Lighting is essential for all marijuana plants, whether indoors or outdoors. It can make a difference in the 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.

When does weed flower?

Outdoor marijuana plants in the wild (or those grown by marijuana growers) generally enter into their flowering stage towards the end of the summer season and continue flowering throughout the end of the fall season.

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 (pic by Growweedeasy)

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 in order to be fertilized and to start growing seeds.


Tip: make sure to download my free Grow Bible for more information about cannabis light cycle


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

Indoor light cycles

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

In fact, it is not really the light that makes as 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.


Hours light marijuana


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.

Those two light cycles are the perfect tools for creating an environment in which you have complete control over how your plants respond.

The science behind the light cycles

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 let’s go over some of that understanding in terms of your marijuana plants and light cycles.

science behind the light cycles
Science behind the light cycles

Marijuana plants are considered to be “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.

The light receptors in your plants are actually Phytochrome Red and Phytochrome Far Red, which are two color pigments in the leaves. They absorb red light of certain measurements, and therefore react to light chemically to either tell the plant to keep vegetating or to begin flowering.

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. Any light that has wavelengths that are far red in color is going to cause the Phytochrome Far Red to be produced.

During times of light, this Phytochrome Far Red and the Phytochrome Red are balanced in number. During the darkness, however, the Phytochrome Far Red becomes Phytochrome Red over time.

phytochrome red light cycle
Explenation about the cycle

Over the course of one 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.

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 vegetation stage

Let’s take a closer look at the vegetation stage of marijuana plants. 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.

During this time, it is also growing out the foliage as much as possible for the purpose of catching 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 a vegetation light cycle should be between 18 and 24 hours of light per day.

Vegetation stage growing marijuana
Vegetation stage

It all comes down to powering the chemical reaction of converting light (and carbon dioxide) into sugar. Let’s get into the nitty gritty of the science behind it. Marijuana plants have two types of chlorophyll receptors: two chlorophyll A receptors and two chlorophyll B receptors.

Both types absorb the blue and red spectrums, so the best lighting systems will cover the entire spectrum for the light to serve as a more complete source of energy, much like actual sunlight.

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.

This process is responsible for informing the marijuana plants where the light is coming from and therefore helps them grow in such a way that they are pointing toward it. If the light you have provided isn’t sufficient, you will begin to see your plants stretching and reaching upwards, growing taller and slenderer, to “find” a good source of blue light in particular. This can be unhealthy for the plant and lead to a disappointing harvest.

phototropism cannabis plant
Phototropism on your plant is unhealthy – pic by Photo tropism

This is why it is so important to choose lights that have a full spectrum. 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

It is best to keep your lights on some sort of automated timer during your marijuana plants’ vegetation stage. This will ensure that the timing of the light changes remains exact and perfectly consistent.

An absolute minimum of about 14 hours of light is required to maintain your plants’ vegetation stage status. Marijuana plants do not need any amount of darkness unless they are going to be flowering so you can amp it up to 24 hours of light every day — and plenty of successful marijuana growers do just that.

There is some flexibility

Some 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 whichever you are more comfortable with and that works for your lifestyle — and keep in mind that 6 hours of darkness has the added benefit of saving some energy costs. That being said, the growth of plants who get 18 hours of light rather than 24 will be a little bit slower.

Check this video from Dutchman’s medical marijuana

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.

The vegetation stage can last as long as you want it to, depending on your schedule and how big your grow room is. Most average would be 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 of height (between 12 and 18 inches, usually) rather than time. Keep in mind that 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.

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 on a daily basis for weeks, as you won’t see the changes right away. After just a couple weeks you should be able to see the beginnings of bud formations.

Check this video from John Berfelo about switching to 12/12

Darkness matters

Don’t ever underestimate how important that uninterrupted period of darkness is for the healthy transition of your plants into the flowering stage. Any split second of light during a predetermined period of darkness 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 periods because even the light from another room leaking in for a second can change things.

Before you are ready to go into this new light cycle, however, there are certain things you will need to pay attention to. The first thing is to ensure that there are absolutely no light leaks in the grow room at all.

The best way to do this is to 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.

darkness cannabis
Darkness is very important while growing

Be sure to double check beneath the door, the curtains, the dehumidifier, CO2 generator, and other such devices. Equipment often has a display that is lit up, and that will mess up your uninterrupted darkness. If there is a lit-up display, cover it with duct tape.

One good way to see if the room itself is too light is to simply hold your hand out right 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 even getting started 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, it can actually be given a head start in another way. 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.

36 hours darkness growing
Result off 36 hours darkness while growing

Once the 36 hours are complete, begin with the 12/12 lighting cycle. The results of doing it this way can be significant. It is also important to remember the nutrients needed for this change in life cycle — 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

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, the flowering will be visible, and your marijuana plants will show their sex. Be sure to take out the male plants as soon as you are able to 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.

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 can work well only for the flowering stage.

The thing to remember is that, for the flowering stage, the marijuana plants require red light in addition to blue. They should be receiving more red than blue while during the vegetation stage they should receive more blue than red.

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 the tops of your plants. 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 steps you can take to achieve depriving your plants of light. It can be used to achieve an early harvest (which many growers are already doing), or even allows growers to harvest more than once in a year.

The key is to build your garden in such a way 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.

outdoor growing light deprivation
Using light deprivation while growing

This, of course, must be absolutely consistent to achieve it, 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 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.

Clones are usually used to a minimum of 18 hours of light per day, which means that clones planted in early spring (when the light is less than 18 hours per day), they will often already start to enter the flowering stage. This means that clones simply shouldn’t be planted until at least the middle of May, depending on your location.

Proper light while cloning
At least 18 hours of  light for clones

If you are willing to put in the extra effort to add supplementary light, however, you will be able to plant the clones in the spring. The way to do this is put a stake out, or a cage around the plants that has a florescent light and aluminum reflector clipped onto it.

This will get 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 frost late in the season, instead, go for 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.

Pacific Power cannabis
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.

If you have a camping lantern, for example, that will already be enough to stop the Phytochrome Far Red from getting too low in number. 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.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible.


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