While everyone knows that cannabis plants need light, not many people know how lighting affects their growth. Although the light cycle of cannabis can seem complicated, we will try to simplify it. We will discuss how lighting affects cannabis’ growth and development. But we also dive into the science behind this. You’ll find some tips and tricks to improve lighting conditions for outdoor cannabis cultivation. You will also learn what to do when you lose a light source.

All cannabis plants need lighting, indoors and outdoors. Lighting can have a significant impact on how your plants grow and also has an influence on their life stages.

This article will explain how light cycles can be used for successful flowering and harvest of marijuana plants.

When does Cannabis bloom?

When does Cannabis bloom

The flowering stages of cannabis plants grown outdoors wild or by growers are generally at the end of summer and last through the autumn.


Seasonal variations are what cause this change in the growth stage. These seasonal changes are mainly due to the variations in light over the years. As the summer ends, the hours of darkness will increase and the hours of sunlight will decrease. This signals to the marijuana plants that it is time to start flowering.

What is the point?

After this signal is received, marijuana plants will begin to focus their energy and resources on producing flowers rather than vegetative growth. This allows the female marijuana plants to attract Pollen, which will allow them to fertilize and begin growing seeds.

This is the indoor cannabis method of manipulating artificial lighting cycles to create a earlier or later flowering season.

Cannabis Light Cycle Indoor Cultivation

Cannabis Light Cycle Indoor Cultivation

You provide the light when you grow indoors.

You’ll need to control the light during the vegetative phase. Some growers like to have as much as 24 hours of daylight per day.

But there is a catch.

The darkness is what makes a big difference in the life of your plants. Your plants will decide whether they want to remain in their vegetative stage or move into the flowering stage based on how long they are left without light.

Indoor Cannabis Plant Lightning Plan

Stage of growthHours of lightingHours of darkness
Cannabis Seedling16-248-0
Cannabis Clone (Cutting)18-246-0
Vegetative Growth186
Flowering Stage1212

Your plants will remain in vegetative status until they have had not more then 12 hours of uninterrupted light. Your plants will begin to flower when you give them 12 hours of light, 12 hours darkness.

To summarize:

You can create an environment that allows you to control how plants respond by using light cycles.

The Scientific Aspect of Light Cycles in Cannabis

If a new grower learns the science behind what they are doing, it is less likely that they will forget something or misunderstand instructions. Understanding something is key to remembering the details.

I will now explain the science behind the cannabis light cycles to help you avoid making a mistake.

Cannabis requires darkness

Cannabis requires darkness

Cannabis plants can be either “short day” or “long night” plants. This means that they require long periods of darkness and no interruptions in order for their hormones change from vegetative growth into flowering.

It gets more complicated:

The light receptors of your plants are , Phytochrome Red, and Phytochrome Far Red. They are both color pigments found in leaves. They are chemically reactive and absorb red light at specific wavelengths. This chemical reaction tells the plant whether to continue vegetating or flowering.

Red and Far Red Light

Phytochrome far-Red can be controlled by changing the time of the light. If your marijuana plant has Phytochrome Far Red, it will not flower. The Phytochrome Far-Red will be produced as long as it has far-red wavelengths.

  • Phytochrome Far-Red and Phytochrome Red are equal in number when there is light. Phytochrome Red is formed when there is darkness.
  • Phytochrome Far-Red is gradually converted into Phytochrome Red during the dark period. Without it, the plant will reach the flowering stage.

It is important to give your plants 12 hours of continuous darkness. Although the conversion from Phytochrome Far-Red to Phytochrome Red is slow, once the plant has been exposed to light, it will make a quick change from Phytochrome Far-Red back into Phytochrome Far-Red. Any amount of (far-red) light interruption can make a huge difference.

Let’s sum it up:

Your plants will not flower if they have Phytochrome Far Red. Your plant will flower once it has a sufficient amount of Phytochrome Far Red. This can only be achieved by long periods of darkness that is uninterrupted.

The growth stage of cannabis (vegetative stage)

The growth stage of cannabis (vegetative stage)

Let’s first look at the vegetative stage in cannabis plants before we get into flowering. This stage serves two purposes: to strengthen the main stem and develop the root system.

But that’s not all…

It is also growing the leaves as much as it can to get more sunlight and converting it into useful energy. (Or sugar, as the plant requires for energy). This growth requires a lot of light. Therefore, the vegetative light cycle should offer between 18 and 24 hours of daylight per day.

This stage hosts the chemical reactions that convert light (and carbon dioxide into sugar).

What is the secret to it?

Two types of chlorophyll receptors are found in marijuana plants: two for A and two for B. Both absorb the blue and red spectrums. The best lighting systems will illuminate the entire spectrum and create a greater source of energy. This is similar to actual sunlight.

It goes deeper, however.

Photosynthesis & Phototropism

Photosynthesis & Phototropism

We all know about photosynthesis. In elementary science classes, we were taught about it. But phototropism is a lesser-known process.

This is how marijuana plants learn where the light is coming from, and help them grow in a way that is best for them. However, a light source to far away can strech them.

Plants can stretch to reach the light they need. They will grow taller and more slender, in order to find good light sources.

Blue light is preferred by cannabis plants at this stage of their growth. But, to let them strech is not a good idea and can lead to disappointing results.

Select the right (Grow) light

Select the right (Grow) light

Full spectrum lamps should be used. High-pressure sodium lamps do not contain enough blue light, and can cause phototropism.

HPS lights have only 3 to 4 percent of the blue light recommended, while the recommended amount is minimum 12 percent. If you grow large amounts of marijuana, (and you know what you’re doing), then metal halide grow lamps that emit a lot of blue light will be the best. They will instead opt for LED grow light that includes the whole spectrum. This makes them ideal for the entire cannabis life cycle.

These are the two most important things successful growers do:

  • Automated timers are useful during the vegetative phase. This will ensure consistency and precision in managing the light cycle.
  • During the vegetative stage, keep at least 14 hours of daylight. Marijuana does not require darkness until they are ready for flowering.

You can give your plants uo to 24 hours of light during the vegetative stage. Many growers believe so.

Optimal Illumination For Your Plants

You can maximize your marijuana plant’s potential yields by knowing the key factors you need to consider when lighting is done correctly.

Flexibility in lighting cannabis

However, some darkness is okay during the vegetative phase. Growers believe that plants need some rest, so they give their plants about 6 hours of darkness and 18 hours of light.

You can do what you like, but 6 hours of darkness will help you save some money. Plants that receive 18 hours of sunlight instead of 24 hours will grow slightly slower.

A seedling can burn easily if it is exposed to too much heat or light. The more light a plant can take, the older and stronger it is.

What is the limit?

You might be curious how long the vegetative stage lasts.

The vegetative stage can be as long or short as you wish. It can be adjusted according to your schedule or the size and layout of your grow space. It is usually between four to six weeks for most growers, but it may vary depending on the strain. Make sure to do your research.

Others will base it on height (usually between 12-18 inches) and not time.

But there is a catch.

Once the flowering stage begins, the size of your plants won’t be the same. They will reach twice or even three times their size at the end of the vegetation stage.

Inducing flowering

Inducing flowering

You would be vulnerable to the weather and changes in the light if you were growing marijuana outdoors. You have full control over their life cycle when you grow them indoors.

Let’s take a look at how to induce the blooming stage in your indoor marijuana plants.

You can alter the light cycle to 12 hour of darkness and 12 hour of light to trigger the flowering stage through what is called “photoperiodism”. It is important to do this every day for several weeks. You won’t notice any changes immediately. In a few weeks, you will begin to notice the first signs of bud formations.

Darkness is a powerful force


Don’t underestimate the importance of a continuous period of darkness when transitioning your plants to the flowering stage.

Any fraction of a second of light during this period will change the Phytochrome Red into Phytochrome Far Red. We know what that means: the flowering stage is postponed.

Do not enter your grow room during 12 hours of darkness. Even if there is a second source of light, it can still cause damage.

How to keep the plants really dark

Before you begin the flowering cycle, make sure there is no light entering the grow room. Here’s how to do it:

  • Turn off all lights in the room and then close the door.
  • Allow your eyes to adjust to the darkness for a quarter of an hour. Then, take a closer look at the room to determine if there are any light leaks.
  • Double-check the doors, curtains, dehumidifiers, CO2 generators, and any other devices.
  • Equipment equipped with light-up displays can disrupt periods of continuous darkness. Use duct tape to cover these displays.

Simply holding your hand in front of you can help you see if there is too much light. If it is visible, you know that there is some light entering your grow room. You will waste your time and your plants’ time if you don’t fix the problem before you start your 12/12 light cycle.

How to jumpstart your flowering season

How to jumpstart your flowering season

While most people would rather change their lighting schedules to start the flowering process they can also use other methods.

That’s how it’s done

Giving your plants extra darkness can help them get a head start. You can take advantage of the darkness in your grow room by keeping it dark for 36 hours.

This will result in a significant drop in the Phytochrome Far Red amount — much more than if it was switched to a 12/12 schedule at the beginning. This is a way to signal your plants that the flowering season has started.

After the 36 hours have ended, you can begin the 12/12 lighting cycles.

This method can have significant results. Also, you’ll need to think about the nutrients your plants will need during this transition to the next stage of their lives. Your plants won’t need to use high-nitrogen fertilizers anymore. Instead, they will need to have more phosphorus to flower. Potassium can help speed up bud growth.

Learn more about the flowering stage

We haven’t covered everything if you think so.

Let’s dive deeper into the details about the flowering stage as well as the associated light cycle. The light cycle must be changed to a 12/12 proportion, as we have already explained.

You’ll start to notice some changes in your marijuana plants after two or three weeks. Make sure you remove the male plants as soon and as possible.

Despite the fact that the vegetation stage has turned into the flowering stage, your plant’s growth will not cease. It will grow, and it could grow to 2 or 3 times its size before you change the light cycle for the flowering phase .

People always ask me when to harvest their plants. This PDF document provides a great idea about when to cut your plants.

Lighting and flowering stage of Cannabis

Lighting and flowering stage of Cannabis

Many growers will choose to switch to an HPS lamp, or to continue using full spectrum LED grow light to give their plants the best possible light. These lights are very efficient and emit a lot of photonic energy. This is great for all the bud growth.

Phototropism won’t be as powerful as during the vegetation stage, so don’t worry about it. This is why HPS lighting should be reserved for the flowering stage.

Remember that cannabis plants need both red and blue light. It is as follows:

  • Vegetative Stage of Cannabis: More blue than red
  • Flowering stage of Cannabis: More red than blue

It is generally difficult to give your plants too much light. As long as the grow lights are not placed too close to their tops, it will be very difficult. While heat damage is more common than light saturation and bleaching, it could be caused by narrow beam angle LEDs.

Light deprivation in Cannabis Plants

It is definitely more difficult to control light in outdoor settings, but there are ways to make sure your plants don’t get enough light.

Why would you deny your plants light?

This little trick will help you harvest early. This trick can help you have more harvests per year.

What can you do to do this?

Your garden should be built so that it can be covered when the sun sets and then uncovered mid-morning. This method can provide 12 hours darkness if the natural light is not there yet.

It is important to be consistent with this method in order for it work. However, if you can increase your harvest by doing it twice within a single season, it is worth it.

Cloning Cannabis in Spring

Cloning Cannabis in Spring

Proper lighting can make it difficult to grow clones outdoors. The results can be amazing if done correctly. You will be disappointed if you do it poorly.

Clones need at least 18 hours of sunlight per day. This means that clones planted too early in spring (when there is less than 18 hours of daylight per day) will not reach the flowering stage.

What does this all mean?

This means that clones should not be planted before the middle of May depending on where you live.

Clones can be planted in spring if you’re willing to make extra efforts to provide additional light. Use a stake or a cage with a fluorescent light and an aluminum reflector attached to it.

The extra light will provide at least a few more hours to the plants. This should be enough to prevent them from reaching the peak of their flowering season too soon. An incandescent lightbulb is better for cold climates that are susceptible to a late-season freeze. It emits more heat and can be used to produce extra heat.

Power outages: Light interruption

Things with lights and other technology can go wrong at the worst possible times. Unexpectedly changing a lightbulb or experiencing a power outage are two examples of such an event. These events can disrupt your light cycle, which can have serious consequences for your plants.

A power outage will not cause any harm to plants that are currently following a 12/12 cycle of light (unless it lasted days and weeks). It is more likely that light during darkness will cause damage than when it is bright. While growth will slow down naturally, there shouldn’t be any problem if it lasts less than two days. Stress could result if the outage lasts longer than two days.

However, if your plants are on an 18/6 cycle of light, there could be problems. A prolonged period of unbroken darkness could cause them to flower prematurely. You need to discover a way to bring your plants into the light.

Lighting problems can be fixed

Have you lost your light source? This isn’t a problem.

A camping lantern can be used to stop the Phytochrome Far-Red levels from dropping too low. The lantern can be kept on during the dark period of the plants to prevent them from blooming.

Remember that the light is only for interruption purposes. Your plants will grow fine with a few days without proper lighting. It is important to not stop the flowering stage too early.

Proper lighting is crucial for marijuana plants, and this article should have taught you that. They will grow slower if they don’t have enough light. Lack of light can also have a significant impact on marijuana plants, which will flower around dark hours. These things will help you grow better marijuana and produce better yields.