The Cannabis Light Cycle

Everyone knows that you need light to grow marijuana plants, but how many people understand how lighting affects the plant’s growth? The light cycle for weed can be a fairly complicated topic, but I will explain it in this article. We’ll talk about how lighting affects the growth and development of cannabis, but we’ll also go deep into the science behind it all. Near the end, I’ll share some ways to control lighting when growing outdoors. I’ll also teach you what to do if you suddenly lack a light source.

Keep reading to start learning or jump ahead to what interests you:

Lighting is essential for all marijuana plants, whether indoors or outdoors. It can make a difference in how well your plants grow, and it has a profound influence on the different life stages of your plants as well.

In this article, we will discuss the ways that light cycles can be used to achieve a successful flowering stage and harvest of your marijuana plants.

This change in the growing stage simply comes from seasonal variations. Generally speaking, these seasonal changes are mostly about the differences in light as the year progresses. The hours of darkness lengthen while the hours of sun shorten starting towards the end of summer, and this serves as a signal of sorts to tell the marijuana plants to begin flowering.

Vegetative vs Flowering
Vegetative vs. Flowering – Image powered by Growweedeasy

Why does this matter?

Once this signal has been received, marijuana plants will start focusing their energy and resources on developing flowers instead of having vegetative growth. This allows these female marijuana plants to attract pollen so that they’ll be fertilized and start growing seeds.

Tip: make sure to download my free Grow Bible for more on the cannabis light cycle

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    This concept is what is behind the indoor marijuana technique of manipulating artificial light cycles to start an earlier flowering season, a later one, or whenever you want.

    Indoor light cycles

    When you are growing indoors, you provide the light.

    During the vegetative stage, you’ll manipulate the light, so there is a minimum of 18 hours of light per day — some growers prefer to go as high as 24 hours a day.

    But there’s a catch:

    It is not really the light that makes much of a difference in your plants’ life cycle, but rather the darkness . The length of time your plants are exposed to uninterrupted periods of darkness will signal to them to stay in the vegetative stage or to go into their flowering stage.

    Lighting Schedule

    Growth stageHours of lightHours of darkness

    Until your plants have 12 hours of complete, uninterrupted darkness, they will stay in the vegetative stage. Once you do give your plants 12 hours of light and 12 hours of darkness, the plants will respond as if the fall season was coming soon, and will, therefore, begin to flower.

    To summarize:

    Using light cycles, you have the perfect tools for creating an environment where you have complete control over how your plants respond.

    The science behind the light cycles

    I can’t say this enough:

    Knowledge is power!

    A new grower is far less likely to forget something or misunderstand directions if they also learn the accompanying science behind it. When you truly understand something, you won’t forget about the details.

    So, I will explain the science of the marijuana light cycles so that you are less likely to make a mistake.  

    Marijuana needs darkness

    Science behind the light cycles
    The science behind the light cycles

    Marijuana plants are “long night” plants or “short day” plants. This simply means that, for their hormones to be triggered into changing from vegetative growth to flowering, they need long periods of darkness without interruption.

    But it gets even more complex:

    Your plants have light receptors called Phytochrome Red and Phytochrome Far Red. To put it differently, they are both color pigments in the leaves. They absorb red light in specific measurements and create a chemical reaction that tells the plant to keep vegetating or to begin flowering.

    Red or Far Red?

    How do they work?

    To begin with, Phytochrome Far-Red can be manipulated through the timing of the light. If there is Phytochrome Far-Red present in your marijuana plant, it will keep it from flowering. So long as the light source has far red wavelengths, it will cause the Phytochrome Far-Red to be produced.

    When there is light, Phytochrome Far Red and Phytochrome Red are balanced in number. During the darkness, however, the Phytochrome Far-Red becomes Phytochrome Red over time.

    Explenation about the cycle
    Explanation of the cycle

    Throughout the dark period, this gradual change from Phytochrome Far-Red into Phytochrome Red continues until there are too few Phytochrome Far-Red to stop the plant from flowering; without it, the plant will enter the flowering stage.

    This is why it is so crucial to expose your plants to 12 hours of uninterrupted darkness. The conversion from Phytochrome Far-Red into Phytochrome Red is slow; however, the change back from Phytochrome Red into Phytochrome Far-Red is instant once it’s been exposed to the light. Therefore, any amount of (far red) light interruption could make a big difference.

    Overwhelmed by lighting? Try easy-to-grow autoflowers in the ILGM cannabis seed shop

    To sum it all up, the basics are this:

    Phytochrome Far-Red actively keeps your plants from flowering. Once there is a low enough amount of Phytochrome Far Red, your plant will start to flower. This decrease can only come from long periods of darkness without any interruption.

    The vegetative stage

    Before we discuss flowering, let’s take a closer look at the vegetative stage of marijuana plants. First of all, the purpose of this stage is to develop the root system until it is solid and strong, as well as strengthen the main stem of the plant.

    That’s not all…

    During this time, it is also growing out the foliage as much as possible to catch more sun and converting that into useful energy (or rather into sugar, which the plant needs for energy). All of this growth takes a significant amount of light, which is why the vegetative light cycle should provide between 18 and 24 hours of light per day.

    Vegetation stage
    Vegetative stage

    This stage hosts the chemical reaction that converts light (and carbon dioxide) into sugar.

    How does it work?

    Marijuana plants have two types of chlorophyll receptors: two chlorophyll A receptors and two chlorophyll B receptors. Both absorb blue and red spectrums. In general, the best lighting systems will cover the entire spectrum, creating a more complete source of energy, much like actual sunlight.

    But again, it goes deeper.

    More complicated than photosynthesis

    Everyone knows about the process of photosynthesis — we’ve all learned about it in elementary science class, after all — but the lesser known process is called phototropism.

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      This process is responsible for informing marijuana plants where light is coming from and, therefore, helping them grow so that they are best oriented toward it. On the other hand, this also means that a too-distant light source can cause stretching in your plants.

      When plants stretch, they will literally reach upwards, growing taller and slenderer, to “find” a good source of light.

      At this point in their growth, cannabis plants prefer blue light. However, stretching plants are never a good thing, and they can also lead to a disappointing harvest.

      Phototropism on your plant is unhealthy
      Phototropism on your plant is unhealthy – Image powered by Photo tropism

      Choose the right lights

      I can’t say this enough:

      You should choose full spectrum lights HPS lamps, or high-pressure sodium lights, for example, do not include the right amount of blue light and therefore will lead to unhealthy phototropism.

      To clarify, HPS lights only have about 3 or 4 percent blue light, whereas the recommended amount is at least 12 percent. Anyone growing large quantities of marijuana (who knows what they’re doing) will, therefore, use metal halide grow lights that are rich in blue light during their plants’ vegetation stage. Otherwise, they will opt for LED grow lights that include the entire spectrum, making them perfect for the entire marijuana life cycle.

      Read the article best marijuana grow lights for more information about different grow lights

      LED grow light cannabis
      LED grow light cannabis

      Successful growers do these two things:

      • Use automated timers during the vegetative stage. This ensures consistency and precision when managing the light cycle.
      • Maintain at least 14 hours of light during the vegetative stage.  Marijuana doesn’t need darkness until they are ready to flower.

      Believe it or not, you can give your plants up to 24hours of light daily during the vegetative stage, and plenty of growers do.

      There is some flexibility

      On the other hand, there is nothing wrong with a little darkness during the vegetative stage. Growers operate under the belief that some “rest” time for their plants doesn’t hurt, so they will still give them about 6 hours of darkness with 18 hours of light.

      Do what works best for you, but keep in mind that 6 hours of darkness has the added benefit of saving some energy costs. That being said, plants who receive 18 hours of light rather than 24 will grow a little bit slower.

      Of course, the same lighting technique for a mature vegetating marijuana plant would not apply to a seedling, which can be burned easily by too much light (or heat, for that matter). The older and stronger the plant is, the more light it can handle.

      How long is enough?

      You may be wondering how long the vegetative stage can go…

      In essence, the vegetative stage can last as long as you want it to. You can alter it based on your schedule, or the size of your grow room. Most growers average between four and six weeks, but it can also vary according to the strain you are growing. Do your research accordingly.

      Other people will decide to base it off height (between 12 and 18 inches, usually) rather than time.

      But there’s a catch:

      The size of your plants will not stay the same once the flowering stage has begun. They will grow to twice or three times the size that they are at the end of their vegetation stage.

      TIP: Hearty genetics lead to hearty plants. Buy high-quality seeds from the ILGM seed shop

      Inducing the flowering stage

      If you were growing your marijuana plants outdoors, you would be at the mercy of the elements and the natural light changes. When you’re growing them indoors, however, you have complete control over what their life cycle looks like.

      Let’s look at the details of inducing the flowering stage for your indoor marijuana plants.

      Through what is referred to as “photoperiodism,” you can change the light cycle to 12 hours of darkness and 12 hours of light to induce the flowering stage. The key is to continue doing this consistently and daily for weeks, as you won’t see the changes right away. After just a couple of weeks you should be able to see the beginnings of bud formations. Check this video from John Berfelo about switching to 12/12

      YouTube video

      Darkness matters

      Once again:

      Do not underestimate the importance of an uninterrupted period of darkness for transitioning your plants into the flowering stage.

      Any split second of light during this time will switch the Phytochrome Red back into Phytochrome Far Red, and we know what that means — the flowering stage will be postponed.

      Don’t even go into your grow room during the 12 hours of uninterrupted darkness because even the light from another room leaking in for a second can change things.

      How to create darkness

      You’ll want to confirm there is absolutely no light leaking into the grow room before you start the flowering cycle. Here is how you do that:  

      • Go into the room yourself, turn off all the lights, and close the door.
      • Give yourself a good quarter of an hour until your eyes are used to the darkness, and then take a close look around the room to see if you can spot any light leaks at all.
      • Double check beneath the door, the curtains, the dehumidifier, CO2 generator, and other such devices.
      • Equipment with light-up displays will disturb periods of uninterrupted darkness. Cover those displays with duct tape.

      Tip: Download my free Grow Bible for more on grow rooms

      Darkness is very important while growing
      Darkness is very important while growing

      One good way to see if a room has too much light is to simply hold your hand out in front of your face. If you can see it, there is light getting into your grow room somehow. Make sure to fix the problem before starting on your 12/12 light cycle, or else you will just be wasting your (and your plants’) time.

      Jumpstarting your flowering period

      Although most people prefer to change their lighting schedule to begin the flowering process, there are other ways.

      Here’s the deal:

      You can give your plants a head start by giving them an extra shot of darkness, to begin with. Take advantage of your completely dark grow room by actually keeping it completely and utterly dark for an entire 36 hours.

      This will drop the Phytochrome Far-Red amount significantly — more so than if you switched it to a 12/12 schedule from the beginning. The idea behind kick-starting the process in this way is to give a loud and clear signal to your plants that flowering season has begun.

      Once the 36 hours are complete, begin the 12/12 lighting cycle.

      Result off 36 hours darkness while growing
      Results of 36 hour darkness while growing

      I can’t say this enough…

      The results of doing it this way can be significant. You’ll also want to consider the nutrients needed during this change to the next  lifecycle — your plants no longer need high nitrogen fertilizers, but rather need more phosphorus for the flowering period. Potassium can also help with faster bud growth.

      More about the flowering stage

      If you think we have covered everything, we have not.

      Let’s delve deeper into the details of the flowering stage and its corresponding light cycle. As already described, the light cycle needs to change to a 12/12 ratio.

      After two or three weeks, you’ll notice changes, and your marijuana plants will show their sex. Be sure to take out the male plants as soon as you can confidently identify them.

      Male vs Female plant
      Male vs. Female plant

      Even though the vegetation stage is turning into the flowering stage at this point, the growth of your plant will not stop. It will continue to grow and could reach up to two or three times the size it was before you changed the light cycle for the flowering stage.

      One thing people always ask me is when they should harvest their plants. This free PDF doc gives a perfect idea of when to cut your plants.

      Lighting and the flowering stage

      Many growers will decide to change lights to an HPS lamp or will continue using their full spectrum LED grow lights to get the plants as much light as possible. These are efficient and give off large amounts of photonic energy, which is perfect for all the bud development that is going on.

      HPS lamp
      HPS lamp

      Don’t worry too much about phototropism here, because it won’t have as strong of an effect as it would during the vegetation stage. This is why HPS lights should only be used for the flowering stage.

      The thing to remember is that marijuana plants require red light in addition to blue light. In terms of the stages it goes like this:

      Vegetative stage: more blue than red

      Flowering stage: more red than blue

      Generally speaking, it is pretty tough to give your plants too much light, as long as your grow lights are set up far enough away from their tops. Heat damage is usually a higher risk than light saturation or bleaching, although narrow beam angle LEDs could cause the bleaching to occur.

      Light deprivation

      Although it is certainly a more difficult process to manipulate anything having to do with light in an outdoor setting, there are some ways to deprive your plants of light.

      Why would you deprive your outdoor plants of light?

      This little trick can help you achieve an early harvest.  It can also help you harvest more than once in a year.

      How do you do this?

      Build your garden so that you can cover it when the sun sets, and then uncover it mid-morning. The correct application of this method should lead to 12 full hours of darkness when the natural lighting is not quite there yet.

      Using light deprivation while growing
      Using light deprivation while growing

      This, of course, must be absolutely consistent for it to work, but if you can double your harvest by doing it twice in one season, then it is well worth the extra effort.

      Cloning in the springtime

      Proper lighting for your clones can be a difficult endeavor when growing outside. If done well, the results will be phenomenal. If done poorly, you will be sorely disappointed.

      “Cultivating several strong, healthy mother plants is the key to having a consistent supply of all-female clones.”

      Jorge Cervantes

      Clones prefer a minimum of 18 hours of light per day, which means that those planted in early spring (when the light is less than 18 hours per day), will start to enter the flowering stage too early.

      What does this mean?

      It means clones shouldn’t be planted until at least the middle of May, depending on your location.

      At least 18 hours of  light for clones
      At least 18 hours of light for clones

      If you are willing to put in the extra effort to add supplementary light, however, you could plant clones in the spring. To do this, use a stake, or a cage around the plants that has a fluorescent light and aluminum reflector clipped onto it.

      This will add at least a few hours of extra light onto the plants, and that should already be enough to keep them from entering the flowering period too early. For colder climates with a risk of a late-season frost, choose an incandescent light bulb because it emits extra heat.

      Light interruption: power outages

      As with any type of technology, things with lights and other equipment always seem to go wrong when you least expect it. A power outage is one such event, or if you need to unexpectedly change a light bulb. These things can interrupt your light cycle and can, therefore, have consequences for your plants.

      For plants currently on a 12/12 light cycle, a power outage won’t do any damage (unless it lasted for days and days). Light during a time of darkness is more likely to mess things up than darkness during a time of light. Growth will slow down, of course, but there should be no issue if the outage lasts for two days or less. Longer than that could lead to stress.

      Buy a backup power generator if possible
      Buy a backup power generator if possible

      If your plants are on an 18/6 light cycle, however, you could run into some trouble. The sudden long period of uninterrupted darkness could spur them to begin flowering prematurely. For this reason, you need to find a way to expose your plants to light.

      Fixing a lighting problem

      Lost your light source? This is not a problem.

      If you have a camping lantern, for example, that will be enough to stop the Phytochrome Far-Red from dropping too low You can keep the lantern on when the plants are supposed to be experiencing their dark period to prevent the flowering cycle from beginning.

      Keep in mind that the light is just for interruption purposes — your plants will do fine without a proper growing light for a couple of days growing-wise. The only concern is stopping the flowering stage from starting too early.

      If you learned anything from this article, it should be that proper lighting is essential for your marijuana plants. When they do not have the right amounts and types of light, they grow slower or not at all. The absence of light can also play a big role, as the marijuana plant times its flowering around darkness. Once you understand these things, you can be a better grower with better yields.  

      Stumped by all this light-management? Start out with a simple windowsill grow powered by our friends at a Pot for Pot.

      Happy growing!

      Robert, founder of ILGM and author of the Marijuana Grow Bible

      FAQ About Cannabis Light Cycle

      When to switch to 12/12 light cycle?

      You should switch to the 12/12 light cycle when you want to induce the flowering stage.

      When to start 18/6 light cycle?

      You should start the 18/6 cycle when your plants are in the vegetative stage.

      What is the recommended light schedule for seedlings?

      Seedlings need 16 to 24 hours of light and 8 to 0 hours of darkness. 

      Robert Bergman

      Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations. His passion for growing lead him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]


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