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The Fridgegrow – closed loop growing with co2 support

Weed in a fridge

Flowering cannabis in a fridge
Cannabis that was grown in a fridge using the fridge grow

So when people think about starting to grow weed, they usually look at it from the beneficial side: It would cost less than the shady park vendor or the dispensary in nearly every case, especially when quality is taken into account. The overall quality is more controllable. Plus: In a world with PGRs, terpene spray, brix or artifical cannabinoids its still recommended by me to rather grow at home, than buying stuff simply because the packaging looks shiny.

The fridgegrow controller tries to fill a gap in the homegrow segment. Its not a fullblown all-in-one closet where you have little to no options in terms of light or nutrition, but its also not a tent, where you usually have to deal with environmental values as they come. When the door to the fridge is closed, its not possible to exchange air or even water from inside the fridge. (disclaimer: this is only half-true, but for simplicity we will assume this for that article.) You will have to bring some ingenuity into the operation!

The fridgegrow setup uses a few key elements to keep the environmental values in check:

The fridge

A fridge is able to lower the temperature inside its volume, without exchanging air inside the fridge. Lowering the temperature lowers the possible maximum of moisture the air can hold, water will condense on the cold backwall of the fridge and trickle down…
The fridges total power has to “work” against the grow lights total power, and therefore should be able to “outfreeze” the heat.
The fridge is not included in the system. I recommend to buy a beverage fridge for commercial usage with a closed front. You want to keep the light out. Make sure, that the fridge has as much power as you can get. Its easier to hold the values this way. Heres a fridge from amazon with a clear front, that has to be masked first.

A small heating element

A heater is able to work against the fridge to keep the relative humidity up. In a perfectly dialed in fridgegrow setup the heater will almost never run during the light-on phases, because light and fridge should cancel each other out AND holding onto certain humidity levels while doing so. This is also dependend on the total amount of water inside the fridge chamber.

The perfect light – crescience modular fluXengine LED built

There is no reasoning here for anything else than a high efficient LED setup. No asian amazon light or HPS light will be acceptable if your plan is to grow cannabis in here. Make sure you are able to remove the driver and try to place it outside the fridge. One thing I love about that built is, that I get to chose the light, and its not prebuilt in here!

LED grow light and flowering cannabis in a fridge
Crescience DIY LED setup -3x fluXengine board-style repping the Samsung LM301B chips aswell as a Meanwell – ELG-150-24A

For my fridge I have chosen a modular LED solution by Crescience. Crescience is known for their modularity and compatibilty to standardsized aluminium-framing. Its super-easy to build and you can actually achieve tailored photon-outputs for every system ever.
I am running 3 of their fluxEngines (link goes to the updated lm301h version) powered by a Meanwell ELG-150-24-A Driver.

Some thoughts about the fridge-light-interaction

The principle of continuity says: What goes in has to come out. This is true for everything that has a flow to it. In the case of the fridge one puts energy as heat into the system via the grow light and the heating element. The vapor-compression element on the back of the fridge is able to get heat out of the system and will transport it away through the heatpipes on the back of the fridge. Thats also the reason a fridge is hot in the back and cold inside. Our limiting factor for our light is the fridges compressor efficiency. If our fridge cant transport the heat away through its compressor the inside of the fridge will heat up endlessly, which will result in darkness since the controller will always try to save the plants, no matter if its “daytime” or “nighttime”. This means we need the most efficient and cool running light setup in here. A commercial beverage fridge for business will have a power draw between 90 and 150W from the wall. The light you use should draw a bit less than that.

Air circulation

Small fans directed at the back of the fridge are also an important element in this setup. It keeps the watercycle running smooth and steady. I have chosen Arctic 120mm PC-Fans . I connected them via a 4-way 3-pin fan-connector in parallel and a 12V/2A powersupply. If you are not sure about that, ask an electrician how to wire it.

fans in front of a fridge grow
Arctic PC-Coolers keep the air circulation going in the fridgegrow.

vertical nets or other vertical scrog systems

Keeps your growth in check! I had a few leafs touching the backwall of the fridge keeping the water from dripping down. This results in considerable pieces of ice in there, although its almost 30°C while the lights are on! Ice is bad. It binds water and takes it out of the waterloop.
I gave my plants only 4 glasses of water – in 67 days of growing. And this is a problem.

Scrog in a fridge
Basket-ScrOG system – the grow-space is very limited, so you have to expose as much buds as possible to direct light.

Feeding plants in a closed loop system

While a small windowsill plant doesnt need much to stay green and vital, cannabis under a blasting sun-replacement will transpire significantly more and will therefore have a much increased need of moisture. The trick is to catch the condensing droplets from the back of fridge and guide them back into the pots. Even if youre not able to do that perfectly, the water will still stay in the system, because of the fridges isolation. All you need to do is to close the small hole in the lower back of the fridge, where drops usually get vaporized through the compressor on the back. We dont want that, so we close the loop with a sheet of metal, guiding it back into the pots. This also has disadvantages: Im used to give diluted water-nutrition solution to my plants. Bio Bizz usually is diluted in a way that about 1-5ml of nutritional concentrate gets mixed with one liter of water. This is of course only true for system where the soil can dry out a bit and this wont happen in a fridge setup. You have to put in higher concentration (same amount of nutrition but on a glass of water) or you go for solid nutrition like greenhouse feeding or pre-grow formulas like Biotabs Organics.

I hold my plants in 3 gal smart-pots. Pathological amounts of greenhouse bio bloom and some biobizz was given, but i wasnt really able to conquer the ramping deficiencies my plants developed.

nutrient defincency on cannabis plant in fridge
Leaf discoloration and washed out leaf-tips: this plant was craving for nutrition.

Controlling CO2 – hacking sodamax/sodastream bottles

This is the first setup I was able to make use of extra CO2 inside a grow room. You first need to buy a CO2 bottle, that gets usually used for gassing up beverages mixed together in the kitchen. This bottle is under a standard pressure of about 6 Bar. The first connection has to go to a pressure-bottle adaptor. This adaptor is then connected to a pressure-reducer unit. The reason for that is the next element: its the actual CO2 valve that lets CO2 flow into the system: The CO2-magnet-valve that can be controlled electrical by the fridge-grow controller.

pressure limiter system
CO2 pressure-reducer: left you can see that the bottle-pressure is below 1BAR, so it will have to be replaced soon! On the left is the pressure measured, that lies between the magnetic-valve and the pressure-reducer: theres still CO2 available! But a second bottle would come in handy…

The Fridgegrow-Controller

This is the heart of the system. Its basically a mixture between a server (with a UI to check data and setup the environment ) and a controller. In principle the controller is just turning bluetooth-powerplugs on and off whenever temperature, humidity needs to be dialed in, lights to be turned on and off, or to open and close the CO2 valve. The system is pretty much plug and play and can be setup even if youre not really techsavvy.

fridgegrow controller
The fridgegrow-controller . In the back you can see the droplets of water that will eventually flow back into the soil. On the right-hand-side of the controller is the casing for the temperture and humidity sensor.

The controller will react to different parameters that are more or less dependent on each other. It will find a more or less stable cycle dependent on set values and the amount of water in the system.


The fridge reacts to two central reading: humidity and temperature. It will turn ON when ever the current humidity rises above the set value AND the temperature is high enough.
In this case that means that the temperature is at its set level or at max. 2x below the set hysteresis. A hystersis-value is the margin a value can go above or beyond.
So if you setup the fridge to hold a temperature between 24°C and 28°C (Thats 26°C with a hysteresis of 2°C) and relative humidity between 65% and 75% (set 70% with a hysteresis of 5%) the fridge will only start running when the humidity exceeds 75% while the fridge is between 22°C and 26°C.
The fridge will turn off when values below 70% relative humidity are reached or the temp drops below 22°C which is 2x the hysteresis. You can see easily that reaching the right relative humidity is dependent on the amount of water in the system. The fridge will never get below certain values if theres too much water to condensate.

Light control

This one seems easy: It just switches on and off at set-times.
But the system is also inhibitng fail-save measurements to save the plant. And heres the tricky part. The fridge can actually overheat while its trying to find a cycle, and thats a nono for the controller. It will turn of any heatsource that exceeds the global temperature limit. And thats the set-tempreature plus 2x the set-hysteresis. So in a running system with a 26°C ± 2°C the light will go out when ever the system breaks the 30°C limit to save the plants.
This can actually happen when theres not enough water around to keep humidity high: the heater will run longer to increase the amount of water in the air and will hit the temp-limit before the humidity levels can be reached. This will perodically lead to on/off cycles of the lights! So be aware of your cycles!

CO2 and Heating

CO2 is kind of entangled from the rest. The gas wont interfere with temperature and humidity and will be dialed in by the controller according to the set value and hysteresis.
The heater is your last bastion to figure out your values/moisture levels in the system. If the controller thinks, that a the heater needs to run WHILE the lights are on you have one of two problems:You need less water in the system!

The heater only reacts to temperature, so it will only go on, while the temperature drops too low. This can only be the case when the fridge runs too long against the light, trying to reduce the humidity. If you can dial up the light to create more evaporation the fridge will have a harder time reducing the temps, keeping the heater off.
If theres no use to it, and the humidity levels wont fall of with a constantly running fridge and a constantly running light, you have too much water in the system, so try to remove some. But this is basically the equilibium you want to achive: Running heater only when the lights are off, trying to get a stable water-humidity-temp equilibrium going.

The fridgegrow-app

One last word about the fridgegrow-app. To set everything up you need to be connected to the controller via the app or the controllers local webiste. If you dont connect the controller to your local wifi (which i recommend for safety concerns) you have to use the internal wifi from the controller. The controller guides you through the first setup of the remote-power plugs and is pretty easy to use with a standard UI and graphs of the sensor-data. Perfect for a nerd-grower like I am! In the graphic below you can see that i turned on three graphs, temp, hum and CO2. In the graphic you can see the exact moment when I opened the fridge at 19:15 or 7:15pm. the humidity and CO2 drops significantly as I pulled everything out of it, when I opened the door. The CO2 controller will turn on immediatly while you can also see the heating element tries to work against the fridge to keep the temp low.

screenshot of fridgegrow app
View through the fridgegrow app – setup your controller, check your values – remotely

Wrap-up: Should you buy a fridge grow?

For the beginners: Yes, its possible to just watch and learn with this system. Its really an accomplishment and a good amount of work to have it running stable for the first time, but you will be given a system were you have full control of every aspect of a run, while still be able to hold smell and noise in place.

Its for the nerd who grew weed in the past, but maybe dont want to have the whole fan/tent etc. infrastructure in his family home anymore. Maybe you want to get most of a limited space? This system got you definetly covered. I will try to harvest 100g dry regurlary out of there, and this should be enought to supply a single consumer out there and I am convinced that this system is working since I got my first 60g of Humboldt Seeds OGKZ out of there and it was a more than decent smoking experience. Welcome to the 21 century growers!

Production cost of homegrown marihuana

stock photo of a power bill

If you are entrigued by the costs of a grow operation, you might be surprised how little it is in comparison to street weed or cannabis from a dispensary!

After the initial investment in a small or a bit bigger setup this is your running cost of a marihuana operation.

Cost factor I: The power bill

Power has the biggest impact on your bill over the course of a grow. Your light, ventilation and air circulation are all ramping up that bill.
To determine the cost of a run (one grow operation from seed to weed) you have to consider the different stages of the grow. 

I will determine the cost of my current run as an example:

Lemon zkittle cannabis plant
For a single plant grow you have to spend more time in the vegetative period. This costs more money!

Im using a light that draws about 310 W from the wall, a fan for air circulation that is worth 40 W and a single outlet fan for 10 W. Im also running  two Raspberry Pi minicomputers in there taking the pictures for my timelapse which will add up to about 10 W in total. (might be less, since they should be on standby most of the time)

The light was on 18 hrs a day for 51 days (vegetative state) and will be on for roughly 63 days for 12 hrs a day. I am paying about 25 ct for a kWh right now.

(51 days  · 18 hrs/day + 63  days· 12 hrs/day) · 0.31 kW · 0.25 €  / kWh = 129.74 €

For the fans and the Raspberry Pis I have to calculate a 24 hrs/day running schedule:

(114 days · 24 hrs/day) · 0.06kW · 0.25 € / kWh = 41.04 €

So in total the power bill will be at an extra 170.78 € after a single run or 44.86 € per month if I consider this a 3.8 month run. 

Cost factor II: not reusable items

If you use seeds, soil, bottled or packed nutrition or any other item that you have to rebuy after a grow, it will go into the final calculation.

In my case I have used 2x 15l packs of Kanamu Pacha Hortisol coming for 43.80 € and a single Dutch Passion Lemon Zkittle Seed, that would cost about 15 € if bought in a pack of three. I am also using Greenhouse Feeding Bio Nutrition that costs 19.95 €. In total I am looking at 78.75 € for the items I had to buy to start this run.

What do I pay for my weed?

In the end it all boils down to efficieny. With my current setup I harvested about 300 gr of dry material in the last two run, and im not expecting less in this one. 

In the end I will have invested 249.53 € in 300 gr of high quality cannabis resulting in a price of 83 ct / gr

Cannabis Critical flowering
Critical! Good weed does not have to be expensive!

How to save even more money!

Here are some suggestions to shrink your budget even more:

  • Use LED lights. Preferably passivly cooled ones with high efficiancy values like COBs or Quantum Boards. AND you save moneyfor NOT needing an AC unit in your tent. 
  • Reuse your soil. Bio Bizz for example states that you can easily reuse their soil with a smell addendum of fresh one.
  • Use clones. Save money on seeds by using cuts from a mother plant. Remember, that maintaining a mother can also be pretty expensive. Suitable for bigger operations.
  • Use Autoflowers. Autoflowering cannabis has a much shorter lifespan than photoperiods. You can harvest more often. But be careful: A light cycle of 18/6 or 20/4 can impact your powerbill surprisingly hard!
  • Use the Sea of Green grow style! This grow style requires you to take care for a lot of plants at once. The advantage: you shorten the vegetative grow time significantly (usually 2-4 weeks) which saves moneyon power. Disadvantage: You will need to spend more money on soil and seeds.

COB LED Lights Simplified

Build your own COB LED light – the absolute basics

DIY COB light – 310 W optimal for a 9 ft² space

Cool, so you are interested in building your own light? Here is what you need to know! We dive a bit into LED tech and how to design a simple circuit, that will enable you to choose the parts for your grow LED to your individual needs. You will also learn about the importance of reading datasheets.

Disclaimer: Please please please DO NOT try to connect any selfbuilt circuits to your electrical circuit at home without being 100 % sure that what you build is OKAY. It is easy to kill yourself in the process if you do anything wrong.

Why even consider building a LED grow light yourself?!

Good grow LED tech is expensive. With 2-4 $ per Watt you can end up paying thousands of dollars on the best lights.
The advantage of this is mostly, that you get a finished plug&play module with things like warranty and product testing. In the case of DIY LED Kits sold by Horticulture Lighting Group, this is not the case. The sets you buy there are great, but its way cheaper to build them on your own.

Comapnies like Timber Growlights or Pro Emit are selling complete builds out of these items. Yes, they are companies, and therefore need to make a profit. But with a little insight and help, you will be able to but together a high quality LED grow light yourself for about 1$ per Watt or even less.

Why LEDs are superior

This is the future.

Wiliam Texier, “Hydroponics for everyone”, Chapter about LEDs

LED tech is advancing at an enormous speed. Companies producing LED light fixtures have to deal with this and need to apply very fast, to not fall behind. Almost every other month there is a new chip out there crushing all numbers that were there before.
LED light is semiconductor technology. Without going any way into solid state physics you can try to imagine that electrical energy gets used to push electrons into another state where they can recombine to a energetic more favorable position. The excess energy  of this recombination is then emitted as light. This is a very efficient process. In 2006 the first LED by Nichia with a light yield of 150 lm/W, equal to the output of HPS lamps. 

Disclaimer: Lumen per Watt is actually not a good way to compare LED and HPS lights. LEDs and HPS have different spectra and Lumen favors lights with more green light, because it weights the spectra with the so called luminosity function. The high output in the yellow/red range of a HPS light is not favored as much as a full spectrum white light from a 3500K LED.

Since then alot has happened. Current tech pushes the the light yield way beyond the 200 lm/W levels for Quantum Board chips (LM301b or LM561c). Current record holder is Cree with 303 lm/W with a 5150K (Cold white) LED chip. (cool tech, but not suitable for growing)

The spectrum of a warm white LED at 3000K or 3500K is also way better to reproduce the McCree intensity distribution to induce photosynthesis than HPS lights. 
HPS lights also tend to heat up alot. Modern Quantum Boards as well as COB lights can be cooled passively with heatsinks. That saves even more power, since air conditioning could potentially cost alot of money in an operation if the room gets too hot.

spectral properties of different lights
A grow light should reproduce the McCree curve as  accurately as possible. HPS light has a spectrum that is fairly limited to yellow and red wavelengths (upper right), while LED light is able to distribute energy to all the wavelengths needed (lower three spectra).

In comparison to HPS, CMH or LEC light, high quality LED systems are still pretty expensive. In the long run, a high quality system will be much superior in that regard, compared to the rest.

The Current Circuit

Current flows when they its enabled to do so by the existence of voltage. LEDs need a distinct workpoint, a combination of direct voltage and current to work efficiently.

The power plug in your house gives usually gives you between 110 V (U.S.) and 230 V (EU) and up to 12 A to 16 A of current before the fuse will have a serious word with you. Short circuits are just a unhinged connection between two poles creating a big ass current peak. The fuse will jump out and the light goes of.

So the first thing you will need is a driver.

Transforming electricity with a driver

To transform the alternating voltage to a usable direct voltage you need (over simplifed)three things: A transformator , capacitor and a rectifier. All of these things are included in a driver. Since you manipulate a form of energy this driver will never be 100% efficient. Good drivers will work with an efficiency of more than 90% though.

The Transformator will change the value of the incoming voltage, while the rectifier is used to reverse the part of the waveform that has the wrong signum. The capacitor is used to smooth out the curve and to fine tune the process. There is much more than this happening in a LED driver, but this is the basic principle. 

Schematic of a rectifier
Simplified circuit: The pink waveform is already rectified. The capacitor C smoothes out the form further to create the rippled curve above. If you use more than one capacitor, you can ease this out even further. 

Important: NEVER connect LEDs to the power plug of your home without a driver. Could end lethal.

The right combination of Drivers and COB modules

Okay, now that we can safely convert our electrical input, we can now proceed and choose the right combination of light modules and drivers.
First you should determine the amount of power you will need in the end. 

Since you will build an awesome high quality COB light you will need about 35 Watt of power per squarefoot.

Example one: A 3’x3′ tent light

For this setup we need approximately 315 Watt of high quality COB light. (3 x 3=9, 9 x 35 = 315)

The BXRC-35E10K0-D-73-SE is the perfect canditate for that. It will run with a current of 2.1 A at a voltage of 36.6 V. Power is determined by multiplying these two values, resulting in 76.86 W for each COB. So running four of them will yield 307.84 W, which is close enough to the desired 315 W we need.
The COB delivers hell of a light!
Since we know, that lumen are not really useful to determine light quality, its an okay ballpark number to compare lights with similiar spectra (3500 K).
This light runs at 147lm/W @ 2.1A and 36.6V

COB efficiency

One quick notice: A COB runs less effecient the more current you push through it. You will get a better lm/W value if you run just 1.4 A. This will result in less power and less absolute photon output, but a cooler COB and higher efficiency. So if you plan to build more efficient, you want to consider to use more COBs at less current.

Now that we got our lights, we need a driver, that can run them properly. For this purpose we need to plan the configuration of the lights we are using. Lets assume we connect all 4 COBs in series. In 9th grade in school you learn about Kirchhoff’s circuirt laws, that tells you: The current in a series connection is constant. On the other hand the voltages will have to be added together.
Conclusion: We need a driver with an output of 2.1 A and 146.4 V.

series connection of 4 cobs
Series connection of 4 COBs

The most common drivers on the market right now are from MeanWell.
The model HLG-320H-C2100B will deliver a constant current of 2100 mA = 2.1A and has an output voltage ranging from 76 V to 152 V which is quite perfect for our cause, because we calculated with 146.4 V!
The driver costs about $100 plus $80 for the COBs. Not a bad starting point for a 310 W light!
You will need some more items before you plug in your light and let it shine on your ladies. (Build guide coming soon)

Grow Lights – Theory of Light

In this chapter you will learn about the different aspects of light in gardening. It clears up all the different keywords and hopefully all technical terms that occur when talking about light.

Definition of Light

“Light” is actually defined as a thin part of the electromagnetic spectrum. Exactly the one we can see with our human eyes. 

Light can be divided into parts called photons, which act in theory as light particles. According to physics you can treat light as a particle and a wave simultaneously. We will need both ways in this chapter!

Short wavelenghts means high frequency. Higher frequency correlates to higher energy per light particle. Long radio waves, which can expand to several meters in wavelength have a very low energy per photon.

Electromagnetic Spectrum with the visible part in the middle. From: Wikipedia

Defining a light source

If you want to learn about lightsources such as Light Emitting Diodes (LEDs), High-Pressure Sodium lamps (HPS), Compact Fluorescent lamp (CFL) or Ceramic Discharge Metal Halide (CMH) you will not only have to look at the specs each of these lamps have, you also have to look at the theory on which the light is created. Every single light source used for growing has its advantages and disadvantages.

All light sources used for growing work with a the same rough principle:

Turn electrical energy (input) into light (output)

The devil is in the details. As always. What I will describe now is awful lot to learn and difficult to have alook through. But its worth the reading, and after this you will be more able to decide wether a light is “good” or “bad” for your cause.

What light does a plant need?

Plants have two main sources of energy: The root environment (soil, water, aeroponics, nutrition…) and the light that shines on its leafs. The process in which the leafs turn light energy to “food” is called photosynthesis.

Not every light that a plant is exposed to, is usable to do photosynthesis. A human eye can basically see every color between 390 nm and 700 nm, but is blind for colors that have lower wavelengths (ultra-violet) or higher ones (infrared). Humans are most sensitive for the color green. As opposed to plants. Keep that in mind when you get to the unit section

Plants have a similar spectrum, but see actualy green light the least. Thats obvious, since leafs are commonly green, so they reflect the green parts of the spectrum. The three ingredients in a leaf that are responsible for the reaction are Chlorophyll-a and Chlorophyll-b as well as Carotenoids.

Bottom: Spectrum of photosynthesis rate –  There is a big dip in the middle.

Top: If you build the sum of the three top spectra, and take into account, that the different parts that are responsible for the photosynthesis are available in unequally you get the lower spectrum. Chlorophyll-a is obviously the main driver for the process.

If you look at the lower spectrum in the picture above you can get a sense of what kind light a plant likes to receive: two broad bands of light between 400 nm and 500 nm and between 650 nm and 700 nm. One is in a blueish region, whilst the other is rather red. 

The Relative Quantum Efficiency

The three molecules (molecule groups) Chlorophyll-a ,Chlorophyll-b and Carotenoids are not the only driving factor of photosynthesis. In of the most cited articels from K.J. McCree (1972) he measured how the plants react to light of different wavelengths. He found, that the area around the green light (550 nm) is actually well used by the plants. Not as good as blue or red light, but up to significant margin! So green light is not inherently bad for plant growths.

Mc Cree curve showing the photosynthetic efficiency of green leaf material. The experiment is from 1972, so it might be time to reevaluate. In this graphic we see the impact of  red and blue lights in growing, but you can also imagine that green light is also important.

Measuring light – A jungle of units

Now that we have the favourite colors of a plants light dinner, we need to figure out, how much they need! This is determined by the amount of light a source can emit. This amount can be described with various units of measurement. Some of them make sense for growing (PAR, PPFD), others don’t (lm, lux), and the rest is not normalized and/or very subjective in a way that it is depending on the experimental setup one uses to measure it.(also PAR/PPFD).

The following part will enlighten you about the different units of measurements scientist and light distributors characterize light.
Note: I tried to sort the units in order if importance, starting with the most important one.

PPF and PPFD – Photosynthetic Flux
Photosynthetic Photon Flux Density

Lets dive right into things. So photosynthesis is a quantum process. Just think about it this way: A leaf is plastered with billions of little molecule-machines trying to catch light particles in the exact right energy range. The more machines are hit per timeframe the better. So PPFD counts the amount of photons (µmol) that hit a certain area (m²) in our timeframe (s). This results in the unit of measurement:  [PPFD] = µmol/m²/s

A PPFD value of 900-1000 µmol/m²/s is needed to saturate a canopy area of plants if you use a warm white spectrum!  How to get an even canopy using LST!

PPF on the other hand only counts the Photons emitted by a light source, but does not take into account any absorption effects. So if a photon is emitted through a source and runs right into a the case of the light, its counted towards PPF but not PPFD. [PPF] = µmol/s

The counted Photons also have to be from a certain spectral area between 400 nm and 700 nm. Everything else is not counted!

Two different lights: Blue has a smaller wavelength and can transport more energy per photon than the longer red. Source:

Example 1: Lets say we have two lights emitting 100 photons a second on a normalized area. One is blue, one is red. The blue photons contain more energy. The PAR value would be higher for the blue light. The PPFD value would be actually dead even because both lights emit the same amount of photons in the active range.
So which light is “better”?
Answer: The red one for flowering, the blue one for vegetative growth. This is also the reason “common grow lights” give you both parts of the spectrum which results in a pink look of the lights. (blurple if you want…).

Cannabis plant under a grow light with a blue/purple spectrum
Cannabis plant under a grow light with a blue/purple spectrum. Source:

Example 2: We add a green light to the show with an output wavelength of 555 nm monochromatic light. Its PAR values would be better than the red light, but worse than the blue ones. PPFD would be dead even again. Do we take the green one now?
Answer: No! A green light is not effective to grow cannabis. The plants will not be able to do photosynthesis with green light, as effective as with the use of fullspectrum systems, because they simply reflect alot of it.

Grower inspects his plants with a green headlight.. Source

Conclusion: When reading about PPFD values in the description of your light, always take a look at the spectrum as well. If the light “wastes” alot of intensity on green wavelength the number is not as useful anymore! If you have a high PPFD value it can mean less if it has mostly a green spectrum.
Its also interesting to look at the light distribution a light creates at certain distances to the source and wether the source created an even output! If there is a disproportion between PPF and PPFD its often times a lack of optical diffractors that distribute the light uneven to the plants. Take a look at a HPS lamp that creates light in a very narrow area inside its tube. This creates a very uneven distribution on the surface area: Buds on the edge of the light cone will develop more poorly.

PAR – Photosynthetically active radiation

This is easy to figure out: You imagine a perfect black surface and shine a light on it. The hypothetical surface (m²) absorbs the energy at a 100% effectivity which you can measure. Within a certain timeframe we can now measure the power (W) emitted by the light in [PAR] = W/m². The counted photons are again in the 400 nm to 700 nm range.

Here is a subtle difference to the PPFD value we learned about in the last section:

Example : Imagine two light sources. One monochromatic red and one monochromatic blue one. The red one has an output of 200 photons a second with 700 nm in wavelenghts, while the blue one puts out 100 photons with 400 nm. PAR does not care if the plant can use this light, it just takes the values because they are in the range where we observe PAR. So regarding PPFD we learned that the red one should be twice as good as the blue one. But on PAR level both are about even, because the blue light can carry more energy than the red one and PAR only cares about energy over time on a certain surface.

Conclusion: PAR can be at a high value, because it gets pushed through high intesity of blue parts in the spectrum (more blue wavelengths). Always check PAR, PPFD and the spectrum.

Light distribution under a metal-halide light: perpenticular to the source direction the light is most intensive. PAR values might look good, but whats measured at a surface level is also important! 
PPFD takes uneven distribution into account

W – Real Power Draw (Watt)

The Wattage of a lamp is a good ballpark number to start searching for a light source. Its actually the first value I look at, and from there you start to compare lights on the same level!

The power draw from the wall can directly calculated into money that appears on your power bill. So this is often the first thing you want to check. After this, you find out which light converts this money to the best possible PAR/PPFD/spectrum combination.

Many light suppliers tend to market their lamps with this value to its customers. 

Blue/purple lights from various distributors will name their lamps after “artificial HPS equivalent Watts” to trick the customer. You might think that a 600 W LED with the an awesome blue/purple spectrum is the deal of the day. Be aware that these companies hide the true power draw in the description at amazon or eBay. And they are good at it. Don’t get fooled.
If you think you can have 600 W of high quality LED plug-and-play light for 100$ you are WRONG.

Always check for the real power draw.

  • If you’re going for HPS/CMH/Lowball LED light: Calculate with at least 50 W/ft² for decent results.
  • If you are using High-Quality LED Lights (COBs or Quantumboards): Calculate with at least 35 W/ft² for decent results.
This Phlizon is marketed as a 600W LED Grow-light for mind-boggling $89.99. Its actually a 108W shit light, thats not even suitable to light up a 2×2 tent. ONLY BUY THESE LIGHTS IF YOU ARE ON A BUDGET! This scam is actually the “Amazons choice”…….

lm, lm/W – Lumen, Lumen per Watt

Now we’re really deep down the cave. But still: Companies try to catch you with values like this! 

Lumen is one of the least scientific units around, but still has some meaning for interior architects and astronomers. The unit is an abbreviation from the units candela (which literally refers to the output of a burning candle) and the so called luminosity function.

Quote:  “[…]
luminous flux is 
weighted according to a model (a “luminosity function“) of the human eye‘s sensitivity to various wavelengths. ” (Source)

So if you are following you might wanna ask yourself something. 
First remember: The human eye can best distinguish green wavelengths, because it is most sensitive around the 555 nm value. A plant is literally blind in this region.
WHY would you buy a light only because it has awesome lumen values?!

As always: check the spectrum! If the light is mostly green it will have awesome lm values. And perhaps, if its a modern LED, also great lm/W values. But it might not really suitable for growing.


How The Pro chooses a light

When it comes to the decision for the grow lights people always hassle. The market is flooded!

  1. Determine the size of your grow area and find out how many of the sweet Watts you want in there. The unit might not mean much, but you gotta start somewhere! Example: 3×3 tents have 9 ft² of space, so we need about 315 Watts of High Quality Growers Masterrrace LED Awesome Lights, or 450 Watts of Lowball LED / Good-old-fashioned HPS or CMH lights. Make sure your light has a PPFD value of 700-1000 at canopy level, has a full spectrum (warm white) and an efficiency of 120-200 lm/W (remember that you can use this number only for lights with similar spectra).
  2. How is your budget? Great LEDs are expensive, but worth the money. Pre build ones come for $1-$4/W in various forms. So youre looking potentially at $315 – $1260 just for the light. If you want to save money you can choose low-quality LEDs for like $250 that will reach 300 Watt if you buy 2-3 fixtures. Or you just go for a HPS setup for about $150 as a bundle with reflectors and balast. (I wont go into the details here)
  3. Lower your expectations! Great, you just want to take a look into growing and chose the entry level LEDs. Thats totally fine! You can grow awesome weed with these lamps! But if you ever feel you are missing out on something, come back and read about the real LEDs.
  4. Did you really get the HPS?! Thats also fine. They are working for decades and give awesome results. Come back if you think about buying an AC unit and have nightmares about your power bill…
  5. You cant decide between two or more lights? Compare PPFD/PAR/spectrum values at the same Wattage and find your favourite! A light should at least bring the PPFD up to 700 – 1000 in the sweet spot.
Spectrum of a Mars Hydro 300 W light normalized by maximum intensity. The spectrum is good if you just consider the PAR spectrum, but misses out heavily between the red and blue lines if you take the McCree curve into account as well.

LEDs recommended by The Pro

I’m not getting paid for it (yet) but here are some lights i will approve. I saw the results from growers and if you want high yield, dense buds and fast growing weed with low internodial distances buy from them. They also tend to run much cooler than HPS lights…

Timber Grow Lights (US)
Horticulture Lighting Group (US)
Cob Shop (US)
Greenception (EU)
San Light LED (EU)
Rapid LED (US)

Build your own!

Don’t try to buy a good grow light on