Understanding the cannabis light cycle

Growing Information

The success of a grow depends on many things, the quality of seeds, the right nutrients and pH level and the growing medium. And one other element, which is arguably more important than all the others – the light. Understanding the impact of the cannabis light cycle at each stage of the cannabis growth cycle is key to enjoying a good harvest.

The two times when you need to monitor the lighting schedule most carefully are during the vegetative phase and the flowering phase. But first, let us explain the impact of a cannabis plant when the season changes from summer to autumn. 

The cannabis light cycle explained 

In the wild, cannabis would naturally grow with the changing seasons in a specific environment. Marijuana seeds would germinate, grow, flower, and set seed according to often subtle changes in the environment, such as alterations to the temperature and humidity levels. However, the most significant factor is almost always the number of hours of daylight. Your cannabis cycles of light and dark roughly correspond to the relationship between daylight and night.

Marijuana plants grow outdoors from spring till the end of the summer. The plants enter their flowering phase towards the end of summer and continue to flower until the end of autumn.

This change in the growth phase (growth to flowering) is simply due to seasonal fluctuations. In general, these seasonal changes mainly relate to the differences in light over the year. The hours of darkness increase, while the hours of sunlight simultaneously decrease as the summer heads into autumn, which acts as a kind of signal to tell the cannabis plants to start flowering.

Related article: Growing cannabis in the woods

Once this signal is received, cannabis plants will concentrate all their energies and resources on producing flowers rather than continuing vegetative growth. This allows a female cannabis plant to ingest the pollen from a male cannabis plant so that it becomes fertilised and forms seeds.

The light cycle of weed when growing indoors

This concept of controlling or manipulating the light cycle is an essential part of indoor cannabis cultivation to trigger an earlier or later flowering phase in the plants.
In general, cannabis plants receive more light in the vegetative (growth) phase and equal amounts of light and dark during the productive (flowering) phase. Cannabis seeds typically germinate in spring, when the hours of daylight increase per day. They will keep growing until the days start to become shorter.

Eventually, the shorter days signal that winter is coming and the plants enter the flowering phase, which allows them to reproduce and give their genes to the next generation. The longest days are the time when cannabis plants have enough time and energy to develop a robust, firm structure in readiness for dense, resinous buds.

Due to the abundance of different cultivation techniques and artificial cultivation systems, these natural cycles can sometimes be modified and exploited. In general, growers want their plants to be as healthy and strong as possible without using energy on seed production or reproduction.

As mentioned above, growers can take advantage of the way plants develop in nature by essentially modifying living conditions such as light and thereby affecting how the plants grow. One of these processes is called “forcing”.

Forcing plants to flower

Forcing is the process in which a plant is exposed to a very long period of light (between 18 and 24 hours), and then suddenly the light is switched to 12 hours. By mimicking the change in daylights from spring to summer, this method causes the cannabis plant to switch to the flowering phase quickly.

This natural sensitivity to alterations in exposure and cycles of light that enables growers to grow cannabis effectively.

The light cycle for indoor cultivation

When growing indoors, you must provide the light. During the vegetative phase, you need to control the light so that it is available to the plants for at least 18 hours a day with 6 hours of complete darkness. Some growers prefer to provide the plants with 24 hours of light each day during the vegetative phase.

But there is an interesting catch:
It is not the light that influences the life cycle or the stage of your plants, but the darkness. The period in which the plants have long periods of darkness tells them to remain in the vegetative phase or to switch to the flowering stage.

Until the cannabis plants are exposed to total, uninterrupted darkness for 12 hours, they will remain in the vegetative stage unless they are an autoflowering variety. Once you provide your plants with 12 hours of light and 12 hours of darkness, the plants will react as if autumn is coming soon and start to bloom.

With the control of the light cycle, you have the perfect tool to create an environment where you can enjoy total control over the ways in which your plants react.

The science behind light cycles

Understanding the relevant science – even in a superficial way – adds a more in-depth knowledge of the needs of your plants. A new breeder is much less likely to forget or understand instructions better if they are also aware of the related science. When you really understand something, you won’t forget the details.

Let’s look at the science behind cannabis light cycles so that the chances of making a mistake are reduced.

Cannabis needs darkness

Cannabis plants are “long night” or “short day” plants, which basically means that for their hormones to switch from vegetative growth to the flowering phase, they require long periods of uninterrupted darkness.

But it gets much more complicated:
The cannabis plants have colour pigments called Pr (Phytochrome Red) and Pfr (Phytochrome Far Red). These are light receptors in the leaves.These pigments absorb red light and, depending on the amount; trigger a chemical reaction that instructs the plants to continue growing in the vegetative stage or to switch to flowering.

How do these phytochromes work?

First, Phytochrome Far-Red can be altered by timing the light. If phytochrome far-red is present in the cannabis plant, it will prevent it from blooming. As long as the light source has a spectrum that includes Far-Red wavelengths, the phytochrome Far-Red is generated.

When there is light, the Phytochrome Far-Red and Phytochrome Red are equally balanced. During the dark, however, the Phytochrome Far-Red becomes Phytochrome Red over time.

During the dark period, the change from Phytochrome Far-Red to Phytochrome Red gradually continues until there is insufficient Far-Red to prevent the plant from entering the flower. Without Pfr the plant enters the flowering phase.

This helps to explain why it is so essential to expose plants to 12 hours of continuous darkness. The conversion of Phytochrome Far-Red to Phytochrome Red is slow; However, the change from Phytochrome Red to Phytochrome Far-Red occurs immediately as soon as the plant has been exposed to light. Therefore, any (Far-Red) interruption of light can make a big difference.

To summarise, the basics are as follows:

Phytochrome Far-Red actively prevents your plants from blooming. Once the amount of Phytochrome Far Red is sufficiently low, your plant will begin to flower. However, this decrease can only occur through long dark periods without interruption.

Before we get into flowering, we need to examine the vegetative stage of cannabis plants. The purpose of this phase is initially to allow the main stem and the root system to develop until they are firm and robust.

During this time, the foliage also grows rapidly in order to absorb more sun and convert it into energy. All this growth requires plenty of light, which explains why plants in the vegetative light cycle need to receive between 18 and 24 hours of light each day.

This is where the chemical reaction takes place, converting carbon dioxide and light into glucose.

Phototropism: more complex than photosynthesis

Cannabis plants have two types of chlorophyll receptors: two chlorophyll A receptors and two chlorophyll B receptors. Both absorb blue and red light spectra. In general, the best lighting systems cover the full spectrum and produce a near-complete source of energy, similar to sunlight.

Almost everyone knows the process of photosynthesis, but the less well-known process is called phototropism.

Phototropism informs cannabis plants of the direction of the light and therefore helping them grow in the best way they can orientate themselves towards it. Think of sunflower plants turning their heads toward the sun each day. You weed plants work in the same way. On the other hand, this also means that a light source that is too far away can lead to unwanted stretching and lankiness.

At this point in growth, cannabis plants opt for blue light. Stretching plants, however, are not a good thing and can also result in a poor harvest.

Ready to grow?

There are many advantages to growing your own cannabis and the more you learn about their requirements such as feminized seeds, autoflowering weed seeds, the best soil for cannabis the more successful you will be in reaping the rewards of your efforts.