Too much Co2 being released at night?! At day? Myths that 2000 PPM+ are bad for plants?

Recently I added a bunch of top dressing to my 3 plants in mid flower. The carbon released from that (perhaps) along with the added Co2 being released from the growing biomass of the plants mean that the level of co2 level in the small 4x4x4 room has gone through the roof at night. All the way up to a steady 2,300 PPM! It is not rising like this during lights-on.

I did some scouring online and was able to read anecdotal reports that this high Co2 at night DOESN'T actually hurt the plants (the conclusion of a grower who tested with many strains and commented on another forum post). I suppose there is still enough oxygen hopefully, so the plants are fine at night?

I've also read that it is bro-science and a myth, not really true that 2000+ PPMs of Co2 can hurt plants with lights on. It may or may not be true, in my estimation. What do you guys think?

As a newbie grower using co2 in a sealed room and without possibility of adding vents/intake/outake, I would like to ask any advice. I suppose I should try to minimize this high Co2 by opening the door to let co2 escape during the transitions of day/night and night/day. I have started to do this. As someone who likes automation, reluctantly!

But there is no way I can reign in the high levels of co2 at night. It crawls back up to 2,300 or so without any gas released from my tank!

I do use a controller to top up the room via the solenoid valve on the tank as needed when the sensor determines it is needed. However there is no way for me to vent the room unless manually opening the door.

2000 ppm isnt that high. You exhale that much or more.

redi jedi said:

2000 ppm isnt that high. You exhale that much or more.

Click to expand...

38,000 ppm I read

All animals and humans inhale oxygen and exhale carbon dioxide. The carbon dioxide level in exhaled air about 3.8%, or 38,000ppm (parts per million). When carbon dioxide is exhaled it is quickly mixed with the surrounding air and, if the ventilation is good, the concentration is quickly reduced to harmless levels.

Im high but I think the OP had safety concerns...

I have a similar issue. My room gets up to like 3500-4000 ppm most nights and my tank only runs the last 5 or so hours of flower because it takes all day for the plants to catch up using all that co2. I try to vent the room as often as I can but some days It does not get vented. My plants do fine. They are super healthy.

redi jedi said:

Im high but I think the OP had safety concerns...

Click to expand...

Not worried about safety. Just agronomy. Optimizing the growth and preventing issues.

Eivets00 said:

I have a similar issue. My room gets up to like 3500-4000 ppm most nights and my tank only runs the last 5 or so hours of flower because it takes all day for the plants to catch up using all that co2. I try to vent the room as often as I can but some days It does not get vented. My plants do fine. They are super healthy.

Click to expand...

You are again re-affirming that the myth of high Co2 might kill or stunt plants.
High levels of CO2 from 2000-4000 PPM at night and possibly at day as well, are not going to really harm the plants seriously, let alone kill them.
This is what I am seeing from what I've read. Nobody has proven otherwise yet. A lot of people use bro science and talk like 2000 PPM+ will kill plants but I've yet to see any evidence of this.

@Eivets00 Im curious how big is your room dimensions? Sounds like we have a very similar issue going on. In my case, the plants eat up all the Co2 in the morning and it goes back to normal levels in just an hour or so.

Maybe I don't need to worry so much about the excess at night. And just monitor the excess at morning to make sure it's not having a negative effect and reduce/vent as reasonable in the morning perhaps as well.

My flower room is 10ft by 7.5ft and about 7ft tall. It’s in a shipping container buried underground. I was originally very worried about the co2 issue and the fact that my vpd is way outta wack, but I’m harvesting my first crop in a week out of this room and the plants are beasts. I’ve been growing for 17 years and never had an issue like this. Keeping the rooms full of plants helps to keep the co2 in check a bit for me.
Some people may not believe me but I’m really under the impression that the wood floors in my container are somehow breaking down and creating the co2. I’ve done a bunch of testing to get to this conclusion but who knows....? Nice thing is a 50 lb tank has lasted me forever

CO2 won't kill your plants but i guess it could maybe stunt respiration during the night in extreme concentrations. Outdoors, CO2 concentrations in the soil can be much higher than in the atmosphere ( i read 10 to 15x ). So i wouldn't worry about it

You would have to get over 5000 from what I've learned to have adverse effects (i belive this info was in a Bruce bugbee video)

They just obviously won't be able to use all of it. Sounds like a good problem to have

Do u have a co2 controller? Ive never seen my co2 go past what it set at during dark phase.

Eivets00 said:

My flower room is 10ft by 7.5ft and about 7ft tall. It’s in a shipping container buried underground. I was originally very worried about the co2 issue and the fact that my vpd is way outta wack, but I’m harvesting my first crop in a week out of this room and the plants are beasts. I’ve been growing for 17 years and never had an issue like this. Keeping the rooms full of plants helps to keep the co2 in check a bit for me.
Some people may not believe me but I’m really under the impression that the wood floors in my container are somehow breaking down and creating the co2. I’ve done a bunch of testing to get to this conclusion but who knows....? Nice thing is a 50 lb tank has lasted me forever

Click to expand...

Careful the rotting floor doesn't cause mould issues for you.


My room gets upto 3500 - 4000ppm every night and plants eat that in a couple hours once lights kick on. Stress less.

please tell me what topdress is doing this, I would love to have your problem too. Thank you

like this

1kg Bonemeal
1kg Crabmeal
1kg Epsom Salt
1kg Kelp Meal
250 gr Humic
750 gr Azomite
laced with our proprietary quantum biostimulant

ya mine goes up to 2500ish late flower in my sealed room from 1200ppm.

they eat it up like a boss in the next day cycle. idk if all that excess co2 ppm is bad or not honestly but its less tank swaps for me late in flower.

Southernontariogrower said:

please tell me what topdress is doing this, I would love to have your problem too. Thank you

Click to expand...

Any top dress of organic matter with a favorable carbon to nitrogen ratio and a high level of biological activity will do it. Bacteria, fungi, protozoa, etc. do not photosynthesize, but instead can only eat carbon containing molecules and "exhale" CO2 like we do. Technically plants do the same also for respiration, but they make their own supply of carbohydrates (food) to work with. I think many might underestimate the amount of CO2 released through biological respiration that doesn't stop because your lights are out. Plants also contribute CO2 at night because they stop photosynthesis and instead just release CO2 from the food they accumulated during the previous lights on period.