The Marijuana Light Cycle: Walking You Through Lighting

The marijuana light cycle is something you NEED to master if you want an excellent yield.

Questions you're probably asking yourself:

  • What type of light is required?
  • How long should I leave my lights on?
  • At what times?
  • And how much light does each plant need?

There are many different components that play into the marijuana light cycle.

And today, we'll go over everything you need to know.​

But the bottom line is...

To produce your desired yield, you need sufficient light.


Get Familiar with Marijuana Light Cycle

Before considering the number of cannabis plants that go under one light, or how many lights you need to have in your grow room you need to understand your plants a little bit better.

By looking at various practices around growing, we’ll help growers and cultivators better understand the light their plants need produce flowers.

Defining Marijuana Light Cycle

Marijuana light cycle refers to the conditions under which light is placed on your plant to promote flowering.

Lighting is important from the moment you plant the seed to the moment you trim the buds.

Improper lighting and failure to pay attention to the details can mean failure for your plant.

This involves understanding the science behind light, the marijuana plant and the processes that occur to make a plant flower.

The importance of light in growing marijuana goes far beyond the basic tenets of photosynthesis, which will be discussed further in detail later.

What Makes Marijuana Flower?

Typically, when grown outdoors, marijuana flowers in the latter parts of summer and the early parts of autumn.

So when growing indoors, you need to be aware of these natural light processes because it's your goal to mimic this process artificially. 

You have to force the cannabis plants to recognize its flowering time.

Without being aware of the stage of growth your plant is in, and the required light to help it grow through its current stage, you could put the integrity of your plant, and its ability to flower, in serious jeopardy.

What are the Stages of Marijuana Growth?

There are two main stages the cannabis light cycle is connected to: the vegetation stage, and the flowering stage.

The vegetation stage is when your plant is growing its strength by developing its root system and the foliage required to move to flowering.

It’s a crucial stage because, without a strong root system, the plant will not be strong enough to go through the critical process that will allow it to receive light.

The flowering stage takes a few weeks to be fruitful, but it's the stage that when light is used correctly, will result in beautiful, smokeable buds.

The key to a successful flowering stage is a perfect balance between light and dark, and being consistent with your lights.

The School of Hard Nugs gives a very helpful and demonstrative overview of a professional growing “flower room” that shows their lighting processes while explaining the marijuana light cycle and the stages very well in this video:

Way Beyond Photosynthesis

The essential part of this process goes way beyond the rudimentary photosynthesislessons we all learned about in our elementary school science classes.

What we need to understand is called phototropism.

Phototropism makes the plant recognize the light source it needs to grow, and forces the plant to grow itself into the best position to receive the light.

Phototropism is required to induce the flowering process.

How Much Light and When?

Now that you understand the stages, you’ll have to learn the ways marijuana growers are lighting their plants indoors to ensure plant strength in the vegetation stage and induce flowering.

  • For the vegetation stage, your plant will require 18-24 hours of light, and six hours of darkness or less, depending on the plant. Most strains, however, will require 24 hours of light.
  • The flowering stage, uses a 12/12 rule: 12 hours of light, 12 hours of dark.

Various growers will share some of their ideal light and dark times for their plants on various growers’ forums.

It's never a bad idea to lean on experience.

How Many Lights Do You Need?

Determining the number of lights you need in your grow room isn’t as simple as knowing how many plants you have and multiplying that by X number of lights.

There are a few factors you need to consider to calculate your lighting requirements:

  • The “canopy” – this is the area of your grow room that the lights will need to be focused on, it's usually measured in square footage
  • Plants per sq ft – growers recommend using a “plant per 4 square foot” rule when calculating their lighting requirements. This should allow your plants enough room to grow as tall as they need to be to go through the vegetative and flowering stages
  • Lumens/Watts per square feet – You then need to calculate the wattage/lumens per sq ft.

Watts per Sq Ft

You can set up your grow room using watts as a baseline using the following equation:

The formula for watts per sq ft

Square Footage of Your Canopy Area (at plant per 4 square foot)


32 watts = Required wattage for your area

As an example, a 60 sq ft canopy area with 1 plant every 4 sq ft multiplied by 32 watts will have you requiring 1900 watts to provide sufficient lighting for 15 plants.

The general rule of thumb for growers is 32 to 75 watts per square foot, but it depends on the type of lights and the distance from your plants.

With these general principles in mind, one can calculate their canopy, arrange the ideal plants per square feet, and build lighting that ensures each plant is receiving the right amount of wattage to promote conditions of growth for their environment.

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Lumens Per Sq Ft

Lumens are the basic unit of light.

​And the sun, for instance, puts off around 10,000 lumens per sq ft.

​Back in the day, growers would use a lights watts to determine how much light their garden needs.

This poses quite a few problems:

  • Wat​ts measure the power of an electrical circuit—not light.
  • Lumens don't directly correlate with wattage

Remember, when growing indoors we're trying to mimic the sun with our lights.

And we can't just go off watts because CFL, HID and HPS grow lights all have a different watt to lumen ratio.

Lumens per square foot:

  • Minimum amount: 2000 lumens per sq ft
  • Mid range: 5000 lumens per sq ft.
  • Optimal amount: 7000-7500 lumens per sq ft.

The formula for lumens per sq ft

Square Footage of Your Canopy Area (at plant per 4 square foot)


2000 lumens = Required lumens for your area

Also, note that cannabis needs different amounts of light at different stages.

As you plants switch from vegetative stage to the flow​ering stage, they will require more light.

Some growers light to start off at half the minimum recommend light and work their way up as they get into flowering.​

Also, you can give your plant too much light.

And you can burn your plants from having the lights too close.​

You'll know if white spots begin to appear on your plant, which happens when it gets an over-saturation of light or too much heat.

What Kind of Lights?

It’s important to note that due to the chemical processes involved in each stage of the cannabis light cycle, the type of lighting is important to be aware of during each stage.

Since your plant will respond to different parts of the light at the various stages, growers recommend using a broad or full-spectrum LED light.

These are ideal because they can cover a large area and provide enough energy required to promote growth and budding.

Avoid LED lights with a narrow beam and ensure that your plants are far enough away from the light so that they don't bake in the heat.

CFLs and LED lights are more energy efficient and ​more cost effective than others, so keep that in mind when planning your grow room.

Marijuana Lighting Hacks

Depending on the stage of the light cycle your plant is in, you will need to monitor when your lights are on and when they’re off.

The lighting NEEDS to be consistent. 

Of course, it’s not overly difficult to turn your lights on and off manually, but this does require scheduling your life around your lights.

Using timers helps ensure plants are getting consistent light, when they need it.

Inducing the Flowering Stage with Lights

The reason lighting is so important with an indoor operation, is you control when your plants go into the flowering stage.

But you also must take a few steps to induce the flowering stage to allow your plants to recognize when time to transition.

Inducing the flowering stage can be done through a few ways including:

  • Starting to introduce your plant to periods of darkness
  • Covering your plant during the vegetative state during different parts of the day to encourage budding
  • Use fertilizers that work with your plant strain to help stimulate the plant for blooming, including phosphorus which is a vital mineral for flowering

Overview on the Importance of Lighting

Use our formulas to determine the number of lights required per cannabis plant and the wattage/lumens needed.

But it can’t be stressed how important it is that your plants get the right type of lighting when it’s needed.

To ensure that your plants move forward through the flowering stage into beautiful buds, be sure you educate yourself on the following topics:

  • Calculating the number cannabis plants per light to ensure optimal growth
  • Understanding the light cycle as a critical component of successful budding
  • Determining the required wattage or lumens for your grow area
  • Ensuring the right type of lighting is being used for both growing stages
  • Remaining consistent and disciplined in lighting schedules
  • Ensuring your plants are not getting too much light
  • Recognizing when you can start inducing the flowering process

Once you understand the marijuana light cycle, the fundamentals of growing become easy to grasp. Put lighting to the top of your priority list, and you’ll be sure to yield buds to enjoy.

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