There are plants that only can absorb carbon dioxide at night and thus require a dark period to complete photosynthesis. However, Cannabis is a C3 plant, which means that it takes in carbon dioxide during the day and does not require a dark cycle to complete the photosynthesis cycle. That said, there is reason to believe that certain strains may benefit from a dark cycle. Furthermore, as we explain below, the benefits of 24/0 lighting are minor and may not be worth the additional costs for electricity.
Just thinking about photosynthesis and the Calvin cycle has led many growers to the conclusion that 24/0 lighting is best for vegetative growth. 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.
While cannabis plants do not require any darkness during the vegetative stage, many growers know that most cannabis plants do require a uninterrupted dark period to produce flowers. The dark period for flowering has nothing to do with photosynthesis or the ability of the plant to use light energy and make carbohydrates. Rather, the dark period needs during the flowering period are a result of photoperiodism. Photoperiodism is an evolutionary characteristic which plants developed to be able to determine when the seasons change. There are many plants that respond to day-length or “photoperiod” and the most common response is the induction of flowering. In this way, plants are able to produce flowers at the optimal time of year and before a winter freeze. Most cannabis plants are photoperiod, which means that they will only produce flowers when a certain duration of dark is received. They actually require a period of uninterrupted darkness to trigger the photoperiod response. In most cannabis plants, the dark period must be 12 hours or half of the total 24-hour cycle. This limits the amount of energy that can be provided to plants during the flowering cycle and is one of the advantages of auto-flowering plants. Auto-flowering plants are day-length neutral, which means that they do not depend on light or dark intervals for the induction of flowering. They are bred by crossing cannabis with a day neutral plant that has a fast flowering cycle, ruderalis. This means that longer light cycles can be used during flowering and therefore you can provide more total energy to the plant during the flowering cycle. Rather than reducing the lights to 12/12, you can continue to provide 18/6 lighting to auto-flowering plants throughout their life-cycle. You can increase your yields, but of course you also increase your electricity use, so it will also increase the expense of the grow. As a result, it is not more efficient in terms of returns to electricity, but auto-flowering plants may allow you to harvest more within a certain time-frame, which may be more important to some growers. What is the Best Light Schedule for Maximum Yields? Typically, the goal of growing weed is to have a large yield when it comes time to harvest. Whether you’re growing indoor or outdoor herb, light is one of the essential aspects of your grow. The light your plants get throughout their entire life cycle will have a tremendous impact on how much they weed they produce. These are the vegetative stage (when plants are growing) and the flowering stage (when plants produce flowers or buds).
Being able to manipulate a plant’s light schedule makes it possible to achieve higher yields when it comes time to harvest. When plants are in the vegetative stage, the more light they receive the larger they will grow. Even when growing outdoors, many growers will start their plants inside to ensure they can grow as large as possible. When in veg, plants should be kept under grow lights for a minimum of 18 hours (commonly known as 18/6). Some growers will keep plants under 24 hours of light during this time (known as 24/0) to allow their plants to grow as big as possible. Marijuana plants don’t begin to flower until they begin to receive 12 hours of uninterrupted darkness.
Until then, they’ll continue to stay in the vegetative stage. You can essentially keep your plants in veg forever as long as they’re receiving 13 hours or more of light each day. Growing your plants indoors means you get to manipulate your environment as much as you want. When growing indoors, you can essentially keep your plants in the vegetative stage as long as you desire by keeping plants under light 18-24 hours each day. It’s important to keep in mind however that it’s not just your light schedule during veg that will ensure a bigger yield come harvest.