How CO2 impacts Ideal Temperature for Growing Plants - Pulse Grow

For indoor growers, it is critical to maximize productivity using limited resources and space, while keeping costs at a minimum. If you don’t understand how CO2 supplementation influences photosynthesis, you’re limiting the productivity of your operation. 

This is a companion discussion topic for the original entry at
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CO2 supplementation also increases how much light plants can utilize for photosynthesis. I’ll put together an article on that sometime soon.

Higher temps and higher humidity (and more light!) are a must to get the most benefit from co2 supplementation, but there are caveats. Yes, these conditions lead to increased growth but we must ask ourselves: what kind of growth?

If you want larfy buds and stretchy foxtails, then a good way of getting there is with high temps and choking humidity levels that prevent your plant from cooling itself via transpiration. You’ll get a lot of growth… unsellable growth.

For this reason I would recommend high temps and high humidity for only the early stages of growth, perhaps up to week 5 or 6 (assuming cannabis), before ramping down to more gentle/ordinary conditions.

The article implies that maximum growth (for Aspen) occurs at around 36C (97F) which I hope nobody decides to follow in their rooms. This temperature is acceptable for ordinary growth in desert hoop houses where low humidity and normal light levels (and atmospheric co2) limit the plant’s ability to grow wacky, but in high humidity indoor rooms with high light and co2 this is asking for trouble and a potentially ruined crop.

While I don’t have any studies to prove it, I would suggest limiting your max temps to 30C (85F), where that is the upper end of the differential, with gradual reductions each week, ending in the mid to low 70s(fahrenheit) by harvest.

If I were growing in steady state conditions I would probably opt for 25C (78F), with around 1200ppm co2, 64%RH, assuming very bright light. If any irregular stretching was occuring at the tips then I would back off light levels and humidity by about 10%, or find a strain that appreciates my growing conditions.

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The thing that drives transpiration is VPD, and as long as you’re hitting your ideal VPD numbers I think you can crank temps and humidity if you’re supplementing with CO2.

As far as what kind of growth, that’s a great question…

I would figure there is probably some strain dependency in how plants respond to CO2 supplementation and the kind of growth you get.

25-30C is probably a good middle ground. Yes - the study of CO2 effects on photosynthesis was for Aspen trees. I’ve read a bunch of other studies about similar things, and saw that different plants react differently, but the overall trend stays the same.

Best thing to do would be to pick a few different strains and run some experiments!

My grow philosophy is a bit different than typical. IMO the ideal VPD is not the same for each type of growing whether it’s atmospheric or enhanced or if you’re in full control of the environment or not.

For instance, take your experience as a human being who controls your body temp to some degree with sweating. Is your ideal VPD the same at 70 degrees F vs 110F? I would argue that just like with animals, the hotter it gets, the higher you want your VPD to be. A plant at 95F is going to want to transpire much more relative to a plant at 78F, and accomplish an even greater leaf to air temperature differential. This means more air flow and less humidity than you’d think would be ideal using a typical VPD chart.

This is my non scientific opinion tho, just based on what I see in the room and in the field.

That’s not correct, stomatal conductance (transpiration) is relatively un-correlated with temperature. VPD is really the driving force for transpiration.

Here at a constant VPD, there’s no real difference in stomatal conductance over wide temperature range, at least until you reach a limit for what the plant can handle, as seen by the drop off at the end.

But at variable VPD, there are huge effects on transpiration:

From this study

Technically yes, as long as the plant can handle 110F, but most cant because photosynthesis breaks down at that point. But at 95F vs 78F, your transpiration depends pretty much 90% on the VPD and not the temperature.

Of course, this probably doesn’t hold true in every single case since plants and strains have their own unique things about them, but it’s generally accepted that transpiration is mostly governed by vapor pressure difference rather than temperature.

For future reference, and for anyone curious: I got this from a couple of studies:
(Ec is canopy transpiration, ue is air flow, VP is vapor pressure of water, a is the size of the canopy)

  • Tang, K., Fracasso, A., Struik, P. C., Yin, X., & Amaducci, S. (2018). Water- and Nitrogen-Use Efficiencies of Hemp (Cannabis sativa L.) Based on Whole-Canopy Measurements and Modeling. Frontiers in Plant Science, 9. doi:10.3389/fpls.2018.00951
  • Von Caemmerer, S., & Farquhar, G. D. (1981). Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves. Planta, 153(4), 376–387. doi:10.1007/bf00384257
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I think we’re not on the same page.

I’m arguing for allowing higher VPDs at higher temperatures to increase the leaf to air temperature differential via increased transpiration (which is independent of stomatal closure/conductance) to reduce plant stress and improve plant growth characteristics. Just because you can grow more pot at higher temperatures doesn’t necessarily mean you can grow more good pot.

More transpiration occurs at a higher vpd, which is what I want, because transpiration is proportional to leaf surface temperature to air temperature dT. If temperatures are high I want my plants to be able to cool themselves. Combined with the information you posted, this means I’m giving up some stomatal conductance in exchange for cooler leaf conditions.

Consider my observations:

  • High humidity at high temperatures, especially coupled with low turbulence, leads to wrecked growth
  • Low humidity at high temperatures (i.e. high vpd), as in the desert in california, allows for perfectly ordinary growth

Regardless of what the scientific explanation turns out to be, this is what I have observed and so it governs what conditions I will use to grow cannabis.

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What about the genetics of the plant, like some genetics prefer cooler temperatures or vice versa of the strain ???

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The thing about plans is that they start to close their stomata to conserve water if VPD gets too high, that is why you see the drop in stomatal conductance. Plants are worried about drying out, and respond to the dry air (high vpd air) by closing stomata.

But like @GeMann brought up, it is very genetics and strain dependent. The stuff i talk about is how things are on a more general level. Your own observations and experimentation with the strains you are familiar with should be the final judge.

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