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CO2 during Flowering

I got a quick question for an experienced grower or researcher. I’m supplementing CO2 @1200ppm during the first 4 weeks of the flowering cycle. I’ve heard there’s very little benefit after 3/4 weeks and was wondering if this is true or not. Also anyone with scientific research that might know, I’d welcome any and all advice. Thanks!


As far as the science goes, I don’t know of any scientific evidence pointing to whether or not there’s benefits of running past the first few weeks of flowering, largely because that is a very specific subject on a rather under-researched plant. Most of the research I’ve come across focuses on atmospheric CO2 increases – from manmade emissions, not artificially elevated CO2 levels.

I did come across this during my search though:

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Thanks Hunter! That pretty much nailed what I was looking for. I get so much contrary advice, even on the THC farmer link there’s some debate, but it gave me enough info to make an educated decision.


We as a community could test this on the online platform we are building. We could all have a project where we these parameters in a scientifically rigorous and accurate way.


Anyone who wanted to this experiment alongside could join, we all contribute data. And we sort alongside variables for treatment A,B,C and we look at end THC/CBD yield and we can tell in a quantitative way what is actually happening.


I only have one flower room so I’m limited in the way I can experiment. I’d love to see a comparison from 3 -4 weeks with CO2 supplementation vs 7 weeks using the same genetics and grow conditions.

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There would likely need to be multiple studies done on a large scale, but on the small scale we could look at something like this:

Null Hypothesis – Elevated CO2 levels during flowering do not affect cannabis buds.

Procedure – Using same strain (IE Blue Dream), create (at least) 3 groups to be tested:

  1. Control group (No CO2)
  2. Elevated CO2 during first 4 weeks
  3. Elevated CO2 during entirety of flowering
  4. If you want to take an extra step and make it super interesting, add a fourth group that only elevates CO2 after the first four weeks are up.

Pre-measure plant and pot mass, record and measure all nutrients added or flushed (measure the effluent). Record amount of water added per plant. The same lighting, growing, harvesting and curing techniques should be followed (all ideally in the same environment). In the event of pest infestation, results for said plant should be thrown out.

Data to collect:

  • Biomass of wet buds post-harvest
  • Biomass of dried buds post-harvest
  • Adjusted index of biomass (based on initial weight, inputs, etc) by group
  • Total cannabinoid content by group (adjusted for total number of plants, should any be tossed out)
  • Total terpenoid content by group (adjusted for total number of plants, should any be tossed out)

I’d recommend that each group contains at least 50 plants, ideally 100 or more.

The final analysis could compare the control group against the experimental groups, as well as the experimental groups against each other. I doubt the null hypothesis would hold, but it would be interesting to see how different groups 2 thru 4 are.


Dude this is a perfect setup for a project design. At the lab my job is to set up research projects for various labs to extract the most info from their projects to see the most differences. This is an excellent setup and something we can certainly test!


We could do any strains we wanted to and analyze it over the broad “all the strains”. I would not limit the data coming in by strain, more the merrier.


This will also give us more confidence that what we are seeing occurs across the spectrum of strains, not just in one. If we could say something about Co2 during flowering, and we had 20+ strains, then fuck that’s awesome. If we find out pumping co2 4 weeks before flowering increases yields by 3-5%, would that not be a worthwhile experiment?

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Long Read Incoming.

I decided to walk through our platform and show you all how we could go about setting up this project, the tools available for analysis and how it all ties together.

When you have a Seshat login you a button to create a project and the first thing you will be faced with is what questions you want to ask. Something like this…

You have options with what questions you can add, short answer or multiple choice.

For our first question we will ask what the CO2 Regime is, and according to the above post from @Hunter we are going to have 4 separate treatments, you can add 200 if you wanted.

Add a short answer for strain and we are off to the races, you can add as many questions as you want here, but you will learn quickly what is and what is not important to ask!

When you have ran your grow and have some final end product to sample, fire up the app, or computer application and find the project you are currently working on. You will be prompted to answer the questions you asked before like this…

You can go through and measure as many samples as you want. Takes about ~15 seconds and is none destructive, so light those bad boys up after partner.

When you are all finished, you can go to your project in our online platform and see how the different variables really affected your grow. Let’s start at the dashboard. From here you can go to other tabs, like the table/spreadsheet, graphing tools, and analysis, but before we do that, we need to break down the data so that it can be analyzed!

Let’s select our CO2 Regime to sort by, if we wanted we could sort by strain and regime, so maybe only look at Blue Dream at full CO2, but lets keep it simple and split the three main treatments into three distinct series. Let’s see first how each treatment affected the average CO2 of each treatment…

Wow! They sure seem different, but we can know for sure by performing some basic analysis. Let’s run an ANOVA test to see if there are actual significant differences between them.

Sure enough you can tell that there was a quantitative difference in THC potency depending on which treatment they received. This is a very simple, yet powerful example experiment to how powerful this device and platform can be for growers. Setting up projects like this are super easy, testing is even easier. Hope this clears up or gives you all a better idea of what we are offering. Questions, comments?



I can only talk about other crops that we had experience with. Two greenhouses exact same crops one with an old co2 generator, one without. We saw at times of the year when the greenhouses where tight that adding co2 improved yield signifcanly, about 25% improvement. The improvements in yield, had us upgrade both houses with better co2 generators.

We only raised co2 levels to 600 ppm at times when co2 was a limiting factor. This was times when the greenhouses where tight and light was not a limiting factor. The colder the outside temperature the easier it was to maintain high CO2 levels. On cloudy days light was a bigger limiting factor then a CO2 deficit. We could not justify adding lights to all 20k square feet. We had light for about 5K sqft. Adding CO2 on cloudy days was not worth the effort, as we used NG burners. On cloudy days, venting the excess humidity negated any positive effects of adding CO2.

We noticed better bud set in anenome, renuculus and snapdragons, along with thicker stems.

We used reasurch for tomato production as our reference model, for timing and programming. The research in tomato looked at light, CO2 no additional, 300ppm, 600ppm and 900 ppm. They saw no significant benefits above 600ppm in tomato. They used total KG of fruit produced in the greenhouse as one messure. The second messure was the dry weight plants after the production.

In our Sweetpea we saw less difference. I think this has more to do with the verities we grew for winter production.

Lots of Rose growers add CO2, but again timing is the trick. We purchased our first CO2 generator from an old rose grower. The NG CO2 generators received monthly maintenance to keep the burners clean. We also replaced the old brass LG nossle with a stainless NG nossle.

So the bottom line for me in deciding to add CO2 today, would be can you document a CO2 deficit and at what times do they occur.

From the voices in my head



Thanks Ethan, that was very interesting. I ran CO2 in my flower room for 4 weeks and turned it off for the rest of the cycle. I’m going to be harvesting this Labor Day weekend but I can tell you I have some very heavy producing plants. My best producing plant has ten large colas that are literally the size of milk jugs. It’s insane, I’ve never seen colas this large before. The rest of the flower room have what most people would call big colas, and it’s a beautiful sight to see a field of giant flowers. This is my first time flowering Cannabis, so I wasn’t expecting such a bountiful beginning. If CO2 for three more weeks would have added any more weight to my plants it quite honestly would have created more problems for me than benefits. I’m worried about my monster plant with humidity and to have a whole room like that would be scary. I’ve got my humidity pretty stable at around 50% with only 3% swings +/-. I would think that if CO2 was beneficial for the first 4 weeks there should be an equal benefit the last 4 weeks. There are a lot of myths, traditions and accepted practices in Cannabis growing and it’s hard to tell what is scientific and what is just accepted practices. Each stage of growing seems to require a careful review of techniques and methods as there are so many accepted practices that aren’t backed up by science. I’m now looking at the “to flush or not to flush” argument, and after listening to both sides of the debate, I’m slowly reducing my nutrients down to 350 ppm and the last four days a flush with just water and a flushing product just to be safe. I’m only willing to risk the last 4 days of the flowering period on what is most likely a myth. (Knock on Wood). Yeah just appeasing my superstitious side.


@Seshat, that is definitely cool software.



What is the debate around flush and no flush? I only seen this in potted plants for shelf life improvement.



The “to Flush or Not to Flush” debate as I’ve researched has two definite train of thoughts. I respect the scientific approach of the authors of “Otoke - Cannabis for Capitalists” - (Kerrie and Kurt Badertscher).
Here is the link to their input on the debate;

Then there’s the ongoing debate on grow boards like this;


I run Co2 from Day clones are rooted to day I harvest. There is a huge difference. Also if you want more of a difference. Use 460 nanometer blue florescent lights under your canopy to increase stomata production and Co2 intake. Co2 intake by the way can increase yields up to 40%
Run it every day as long as the lights are on… Raise your ppm if you raise your temps on top but beware keep those roots cool.
Max at 91 degrees @ 2200 ppm
Root temp 72
lights on during flower cycle 13 hours 45 min
Plasma supplimental lights
far red LED’s for emmerson effect
460 nanometer blues for increased stomata production
cuts off 1 week to 12 days off flowering time using my setup


That sounds like a very custom recipe. I’m trying to get some far red into my grow, so I’m glad to hear you’re a fan. I run double ended HPS & MH in my flower room and want to add far red and some extra UVA/B for extra resin production. Thanks for sharing, I’ll try this out when I have the ability to. I run water chillers, so my roots are usually 69 degrees at the start and down to 65 at the end. The 460 nm for increased Stomata production. Can you explain this or send me a link that explains this? It sounds interesting and I haven’t heard of it before.


Sure let me just get done taking my cuts watering and all the other junk I
got to do take me a few hours honestly and then I’ll see if I can find the
link or a link to some reputable research done on it


Sorry been a while yea I️ use plasma for my UVA and UVB works really well as far as the stomata goes. It’s the same blue that prevents stretching but it makes the stomata closer together think of it like the plant stretching. Add that sky blue and you get tighter internodes same theory as underneath the leaves on the stomata