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How Light Cycles Affect Cannabis Growth Stages

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October 01, 2020
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Cannabis plants need light to photosynthesize, and the light cycle is an integral part of this process. During the flowering phase, you can only go 12/12 (12 hours on, 12 hours off). As for the vegetative stage, most people opt for 18/6 while some others prefer 24/0. Scheduling light exposure is not only a matter of necessity, depending on the growth stage. It is also a means for you to control when to transition in an indoor setting.

Light Cycles and Plant Processes

Photosynthesis is a two-part process and can be confusing to understand for beginners. You need to tailor the amount of light to the particular process the plant is trying to perform.

The first stage – called the ‘photo’ or light-dependent cycle – is when plants use light and water to create energy (in the form of ATP and NADPH).

The second stage – also called the Calvin cycle – alludes to ‘synthesis’ or light-independent cycle. In this ‘behind-the-scenes’ process, the plants use the energy produced during the first stage and carbon dioxide to create carbohydrates.

The synthesis of carbohydrates relies on the energy created during the light-dependent cycle. And since the first stage depends on light, it then follows that the longer the exposure to light, the more energy produced. Cannabis plants, in particular, can grow under 24 hours of continuous lighting.

Not all kinds of plants can grow well under 24/0 lighting. Cannabis is among the exceptions as it is a C3 plant. They only take in and utilize carbon dioxide during daytime (when outdoors) or lights on (if indoors). Because of the non-reliance on dark cycles, one of the common perceptions is that a 24-hour continuous exposure is best for maximum yields. On the contrary, actual results show that the gain over an 18/6 schedule is minimal and does not justify the additional cost of electricity.

Vegetative Stage Light Cycle

More lights equate to higher yields. With that in mind, the logical thinking is that a 24/0 light cycle is best for this growth period. In practice, however, it does not work that way. That is because you also have to consider the entire ecosystem, especially the beneficial microbes.

A circadian rhythm governs the living organisms that are essential to plant growth. This natural, internal, hormonal process regulates the sleep-wake cycle and repeats every 24 hours. Although C3 plants, including cannabis, are exempt, the living organisms they rely on do depend on a regular day and night cycle. A disruption, such as taking away the dark period, disrupts biological functions critical for survival, which also impacts the plants.

During the vegetative growth stage, the plants increase in mass, developing more branches, stems, and leaves. They do not produce buds, not until after they transition to the bloom phase. It is the best time to train the plants, manipulating their structure to increase the number of bud sites and the proximity to the light source.

Indoor Notes

Since cannabis plants do not need darkness to function during the Calvin cycle, should you do 24/0? Most people do 18/6, and that makes sense. If you do 24 hours, the electrical consumption will increase by at least 25%. Unfortunately, the growth rate of the plants would not be anywhere close to 25% faster.

Outdoor Notes

The usual practice is to start the seeds, nurture the seedlings (or clones) indoors. Once the young plants are ready, then it is time to move them outdoors. You cannot manipulate the duration of sun exposure, which is why you should start new growth during March or April (in the northern hemisphere). This way, it would give the plants enough time to grow in the vegetative stage before flowering.

Flowering Stage Light Cycle

As part of their natural progression, cannabis plants enter the flowering stage after spending time vegging. You have absolute control over this transition indoors, while it happens naturally outdoors.

Indoor Notes

After 4 to 8 weeks in the vegetative stage, depending on the strain, you can trigger flowering by switching from 18/6 (or 24/0) light cycle to 12/12. There is no other choice concerning the duration of darkness. Your plants need 12 hours. If they receive 13 hours or more of light each day, then the plants will remain vegging.

Outdoor Notes

Nature takes its course, and there is nothing you can do about it. By the third week of June, the plants begin their transition as daytime starts getting shorter.

One concern that you would need to address is if light (e.g., floodlight or street lamp) can reach the plants during the dark cycle. If so, the disruption can stress the plants, affecting the flowering stage negatively.

Light Cycle for Autos

Auto-flowering cannabis plants is a class by itself. They do not depend on light cycles to shift from vegetative to the flowering phase. Because of that, there is no need to switch to a 12/12 schedule. Throughout their life cycle, you can provide 18 hours light on, 6 hours light off daily.

How Do You Use Light Cycle to Control Cannabis Plants?

As you have learned, the light cycle affects your cannabis plants. It is the determining factor in transitioning to the flowering stage. That is why you time the start of new growth – seedlings or clones – from March to April, giving the plants sufficient time to develop in the vegetative stage. Indoors, you merely flip the switch going from either 18/6 or 24/0 to 12/12.

Concerning light cycles throughout the growth stages, there are two things you need to consider.

First is the length of the lights on time. For maximum yields, you can opt for 24/0 as long as the additional cost of electricity incurred is not a concern. For optimal cost/yield ratio, then choose 18 hours on, 6 hours off.

The second thing you need to think about is ensuring that the plants do not receive light exposure during the dark cycle. That means that if you are growing the plants inside the house, seal the grow room completely. And, if outdoors, then pick an area where that is secluded from lamp posts or floodlights.

Light cycles are integral to cannabis plant growth. And with a better understanding of how it impacts the life stages, you can adapt or control the transition from one to the next phase. You could even keep some plants in a vegetative state and use it as a donor for cuts.

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