What is PPFD for Growing Cannabis?

Weirdly, growing cannabis is more about controlling the environment and less about controlling the plant. And one of the leading environmental factors you need to focus on while growing an indoor cannabis plant is light.

Give adequate light and your plant will bask in it and grow tall and bushy. On the other hand, it can be disastrous with the wrong type/spectrum of light. Or, if there isn’t enough light. Either way, it’s crucial to measure the light output as per your plant’s growth stage.

But if you are still measuring light intensity with the back of your hand or simply winging it, you are in for a treat. Learn about the most reliable way to measure light — PPFD — and maximize your cannabis yield.

Here is everything you need to know about PPFD for growing cannabis.

Understanding the Basics: What is Photosynthesis?

Before we proceed to what PPFD is and why it is important for cannabis, let’s clear some basics. Plants rely on photosynthesis for their survival — it is a process where plants use the light from the sun, carbon dioxide, and water to create energy that is used by the plant to grow and sustain itself.

The “photo” in photosynthesis refers to the light and “synthesis” refers to the process of new compounds being created.

When sunlight (or artificial light) hits the chlorophyll within the leaves, the chloroplast membranes (thylakoids) convert heat from the light and apply it to carbon dioxide, thereby creating sugars. This process is the Calvin cycle. 

What Kind of Light do Cannabis Plants Need?

Cannabis plants need light, but not any random light will suffice. Like other plants, cannabis has specific light requirements. Plants need light for various crucial processes, like photosynthesis and photomorphogenesis, which are determined by the light’s intensity, spectrum, and duration. Here’s more on the same.

Light Intensity

Light intensity is the first aspect any serious grower needs to think about. If the grow lights are too dim, the plant may experience stunted or stretchy growth, and if the light is too bright, the plant may experience light burn or heat burn and bleaching. 

Light Distance

The distance of light is similar to how light intensity works for cannabis plants. If the lights are too close, your plant may experience bleaching, and if it’s too far, the plant may experience the same symptoms as low light intensity. This is because light falls off with distance, so placing the grow lights at an ideal distance from the plant is crucial.

Light Spectrum

The spectrum or color of light is another crucial aspect because different spectrums of light encourage different growth in the plant. The light that the plant needs is broader than the spectrum we see, i.e., visible light from 380 to 680 nm. 

Cannabis plants contain photoreceptors and pigments that are sensitive to the wavelengths of light. For example, blue light encourages chlorophyll production, root development, and healthy leaf and stem growth, making it ideal for seedlings. On the other hand, red light is ideal for the flowering stage because it encourages the development of high-quality buds. 

Light Duration

For photoperiod cannabis plants, light duration is also important. This is because plants rely on light duration for growth. When the light is supplied for 18 hours, the plant continues in the vegetative stage, and when you switch the lights to the 12/12 cycle, it switches to the flowering stage as a defense mechanism.

The same principle applies when you are growing cannabis outdoors. During the summer months, plants get light for long hours, which encourages vegetative growth, and during the winter months when daylight hours shorten, the plant starts flowering.

Why is this important to understand PPFD? Because PPFD helps you understand light sources better than any other method of light measurement, as you’ll learn below.

How is Light Measured for Cannabis Plants?

There are various ways to measure lights — some require basic mathematical skills whereas others require your phone. Here are the different ways growers measure light and how they compare to PPFD.

Foot Candle

Using a foot candle is one of the most basic ways of measuring light. It is a measurement of light intensity. In short, it measures the brightness of the area that is being illuminated by the light.

One foot-candle is equal to one lumen of light per square foot when measured one foot from the light source.

Foot candles can be measured using a light meter.

Lux

Lux is similar to a foot candle — it measures the amount of light a surface receives. Think of lux as a measurement of light brightness. But unlike foot candles, lux is measured in square meters. So, one foot candle is equal to 10.76 lux.

Since foot candles and lux are so similar, most light meters can measure both.

Lumens

Lumens is another common measurement in the growing community, albeit a little outdated, which measures the amount of light being emitted by a light source. Essentially, the light output of the source.

For example, if you place a light with 50,000 lumens in a grow tent of 1 square meter, the surface will receive 50,000 lux of light. If the same light is in a 2 square meter tent, the surface will receive 25,000 lux.

Note that lumens are not the same as wattage. Wattage is merely a measurement of how much electricity a light source consumes. And given the abundance of LED lights, wattage is no longer the best way to measure light for growing cannabis. 

Some light meters can measure lumens, but if your light meter only measures lux, don’t worry. Multiply lux by the area and the answer will be in lumens. 

PAR and PPF

Photosynthetic active radiation (PAR) refers to the wavelengths of light that plants require for the photosynthesis process. Originally defined by Dr. Keith McCree in the 1970s, PAR can help you understand the volume and type of light that is best suited for your plant’s growth. 

PAR is often used by grow light manufacturers to produce systems that mimic natural light. 

So, when you are purchasing a grow light, you must focus on its PAR rating — how much PAR does the light source produce? 

Measuring PAR is not as easy as measuring lumens or foot candles. This is because you need a PAR meter or a spectroradiometer, which uses a specially calibrated sensor to measure the quantum flux. These meters can sometimes be too expensive for most growers. 

• PPF

PPF (photosynthetic photon flux) is used to define the PAR measurement of a light source. The PPF value of a light source determines the amount of PAR being produced in a second. 

Understanding PPF brings you one step closer to understanding PPFD and why it is the ideal way to measure light for cannabis plants. PPF helps you understand the amount of PAR being produced by your grow lights. 

PPF is measured in micromoles per second (umol/s), where one micromole equals 602 quadrillion photons. 

But PPF is not all perfect. It measures the light produced by a grow light in a second, but it can be misleading depending on your light’s shape and size. It merely tells you how much useful light your panel is producing, not how much your plant will receive. This brings us to the finale — PPFD.

What is PPFD?

PPFD stands for photosynthetic photon flux density, and it refers to the useful light that can be absorbed by the plant. It is the only measurement of its kind that lets you measure how much light your plant’s canopy receives and how much of it is useful.

PPFD tends to be the highest in the center of the light beam, and the farther the canopy is from the light source, the lower the PPFD gets. This measurement is a great way to not only figure out how bright the lights need to be but also how far you need to place the light source from the plant. 

To measure PPFD, you need a spectroradiometer, but you can also convert lux into PPFD, but it can get confusing, especially if you are not so good with numbers. For that, we recommend using lux to PPFD calculators online as they can give you a reliable estimate. 

DLI

Daily Light Integral (DLI) is a measurement that helps you measure PPFD more easily. Essentially, DLI refers to the accumulation of PPFD over 24 hours — hence, it is measured in moles per square meter per day. It measures the plant’s daily intake.

How to Use PPFD to Grow Cannabis?

When you are growing cannabis, understanding light can get a little daunting. But you need not worry as long as you focus on the important aspects of light, like PAR, PPF, and PPFD. These values indicate how much light your plant receives and how much of it is useful for photosynthesis.

Here are some tips on using PPFD to grow cannabis.

Aim for 20 to 40 Moles per Day for Photoperiod Cannabis

For the best results, you should aim for a DLI of 40 moles per day (or 40,000,000 μmol per day). Here is how to calculate the required PPFD for this level of DLI based on the plant’s growth stage.

• Seeding Stage

Cannabis plants in the seedling stage don’t need a lot of light since they can be easily overwhelmed, which can damage their health or stunt their growth. Once they progress into their vegetative stage, they grow more tolerant to intense light.

So, you can stick to a PPFD of around 200 to 300 μmol/m2/s with a light cycle of 18 hours. 

• Vegetative Stage

When the cannabis plant enters the vegetative stage, you should aim for 20 moles of light per day for optimal growth. For this, your grow lights need an output of 308 μmol per second in an 18/6 light cycle.

During this stage, the PPFD levels on the canopy should be 300 to 500 μmol/m2/s. Some growers even raise the PPFD to 600, but it is not usually required because your plant is not producing any buds at this point.

If you are growing your plant in a 20/4 light cycle during this stage, you should aim for slightly lower PPFD levels, around 700 to 800, to prevent your plant from experiencing light burns. 

• Flowering Stage

When your cannabis plant enters the flowering stage, the grow lights must put out around 462 μmol per second in a 12/12 light cycle so that it can produce 20 moles of light per day.

During this stage, you should aim for a PPFD of 1000 μmol/m2/s.

Remember, it could be different for you, depending on your plant’s strain and genetics. You may need to give more PPFD, even up to 1500, especially if you are supplementing carbon dioxide to your plant. 

Keep PPFD Levels Lower for Autoflowering Plants

If you are growing autoflowering cannabis, you need to keep the PPFD levels low compared to photoperiod plants, as mentioned above. This is because autoflowering cannabis plants grow with 20 hours of light (usually), which is a lot of light. You don’t want the DLI levels to get so high that your plant starts experiencing bleaching or light burns.

When growing autoflowering strains in a long light cycle, aim for PPFD levels below 700. In some cases, an experienced autoflowering plant can even handle PPFD levels of 1000 during the end of the bloom, but work your way up to it and turn down the PPFD levels as soon as you see even the slightest hint of excess light exposure on your plant. 

Increase PPFD If Supplementing CO2

One of the ways experienced cannabis growers improve their yield is by supplementing extra carbon dioxide to the plant. This is a terrific way to encourage bud development, but this also makes the plant absorb more light since it is growing faster and bigger.

So, when using a CO2 system in your grow room, you should increase the PPFD level to ensure your plant does not experience any signs of light deprivation or CO2 poisoning. 

Usually, cannabis plants with supplemental CO2 require a PPFD of around 800 to 1000 μmol/m2/s during the vegging stage and 1000 to 1500 μmol/m2/s during bloom. 

Learn How to Read a PPFD Chart

When you purchase a LED grow light, especially a high-quality one, it will come with a PPFD chart that shows light readings at various spots under the light source. 

Therefore, you should choose a light that has adequate output in an area big enough to fit your plant’s canopy. For instance, if your plant will grow in a 1 square foot area, the PPFD chart should show adequate light in the 1 square foot area. Usually, weaker light sources will only show adequate PPFD in the center with a significant light fall around. 

• Calculate Light Requirement

First, you need to convert the minimum light requirement from moles to micromoles. For instance, for a vegetating cannabis plant, the light requirement is usually 20 moles, which turns to 20,000,000 micromoles.

The next step is to divide the light requirement in micromoles by the duration of the light cycle. That is, 20,000,000 micromoles / 18 hours / 60 minutes per hour / 60 seconds per minute, which equals 308.6 μmol. This is how much light an average vegetating cannabis plant needs to hit 20 moles during a single 18-hour light cycle.

During the flowering stage, if your goal is to reach 20 moles of light per day in a 12/12 light cycle, you would need 463 μmol of light.

Compare the calculated light requirement with the PPFD chart of a grow light to see whether it is adequate for your plant or not.

Measure the PPFD of the Light Properly

A PPFD chart of the light only gives you an estimate, but if you want to be sure of the PPFD levels for your plant, you can measure the PPFD manually with a PPFD meter.

A PPFD reader is a PAR meter that tells you how much PPFD of light it receives at a specific point where it is placed. So, place the meter just above your canopy and it will tell you the PPFD levels of that point. 

PPFD meters are accurate but can get expensive. Instead, you can even use your smartphone! Most smartphones have a light sensor (that measures the ambient light to adjust screen brightness). Simply download a lux meter on your phone and use it to measure the lux. 

Once you have the lux measurement in your phone, use an online lux to PPFD calculator to convert it to PPFD. 

Choose the Best Lights for PPFD Output

The kind of lights you use also dictate the quality of light to some extent. Here are the most common types of grow lights and what kind of PPFD levels they produce:

• HPS Grow Lights

HPS lights are cheap and have been the most popular type of grow light until a few years ago. But HPS lights consume a lot of electricity. So, if you are looking for a grow light with high PPFD levels, you should avoid using HPS lights because they can raise your energy bills significantly.

That’s not the only reason why HPS lights are not the best option for high PPFD — HPS lights heat up a lot, which can increase your grow room's temperature and they produce yellow or orange light, which is not so useful for cannabis plants. 

• CMH Lights

CMH lights are popular in some growing circles because they come in blue and red-dominant variants and are a lot more efficient than HPS lights. So, if you want high PPFD output, you can choose CMH lights.

Note that your options are limited and you may have to replace the lights with every growth stage of the plant since you can’t control their color spectrum.

• LED Lights

Controlling the color spectrum of the light brings us to LED lights — the best grow lights for cannabis. LED is popular as it can offer high PPFD levels without burning a hole in your pocket or plants (literally). However, the initial investment can be a bit high.

If you want to use PPFD to grow cannabis, your best option is to use LED lights. You can not only change their color spectrum but also easily control their light output with the dimmer function (only available in high-quality LED lights).

Increase or Decrease the PPFD of Your Grow Light

Sometimes, you may either have to increase the PPFD or lower it, for multiple reasons. Perhaps your grow lights are not strong enough or your plant grew bigger than expected. And you can do this in a couple of ways.

One, you can use the dimmer function on the grow lights, if present, to control the light output.

Two, you can simply move the light closer or farther away from the canopy of the plant by adjusting the light source’s height. When you increase the height, the PPFD levels will decrease, and when you decrease the height, the PPFD levels will increase. 

In fact, there’s even a physics law regarding light height and intensity. For example, if your light is 18 inches away with an intensity of 1000 micromoles, every time you double the height, the light intensity will reduce by 75%. At 36 inches, the light intensity will only be 250 micromoles. 

Summary: What is PPFD for Growing Cannabis?

PPFD is the key to unlocking the formula for the best light output for your cannabis plant. Without PPFD, figuring out the light panel seems more like guesswork and less like science. So, if you want to optimize your plant’s growth by ensuring it gets enough light at all times, you need to focus on PPFD, not other old-school light measurement units.

PPFD for growing cannabis is beneficial, but there are a lot more aspects to maximizing your cannabis yield. Stay tuned to know more about how to improve your cannabis yields.