How high can I go with the temps in sealed with CO2?

Currently I am running sealed (SOG with rockwool), with CO2. and temps at level of the rockwool cubes when lights on are at almost 30 degree Celsius or 86 Fahrenheit . Actually most of the time its 28 Celsius which is 82 Fahrenheit. I have no air con but can switch to external air easily (built my setup so it can run in winter sealed and in summer with external air).

  • How much can the plants handle? How does heat stress look? Is it risk the worth or will I decrease my yield by the heat? Is there a best time for CO2 in the plant life cycle? Maybe I could run only part time of the grow with CO2?

At 1500 ppm you can get into the upper 80’s. 30-32 C, but slightly lower is better.

You are going to want to pay close attention to humidity and VPD. If you really start pushing plants to their max in sealed rooms with supplemental CO2, you are going to run into bottlenecks, like VPD.

You need higher temperatures to take advantage of CO2 in the first place. All through flower is the best time to run it, especially if you’re running high temps.

Heat stress makes the leaf margins curl upward. You’ll see immediately if they are unhappy.


You dont need co2 after lights out, they are switching over to oxygen then.


can you shortly explain me vpd (on what do I need to watch). Never heard about it before. thanks :wink:


Oh man, this is such a great question CO2, VPD, and such. In fact, I’d call this quite advanced. Doubly so for a sealed room.

Certainly, look at VPD first, then worry about the addition of CO2. But, like Shady says, CO2 during lights on. Its part of the photosynthesis cycle (carbon fixation). No lights … no photosynthesis.

For VPD, the easiest way is to refer to VPD charts. You’ll need to be able to measure temperature and humidity.

Simply, there are “optimal” bands where the plant will excel by optimizing transpiration (exchange of moisture and carbon dioxide). The plant responds to the environment by opening and closing stomata that allows the exchange of CO2 and moisture (mostly). If the conditions are “off”, the stomata (pores) will close and limit several things in a chain reaction. By closing the stomata, the plant will slow the release of moisture into the surrounding atmosphere. Because the moisture is no longer being released from the leaves, the plant slows the uptake of nutrients from the roots. Closed stomata will also limit gas exchange which means the plant will not be able to perform carbon fixation efficiently. Without the necessary carbon and phosphorus (nutrients), photosynthesis stops or is significantly slowed.

By getting VPD optimal along with sufficient light and carbon, you’ll get the plant into the optimal growth mode.

This is a good one on VPD and the VPD charts:

And, source code and technical details for geeks (shameless OG plug):

This stuff is especially pertinent for a sealed room. Humidity and temperature may be limiting factors…

p.s. VPD charts are guidelines not rules. Adding CO2 will likely shift the chart somewhat.

Also, see this:


What @Northern_Loki said. Vapor-pressure deficit. The more moving parts to your grow, the more intense the inputs, the harder you push the plants, the more you’ll have to keep everything running in tip-top shape. Think of it like an ecosystem, you can’t push one thing without pulling another. If you can nail every factor, you’ll get incredible speed of growth and massive yields. It may take a few cycles to get everything in order, this is considered very advanced growing.


Thank you very much also for the awesome document that you attached :wink:


just finished reading the document which you provided. thx for that. but pls have a look about my question about VPD and humidity. thank you!

I switched to external air without additional CO2. But right after some problem occured.

Have you checked the temperature of your nutrient solution?

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Temps of nutrients didnt change. It is outside of the room. always around 15 to 19 celsius. ph 5.7 now.

Been through the Co2 scene many times…Still have a full tank of Co2, and all the gear.

Co2 is ideal for soil grows in the winter time, or a hydro setup with a chiller… But, never tried a rockwool gig… hmmm. Once the nutes goes past 75 degree’s, your just asking for trouble.

Also,… You must remember, if the plants get too hot, the buds will run, and the yield will be far less.

When it gets warm, bugs can and will reek havoc when the temperatures rise. I dont care how clean the setup is, or how tight the room is, They will indeed find a way into your grow space.

Because of what I mentioned, I put the Co2 away… Seems like more trouble than what its worth.

But, if its freezing outside, Co2 can be very beneficial… And well worth the initial expense.


At this point i just have a batch of beer or wash fermenting in the basement at all times and that brings the tent co2 to about 850-900. I vent as normal and cool as normal. No energy to do anything more.

But if I did? I agree, I would be keeping temps lower than 28C. And would be chilling the rez so that the root zone is substantially colder.

$0.02CAD, YMMV


I run c02 at around 1000 with 82 /86 f temps now that it’s cold . Summer time I’ll run 78/80 f and 800 ppm c02 natural gas so not a big issue if I was filling tanks I’d probably run it around 600 .

I’d been told c02 works best with higher temps haven’t really noticed much difference between summer and winter runs .


Gobs of Co2 Info from down under. I’m still digesting it.

Great info on air quality - O2, Co2, exchange during lights out.

I run my Co2 from around 600-1000max in my lil 4x8x8.
They say no Co2 during stretch help to reduce stretch. Plan to try that next run.
My canopy temps were to high last run - not good for terps, foxtailing, etc.

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they say double yield with less humidity in cotton? :smiley: Less yield with high humidity??? Really?