A new toy: CO2 meter for the greenhouse

I just bought a meter to measure carbon dioxide. I have been running the greenhouse warmer than normal during the day. And it’s been cold outside so that means little or no ventilation. So I was concerned that CO2 levels might be falling to near zero. There is a fair amount of green vegetation. Nothing like summer but still it seems like enough to drop CO2 quite a bit.

What I found out so far is that the CO2 does drop by day but not as low as expected. Apparently my pots and other CO2 sources are producing as much as the plants are using.

When ventilated CO2 is near the outside level of 370-400 ppm. At night with the GH closed CO2 rises to near 525 ppm. During the day with the GH closed, sun out, and near 85F inside the CO2 level does drop to around 400 ppm but not below outside. So the sources are about keeping up with the plants use during photosynthesis.

What I’m going to try next is to boost the CO2 production by adding more sources, ie decomposing organic matter. I’ll see if I can hit 1,000 ppm at night and what that means during the day. 1,000 ppm should boost photosynthesis substantially.

My next use for the meter will be to see what’s happening to CO2 levels in my house and grow room. At times I have lots of cuttings rooting in a small enclosed space without the CO2 sources in the GH.


Thanks for posting this. I have a small greenhouse full of citrus, tropicals, and others, and never even considered CO2 levels.

1 Like

I wonder what the optimum CO2 amount is for plants?Are mass concentrations better or can they be overloaded?
If higher levels are needed for your grow room,maybe try a little dry ice,evaporating in a bucket.bb

1 Like

Plants probably can be overloaded. At least to the point that photosynthesis stops increasing. I haven’t looked into it lately. But the current atmospheric level of 400 ppm is clearly better for plants and crop yields than the 300 ppm when I was in grad school. I’d expect 1,000 ppm to nearly double photosynthesis vs 400. Not that you could tell for most things we do. But I’m going to try increasing mine by decomposing organic matter in the GH and if needed in my indoor grow room.

I did an experiment in grad school using dry ice to increase soybean yield. But it wasn’t a real experiment, no control. I did win the yield contest vs about 12 other contestants.


The CO2 levels matter but air circulation also matters equally as much.

Lets consider measured CO2 levels at 1000 ppm but the air is stagnant, the CO2 levels are completely depleted near the leaves even though they could be higher elsewhere.

On the other hand CO2 levels could be just over 400 but with healthy air circulation, fresh CO2 is constantly brought to the leaves. This is why plants grow very well outdoors assuming light levels, temperature etc are adequate.

Arguably though, the air circulation in a greenhouse will be very good even though in a grow room environment, without a fan, it could be pretty stagnant. So CO2 levels could become the limiting factor in a greenhouse where air circulation is good.


I run an air circulation fan in the GH whenever the exhaust fans aren’t running. Wind speeds probably run from 1 to at most 5 mph in my setup. I suppose that could be limiting things a bit but probably not much.


These greenhouse pics make my mouth water.

Im so jealous : )


My understanding is photosynthesis maxes out around 1500 ppm and plant respiration can double around 900 ppm. I know they are expensive but the Schaeffer fans are really nice and last a good long time if you are trying to grab co2 from the ground and mix it into the air. What about growing oyster or other mushrooms for extra Co2? Or beer or wine brewing dug down into the dirt or shed on the side for a good environemental space for brewing and letting the Co2 escape into greenhouse?


Wow, always fascinating information from you.



1 Like

I would be cautious about large amounts of decomposing organic material in there. It can give off a lot of ethylene gas.
I made a hot bed on a compost pile to start seedlings,
It suffered badly, apparent ethylene damage.
Somethings more sensitive than others.
Just saying…
You are doing interesting stuff there.! Good job!

1 Like

I read that plants grown in high CO2 develop fewer stomata and use less water, could that work against you later in the year when it is hot (less able to cool their leaves) and you are trying to limit soil moisture (less water pulled from the ground)?

I’ve been wondering about whether fruits have stomata also, since figs split in when humidity increases it seems like they transpire just like leaves… Can’t find any valid info for that though.

I setup a feeble attempt to increase CO2. One 25 gal fabric bag full of leaves. Anything more than that would cost me money. So I’m probably done at least for now.

Good question. I don’t have an answer. I have answered the main question I had. I don’t have to have as much ventilation as I thought to maintain CO2 levels. By the time I have a lot more leaves it won’t be so cold outside and I can run the exhaust fans. At that point there’s nothing I can do. The air is exchanged every 1-2 minutes so inside will be near outside.

Fruits do transpire, at least grapes do–https://www.researchgate.net/profile/Markus_Keller8/publication/281604765_Grape_Berry_Transpiration_Is_Determined_by_Vapor_Pressure_Deficit_Cuticular_Conductance_and_Berry_Size/links/570980e208aea66081358a71/Grape-Berry-Transpiration-Is-Determined-by-Vapor-Pressure-Deficit-Cuticular-Conductance-and-Berry-Size.pdf

There is more accessible research on this, mostly from Washington State, but I can’t find it now. They’ve also found, through manipulating irrigation and using dyes, pressurizing roots, that the vast majority of splitting (again with grapes) is due to rain on the grapes rather than uptake through the roots. The relative humidity can play a huge role in berry transpiration as there needs to be a pressure differential for the berries to breathe–in 2018 we had horrible splitting leading to sour rot in grapes because of historic levels of high humidity in September. Another scenario that can cause splitting is when the first part of the season runs a irrigation deficit (semi-drought) then the rains pour on (again like 2018 for the Finger Lakes). The other interesting thing that came out of their studies, and ones before it, was that post veraison (onset of ripening) irrigation did not dilute fruit.