When You Should Use Co2

Dr Photon
Member Moderator
Expert Grower
Completed Grows

20190421 232107

Many growers are not aware when the right situation calls for Co2 supplementation. I will go over the basics for when Co2 should be used and how this improves plant performance.

Plants produce energy through the process of photosynthesis. Any fundamental property that is required for photosynthesis that becomes limited is said to be the limiting factor. The main factors required for optimal photosynthesis is light, water, Co2, temperature and nutrients. 

Nutrients and water are typically abundant and usually are not limiting factors for photosynthesis. However light and co2 are. Outdoors plants constantly shift between light limited to co2 limited as the sun shifts its angle and cloud activity occurs. This is because during overcast or low light conditions, light levels are not strong enough to fully utilize the level of available co2 in the air. During clear skies or high light conditions, ambient co2 concentration is not high enough to fully utilize the level of energy provided by light.

With indoor cultivation, we can provide light levels which exceed the co2 sufficiency and push plants into co2 limited states. Which mean plants under these conditions are potentially wasting usable light to heat through non photochemical quenching.

So how do we asses when co2 supplementation should be used ?. Using plant science and research data on the cannabis photosynthesis characteristics. Cannabis has been shown to start becoming co2 limited at around 500umols PPFD. But significant limitation does not occur until around 700-1000umols PPFD. Because we know at what levels of light intensity cannabis leaves start to become co2 limited. Using this data we can use basic mathematic calculations to work out how much light is required to take advantage of co2 efficiently.


Average light level calculations:

Calculating the average light levels is the most ideal way for assessing light requirements for co2 supplementation. Using single measurements at center points of the canopy is not ideal because the light intensity can drastically drop off as the measurement is taken further from this point. In order to take advantage of co2, we must provide average light intensities that makes plants co2 limited not just in the center, but all over the canopy area. This does not mean co2 cannot be beneficial in applications where average light levels are not high enough. As users will find that they can bring their light source closer, reducing the level of wasted energy from radiation and reflection losses. But this level of use is not maximizing the supplementation of co2 and so should be avoided.

I will provide the light requirements for various grow sizes. Which are transcendent across both LED (actual not equivalent) and gas discharge.

2 x 2 (4 square meters)
200w-250w  = 800-950 average PPFD

2.5 x 2.5 (6.25 square meters)
330w-400w  = 800-950 average PPFD

3 x 3 (9 square meters)
480w-570w = 800-950 average PPFD

3.5 x 3.5 (12.25 sauare meters)
640w-785w ÷ = 800-950 average PPFD

4 x 4 (16 square meters)
850w-1000w = 800-950 average PPFD

4.5 x 4.5 (20.25 square meters)
1070w-1280w = 800-950 average PPFD

5 x 5 (25 sauare meters)
1350w-1570w = 800-950 average PPFD

8x8 (64 sauare meters)
3500w -4000w = 800-950 average PPFD

For custom sizes not included here, use the square area that is closest to your grow or request calculations by me for areas larger or not closest from the provided.


As always please comment your thoughts or any questions on the subject.

Being wrong, is an opportunity for getting things right.
Posted : 04/11/2019 8:50 pm Txweedhopper, Heyron420, Rootz369 and 2 people liked
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New Member
Cannabis Fan

Hi @drphoton,

I have a filled, sealed 4x4 tent with supplemental CO2. I have 1500W of LED to play with but not running at 100% atm as not sure where my temp/humidity/VPD limits are.  How hard can I push them? I'm having a bit of trouble keeping humidity in check atm but sitting at 30C atm so don't want to add more dehumidification yet and raise the temps more without some advice.  

Due to the light available can I be more aggressive on the pruning to drop the humidity a bit  and keep the DLI up?

Many thanks

Posted : 10/26/2019 10:34 am Crusader1427 liked
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I have a 3.5x3.5 with 2x HLG 300 RSPEC .. how long should a 20#tank last and how much closer can I bring the LEDs?? I'm at about 22" from canopy now and does co2 speed up or prolong the flower phase?? As always thanks for the science ....RB420

Posted : 02/04/2020 9:46 am Crusader1427 liked
New Member
Cannabis Fan
  • I’m old but this is my first grow. I decided to grow last summer with no idea how to start. I bought a used setup with a over size tent,12 pot big boy set up,hanna flow line monitor,Clay pebble medium,500$ worth of cyco nut.,1000 watt.hpsbulb hood set up,8 inch carbon filter and fan,at that I figured I would start growing-at 65 years old not even ever hearing of ebb and I was going back and forth with different ways to get the thing into motion and found this site. I’m going to have a lot of questions and I hope no question is a stupid question falls into play. I started buying LED lights last week with out knowing nothing about nothing, I bought two king lights last week before I found cocoforcanabis and read your thing on lights-you recommenced the Mars-I did buy the 1000 watt on amozon-from you’re site-hope you get t credit-next day I sent the twoeking lights back and I am going to buy 2more 1000 w mars. I ripped down the over sized grow tent and (it was 25 sq feet ,and built a 48 sq foot air tight grow room from heaven. I’m going to take my time on this (it’s all ready been 6mo)and do it right. I’m going to try of my first question is can I burn my LED and my big bulb lights in the same room with out any issues? I plan on having the 1000watt big bulb and 4-1000 watt mars LEDs ,should that enough light for that size space? I hope I get reply’s 
  • thanks
  • Crusader1427
Posted : 02/07/2020 10:47 pm Ratboy420 liked
Eminent Member
Known Grower

 hi, i am going to start a grow up soon, but ive never used co2, i might try a diy and make it before i get into buying a tank a regulater etc... my area is: L 9 feet x 2 feet x 9, basically a closed closet area, how much co2 do i need in that area per hour to reach an  adequate constant amount like 1500PPM, or even an amount of 1200PPM, whats the suffient flow amount when the lights are on and i plan on growing 4 feminized autos in 3 gallon containers with the light 20 hours on and 4 hours of light off?, The co2 will flow constantly as long as im awake by the way, Thanks  

Posted : 02/14/2020 7:56 pm
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Great article! 
When CO2 supplementation is more efficient?  In Veg stage or due flowering? Im trying use CO2 and im running 900-1100ppm inside the tent. I have enough light only when half of the tent is full (central point of the lights - 22") where im reaching about 800-1100 ppfd. So basically i want run CO2 only due Veg stage cuz its only part of the grow when i have enough light intensity. Can i benefit from this setup? 

Posted : 02/23/2020 12:27 pm Brenpeyton liked
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New Member
Cannabis Fan
My Current Grows

Yes very good , co2 , closed sealed room or tent , air flow , air movement on the ground is important co2 is heavy and drops as it is a gas .. so you want air moving to circulate the co2 , 


aggresive grower 1300-1500 ppm co2 levels flower wk #1 - 5 


safe beginner grower 300-1050
till flower wk # 5


after flower wk #5 plants don’t use co2 they also don’t use co2 during sleeping . So if u have lights off schedule u need to put a timer on the co2 system, 

Posted : 03/11/2020 2:09 am Pond Racer liked
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Cannabis Fan

Which CO2 meter do you recommend?

Posted : 07/08/2020 6:23 am
Eminent Member
Cannabis Fan

High levels of CO2 in a cannabis garden can result in stronger plants with increased yields

Posted : 04/14/2022 2:48 am