When browsing grow lights, you’re bombarded with confusing acronyms and power metrics. Photosynthetically Active Radiation (PAR), Daily Light Integral (DLI) and Photosynthetic Photon Flux Density (PPFD) are the three most common (and confusing) phrases used by grow light manufacturers and resellers.
It’s important to understand cannabis plant light requirements if you plan to grow the highest yielding cannabis plants (or other plants) that you can.
By understanding these fairly simple cannabis light requirements 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.
I’ll answer questions such as:
- How many moles/micromoles of light do my cannabis plants need during each growth phase?
- How to determine the true effective coverage of an LED grow light
- What LED grow lights offer PPFD values high enough to satisfy my plants?
- How to measure the PPFD output of your current grow light
- What to do if your current grow light isn’t strong enough
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 light color requirements for your cannabis plants. 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 and 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.
Ultraviolet 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.
What Is PPFD As It Relates To Grow Lights?
There are two other phrases related to photosynthetically active radiation (PAR) that you need to understand when growing cannabis. Those phrases are Photosynthetic Photon Flux Density (PPFD) and Micromoles (μmols). This is the light intensity requirements for cannabis plants.
PPFD is the amount of light photons that hit a surface (your plant canopy) in one second. It’s measured in units called micromoles (μmols). Simply speaking – it’s the strength of the light that reaches your plants.
The light loses strength as it travels from source to endpoint (from grow light to plant). You’ll notice on most grow light intensity charts, the PPFD numbers decrease with increased distance from the light.
That’s why paying attention to your grow light hanging height is so important when growing cannabis.
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 levels off.
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 Light (PPFD) Do Cannabis Plants Need During Seedling, Veg and Flowering?
|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|
Cannabis seedlings require only 200-300 PPFD per second during the first three weeks of life.
To deliver 20 moles of light per day to your plants, your grow light will need to put 308 umol per second on your canopy for 18 hours straight during the cannabis vegetative (veg) phase.
During the cannabis flowering phase, the grow light will need to deliver 462 umol per second for 12 hours straight in order to achieve 20 moles of light per day.
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 – 308 PPFD to 616 PPFD during the vegetative phase, and 463 PPFD to 925 PPFD during the flowering phase.
How Much Light (PPFD) Do Autoflowering Cannabis Plants (Autoflowers) Need?
If you’re growing autoflowers you should stick to the recommended vegetative phase light intensity of 306-617 PPFD for the whole 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 non-autoflowering 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 charts, or 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 1x1 foot space.
The PPFD values outside of that 1x1 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 1x1 square foot space. But it’s not enough for that plant once flowering phase rolls around. Unfortunately some grow light manufacturers would actually market this grow light as sufficient for growing cannabis in a 3x3 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 Did I Calculate These PPFD Values (Grow Light Strength)?
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 Intensity (PPFD) of My Current 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 Do I Measure My Grow Light Intensity (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:
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 Provide Sufficient PPFD Output?
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 4x4 feet:
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 4x4 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 4x4 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 2x4 feet:
Does the Green Sunshine ES300 grow light provide enough PPFD intensity for your plants?
The Green Sunshine ES300 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 2x4 foot grow space.
Owners of this light absolutely love it, and to paraphrase some of their reviews from Green Sunshine’s site, a grower with 10 years of growing experience said the ES300 resulted in the fastest growth they’ve ever seen. They also complimented the great customer service from the Green Sunshine Company.
There are many other reviews from people who switched from cheaper LED grow light models like Viparspectra or Meizhi. They say the difference is night and day.
If you need an strong and efficient grow light that truly covers your entire space, Green Sunshine’s Electric Sky 300 (ES300) is the way to go.
High Yield/Budget Option – for 2x4 feet:
These new Mars Hydro TSL-2000 watt LED grow lights will spread sufficient PAR light intensity onto your 2x4 canopy. They also happen to be my budget choice because they’re so damn affordable.
I own one and you can read my full review for it here.
The only difference is you’ll be trading lower price for more of a moderate coverage area and intensity compared to lights emitting the maximum light intensity.
I think you can throw two of these units in a 4x4 and have a very solid setup.
The quantum board design features a sleek, fanless, lightweight, and energy efficient setup at an entry level price. This light is hard to beat on value.
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:
This red wavelength booster light will add considerable PPFD value boost for a 2 foot space. People who bought this bloom booster left some pretty enthusiastic reviews, including that it noticably enhanced the buds and their plants love it.
That’s right, I’m recommending the TS-600 Mars Hydro for use as a bloom boosting LED light to bring your PPFD values up to sufficient levels. Why does this light serve as a perfect PPFD booster?
The cost of this light is quite low, yet the PPFD values up to 1.5 square feet are impressive enough to considerably boost light intensity in your existing grow space.
The white light spectrum plus infrared will boost any phase of plant growth. Can’t go wrong with a few of these to brighten up your corners!
What’s the Deal with Grow Light Wattage?
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.
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.