It’s important to understand the meaning of PPFD if you’re growing cannabis. You should also understand how the Daily Light Integral, the PAR light spectrum, and wattage play into growing cannabis plants.
By understanding these phrases you’ll be able to purchase a LED grow light that’s appropriate for your plants’ needs and the amount of grow space you’re using.
Keep reading if you want to know…
- How much PPFD for seedlings? How much PPFD for Veg? How Much PPFD for Flowering? How much PPFD for Autoflowers?
- How to determine the true coverage area of a grow light.
- What LED grow lights offer PPFD values high enough to grow cannabis plants?
- How to measure the PPFD output of your current grow light.
- What to do if your current grow light isn’t strong enough.
What Is PPFD When Talking About Grow Lights?
PPFD, or Photosynthetic Photon Flux Density (PPFD), is the amount of light particles (photons) that hit a surface (plant leaves) in one second.
PPFD is measured in Micromoles (μmols).
You’ll notice on most grow light intensity charts, the PPFD numbers decrease with increased distance from the light. That’s because light loses strength as it travels from source to endpoint (from grow light to plant).
This is why a grow light’s distance above plants can affect how much light the plant receives.
What is Daily Light Integral (DLI) and What DLI Does Cannabis Require?
The daily light integral is when you take those PPFD numbers and add them up over the course of one day. Since PPFD numbers represent photons measured per second, in the course of one day you can end up with millions of micromoles.
1,000,000 μmols = 1 Mole of light.
Fruit or vegetable bearing crops need between 20 and 40 moles of light per day. That’s true for cannabis as well.
The bare minimum total light may be 20 moles per day as described above, but most growers aim for 40 moles, or 40,000,000 umol per day to maximize their yield.
As P.L. Light Systems elegantly puts it,
The production curve from a crop in relation to the amount of moles delivered per day sharply increases between 20-30 moles, begins to really level off between 30-40 moles and over 40 begins to hit a point of diminishing returns for your crop.
So ideally, you’ll want to provide your cannabis plants 40 moles of light if you really want the maximum yield. Above 40 moles per day the additional crop yield will be less and less.
Below I break down how much PPFD is needed for each phase of cannabis growth in order to achieve 20 or 40 micromoles of light per day.
How Much PPFD Does Cannabis Need During The Seedling, Veg and Flowering Growth Phases?
|PPFD During Seedling Phase|
(18 hr daily light cycle):
|PPFD During Vegetative Phase (during whole life for autoflowers)|
(18 hr daily light cycle):
|PPFD During Flowering Phase|
(12 hr daily light cycle):
|Moderate Cannabis Yield (minimum light needed):||200-300 μMols||306 μMols||463 μMols|
|Max Cannabis Yield|
(max light needed):
|200-300 μMols||617 μMols||925 μMols|
If you want to increase the amount of light from 20 to 40 moles per day, resulting in higher yields, you simply need to double the numbers above – 306 PPFD to 617 PPFD during the vegetative phase, and 463 PPFD to 925 PPFD during the flowering phase.
How Much PPFD Is Needed For Cannabis Seedlings?
Cannabis seedlings require only 200-300 PPFD during the first three weeks of life. This level of light can easily be achieved with most grow lights. Make sure you hang the grow light at the correct distance from the seedlings.
How Much PPFD Is Needed During Veg?
To deliver the minimum required 20 moles of light per day, your grow light will need to emit 308 PPFD for 18 hours straight during the cannabis vegetative (veg) growth phase.
To achieve better growth, you can increase the PPFD up to 600 PPFD during vegetative growth. This is assuming 18 hours of light per day.
How Much PPFD Is Needed For Flowering Cannabis?
During the cannabis flowering phase, your grow light will need to deliver 462 PPFD for 12 hours straight in order to achieve 20 moles of light per day. If you want better yields, you can increase the PPFD up to 925.
How Much PPFD When Growing With CO2?
Data from one study actually found the ideal PPFD to be around 1500 micromoles, which is much higher than the 900 mentioned above. But this was done with CO2 supplementation which allows plants to withstand higher heat and more intense light.
I definitely wouldn’t recommend using 1500 micromoles of light unless you have the right environment and growing experience to do so. Also remember when you increase light and temperature that much, your plants will require more water.
How Much PPFD Is Needed For Autoflowers?
If you’re growing autoflowers you should stick to the same PPFD that’s recommended for the vegetative phase. That’s 306 to 617 PPFD for the entire life of the autoflowering cannabis plant.
That’s because autoflowers require 18 hours of light per day for the whole life cycle, instead of switching to 12 hours of light for flowering like photoperiod cannabis plants.
Due to the longer light days, the light doesn’t need to be as intense for autoflowers as it needs to be for photoperiod plants, which have 33% less time to absorb all the light they need during flowering.
How to Read a PPFD Chart:
The image below shows an LED grow light’s PPFD map, which shows PPFD readings at certain spots beneath the grow light. These PPFD maps are often supplied by manufacturers and resellers on the product listing page to give growers an idea of a grow light’s strength.
This example below shows a grow light that’s too weak to grow cannabis outside of a 1×1 foot space.
The PPFD values outside of that 1×1 ft area shown on the chart below are barely even enough to hit a bare minimum 20 moles of light during either growth phase.
So when you’re purchasing a grow light, make sure it’s strong enough. Don’t opt for the most inexpensive model, or you’ll regret it.
Technically you’d have enough light for the vegetative phase if you were growing 1 plant in that 1×1 square foot space. But it’s not enough for any bud sites outside of that small center area.
Unfortunately some grow light manufacturers would actually market this grow light as sufficient for growing cannabis in the entire 3×3 foot area. It’s not!
It’s best to measure your effective coverage based on the area where your light is emitting the bare minimum calculations we described above. If your light isn’t hitting those levels, you need a more powerful grow light.
How To Calculate PPFD:
First I converted the bare minimum amount of moles of light needed per day into micromoles. The bare minimum 20 moles = 20,000,000 micromoles.
Next you need to divide by the amount of time your grow light would be shining on your plants.
During the vegetative phase most people set their grow lights to 18 hours on, and 6 hours off.
So 20,000,000 micromoles / 18(hours) / 60(min per hour) / 60(sec per minute) = 308.6 umols needed to hit 20 moles during an 18 hour vegetative phase lighting period.
20,000,000 micromoles / 12(hours) / 60(min per hour) / 60(sec per minute) = 463 umol of light needed for 12 hours straight to reach 20 moles of daily light during a flowering lighting period.
Repeat this calculation with 40,000,000 micromoles to see how we arrived at the intensity numbers for the 40 mole per day target.
Can I Increase or Decrease the PPFD of My Grow Light?
Yes, and there are two ways to do this. The first is by using the dimmer function, if your grow light has one. The second method would be to simply adjust the hang height of your grow light.
Keep in mind the higher your grow light is hanging, the larger area it covers but the weaker the light will be. The lower your light hangs, the light will be more intense, but it will cover a smaller area.
There’s a law of physics called the Inverse Square Law. This law says if you know the intensity of your grow light at one height, for example 1000 micromoles at 18 inches, then every time that height is doubled, the light intensity is reduced by 75%.
So if your 1000 micromoles of light at 18 inches is raised to 36 inches, the intensity will be reduced to 250 micromoles.
This makes it easy to estimate the intensity of your grow light without using any tools. As long as you know the manufacturer provided (or self measured) intensity at one height, you can know with certainty the intensity at another height.
To read more about grow light hang height, read my article on the topic here: Grow Light Hang Height.
How To Measure PPFD:
If you already own a grow light and you’re concerned whether the PPFD output is high enough, you can measure your grow light’s PPFD output with this PPFD meter:
- Accurately measure the intensity of your grow light.
- Measures full light spectrum that less expensive sensors don’t capture
This is a highly rated PAR meter that’ll tell you if your grow light PPFD is strong enough for your cannabis (or other) plants. The wavelengths of light detected by this meter are the exact wavelengths you want on your plant. It measures 390nm to 690nm wavelengths – the PAR wavelengths.
This Apogee MQ-500 PAR meter is tested to work with LED grow lights as well as high pressure sodium (HPS) and metal halide grow lights (MH).
Unfortunately the Apogee meter seen above is a bit expensive. There’s also another less expensive (but slightly less accurate) method of measuring your grow light intensity – by converting Lux to PPFD.
Some smart phones are able to measure lux because modern smart phones have a light sensor on the screen-side of the phone. If your phone doesn’t have a light sensor (download the Lux meter app and try), or if your phone isn’t measuring properly, you can purchase a Lux meter for next to nothing.
Once you’re able to get a lux reading, take that number over to Waveform Lighting’s Lux to PPFD calculator, select the light source closest to the light you’re using, enter the number of lux you measured, and press calculate!
You can also try the Apogee conversion calculator as well.
Just to reiterate, you’ll want the following PPFD readings on your plants:
20 mole/day (the bare minimum PPFD): 308 umol across whole coverage area for 18hr during the vegetative phase. 462 umol across whole coverage area for 12hr during flowering.
40 mole/day (the maximum PPFD): 617 umol across whole coverage area for 18hr during the vegetative phase. 926 umol across whole coverage area for 12hr during flowering.
If you aren’t getting these minimum levels of PPFD then you need a stronger grow light.
Which Grow Lights Emit Good PPFD For Growing Cannabis?
Many grow light options provide sufficient light strength for growing cannabis. To be safe, I recommend avoiding the least expensive grow light models, and sticking to a major brand like HLG, Viparspectra, Mars Hydro, or another brand vetted and named on my blog.
You can watch the informational video I made on this topic below, or take a look at one of the recommended lights on the list below. I also wrote a separate article on calculating grow light coverage.
Either way, as long as you follow the steps laid out here, you should be able to find yourself a high quality LED grow light!
Below we’ll go through three LED grow lights that provide sufficient PPFD for your marijuana plants (or other plants). For a full list of the best LED grow lights of the year, check out my Best LED Grow Lights post.
These lights below are among the best LED grow lights due to their PPFD capability.
High Yield/High Quality – for 4×4 feet:
- Delivers strong light and strong wattage in a 4×4 foot area.
- Reliable product from Mars Hydro at a great price.
There’s nothing not to love about the Mars Hydro FC-4800. Mars Hydro has always been a brand that offers a great bang for the buck, and that’s why I’m listing the FC-4800 as a great candidate for a 4×4 foot grow space.
If you’re on a budget you can also look into the slightly less expensive Mars Hydro TS-3000
The new bar style design of the FC-4800 spreads light in a uniform manner across your entire 4×4 foot canopy, beating out older boxy and purlple spectrum grow light designs.
Does the FC-4800 provide enough light intensity (PPFD) for your plants?
The answer is yes, and perhaps more importantly, it does so over a considerable coverage area. This is one of the best LED grow lights on the market. I’d like to focus on the images below showing the flowering and vegetative light modes:
Hang this light at the standard 18-inch level, and you’ll receive strong enough PPFD to get your plants in a 4 by 4 foot coverage area through the vegetative and flowering phases.
High Yield/High Quality – for 2×4 feet:
- Modern Design
- Integrates easily with all AC Infinity grow products
- UltraViolet and Infrared LEDs included
- 280w Samsung 301H LEDs perfect for 2×4 foot areas
Does the Ionframe EVO3 grow light provide enough PPFD intensity for your plants?
The Ionframe EVO3 is a very powerful LED grow light that will get you an amazing effective coverage area. The strong light intensity lasts all the way to the edge of the 2×4 foot grow space.
In addition to the great light coverage you’ll have easy integration into the AC Infinity ecosystem of growing products. The Ionframe EVO3 comes with a dimmer, timer, and hanging kit.
What if you own Grow Lights that don’t have strong enough PPFD levels?
Maybe you’re reading this information after you’ve purchased grow lights and you want to improve your situation. If that’s your case, don’t worry, you can easily add PPFD intensity without throwing out your current grow light.
Bloom booster grow lights are an affordable yet effective way to increase PPFD intensity over a small area.
Hitting that minimum 20 moles per day can be tough with a cheap grow light. Even moreso when you switch to the flowering phase and have 6 less hours of light exposure.
The flowering phase is equally important to the end quality of your cannabis harvest. The vegetative phase sets the stage, but red light intensity is directly correlated to the weight of your final harvest.
If your canopy simply needs a PPFD “boost” instead of a whole new light, you can consider one of these grow lights below:
Does Grow Light Wattage Matter?
Figuring out wattage is especially confusing with grow lights. It’s the unfortunate result of marketing tactics companies use to make somewhat similar grow lights look different.
True grow light wattage can be determined by figuring out how many watts your grow light pulls from the outlet.
Metal Halide (MH) and High Pressure Sodium (HPS) grow lights are simple because the watts number you see in the name is typically what’s being pulled from the outlet.
But with many modern LED grow lights, there are two wattage numbers to understand.
The first is the total number of watts in the LED chips themselves. A “2000 watt LED grow light” is probably using two hundred 10 watt LED chips, and that’s why it’s called 2000 watt. But a grow lights’ power is better measured by how much electricity it pulls from the wall, and that’s the second number.
The wattage at the wall for a 2000 watt LED grow light might only be 500 to 600 watts. This is a more accurate number because the light intensity (PPFD) is more closely correlated to wattage at the wall. This explains why the light intensity can vary widely among grow lights all claiming to be xyz number of watts – they’re each consuming varying levels of wattage at the wall.
Photosynthetically Active Radiation (PAR), UV-A and Infrared:
Photosynthetically active radiation (PAR) is the range of light wavelengths that enables photosynthesis in plants. The actual range is from 400 to 700 nanometers (nm). Think of this as the specific band of light used by cannabis plants (and other plants) to grow.
In the 400 nanometer range, light is blue colored. In the 700 nanometer range, light is red colored. Both blue and red are important to plants and both of those ranges are included in grow lights.
The Relevance of Ultraviolet an Infrared Light to Grow Lights and Cannabis Plants:
Ultraviolet and infrared wavelengths are just below and above each end of the 400-700nm PAR spectrum, and are often included in grow lights because they have specific benefits to plants.
Ulraviolet light is divided into three distinct wavelength ranges (UV-A, UV-B, and UV-C). Of those, only UV-A is beneficial to plants. UV-A light measures at 310-400 nanometers.
Ultraviolet light is a damaging wavelength so it’s included in small amounts in grow lights. UV-A light triggers the same defensive proteins in your plants’ as if they were being eaten or attacked by insects, so it makes your plant more resistant to insects as a result.
Perhaps more importantly, UV-A has been shown to increase THC, CBD, and terpene production in cannabis plants.
Infrared (IR) comes in at the top range of the PAR spectrum at 730nm. It can also be damaging if your plants get too much exposure, so only a couple infrared diodes are included in LED grow lights.
The plant protein phytochrome is sensitive to infrared wavelengths. Plants use this protein to regulate the switch from vegetative to flowering phases.
A grow light that incorporates the proper amount of infrared light will result in full stem growth, proper node spacing, and higher yields.
Having a sufficient PPFD intensity for your plants is the most essential factor to consider when buying an LED grow light or setting up a grow space. We tried to make it easy for you to dig through the marketing and figure out what PPFD values you’ll truly need to maximize your cannabis yield (or other plant’s yield).
Aim for a minimum of 20 moles of light per light session. Aim for 40 moles if you want to really maximize yield. Follow these guidelines for obtaining a proper PPFD output and you’ll be growing like a pro.
Once you’re finished choosing your perfect grow light, be sure to head to one of my other helpful home growing guides, such as picking the right size grow tent, choosing a grow tent fan, or understanding nutrients for cannabis.
What is light PAR?
PAR stands for Photosynthetically active radiation. It’s the range of light wavelengths that enables photosynthesis in plants. The actual range is from 400 to 700 nanometers (nm).
What is DLI?
DLI stands for Daily Light Integral. This is the amount of light fruit, vegetable, and cannabis plants need each day to produce fruit vegetables or marijuana. The units of measure for DLI are Moles and micromoles (one millionth of one mole).
What is PPFD?
PPFD stands for Photosynthetic Photon Flux Density. This is the term for the intensity of light at your plant leaves’ surface. The unit of measure for PPFD is micromoles (μMols or uMols)
How much light do cannabis plants need each day?
Cannabis plants need a minimum of 20 moles of light per day, or a maximum of 40 moles per day. Over an 18 hour vegetative light cycle that equates to 306 uMols per second (20 moles per cycle) or 617 uMols per second (40 moles per cycle). Over a 12 hour flowering light cycle, that equates to 463 uMols per second (20 moles per cycle) or 925 uMols per second (40 moles per cycle).
Which color light to grow marijuana plants?
You can grow marijuana plants using purple or white grow lights, as long as the intensity (PPFD/uMols) is strong enough.
Does grow light wattage matter?
Grow light wattage doesn’t matter. Light intensity matters because that determines whether your plants receive enough light to grow or not. Light intensity is measured in moles per day or micromoles per second.