a lawyer or qualified legal advisor, while decisions regarding cannabis use should be made in consultation with a doctor or medical professional.

The Cannabis Light Cycle: How much light do my plants need?