When to use extra C02

HI guys, I just need to know when CO2 is needed for a closet grow?

My previous attempt was a small scrog (1.5 x 2 foot) with 3 plants and 250w hps for flowering. 2 or 3 week veg time in peat moss/vermiculite/perlite with Biogrow nutes.

My harvest was particularly small though. Think I got slightly over 2.5oz. Below is a picture of a small section of that srog.

I have notice that the lower half of all my buds were extremely airy while the upper half were much denser. Basically the lower portion of the bud above the screen were almost just sugar leaves. The picture makes it look better though. It was extremely sticky but just didn’t have any weight. A lot of the nodes had nothing there, just stem, covered in crystal, but no bud material at all.

I’m sure at 2.5inch tall, that 250w hps should of still been able to penetrate through the entire bud, so my question is could the lack of CO2 be the cause or is it the light? My closet was standing almost right up against the wall so some of the exhaust air would move back to the intake and although it would be cool enough, I’m sure the co2 would be completely used after a few hours of this.

My current setup is similar, but using a slightly bigger scrog with a 400w HPS, but here, the closet is in a closed bedroom with NO access to outside air. I exhaust into the room, so that air probably circulates quite a few times through the closet, obviously loosing C02 in the process. The only air coming into the room is the gap under the room door so I’m kinda thinking that after circulating for some hours, the plants would of used up all or most of the CO2 in the room.

4th week flower now (flipped to 12/12 on the 10/07 but started counting day 1 from the 17/07), Hairs and bud structure looks good, but I know they start packing on material in the last 2 or 3 weeks so I’m wondering if 2 DIY, 2-liter CO2 bottles could help - just in case I’m right.

Has anyone ever used these DIY CO2 bottles (water/sugar/yeast) and could you see any difference in plant growth?

I found many videos on how to make it and people using but non really say if its made any difference.

I’m not trying to get super yields here, just by logical thinking, it seems that if a closet is in a closed bedroom, I would need a source of C02.

I’m sorry, even HPS doesn’t have the magic to penetrate through that thick of a canopy. Watts is watts, lumens is lumens, and PAR is PAR. It doesn’t matter what type of light is producing the light, if you have say 5,000 lumens at the bulb, no matter if it is made from a HPS, MH, LED, plasma or even a florescent, it is still 5,000 lumens and will drastically reduce the further you get away from the bulb, then on top of that you grow a very dense scrog, the undergrowth just didn’t get enough light, plain and simple, sorry. This is why most people underhcuck and lollypop, even the professionals, and even under 1000 watt HID lights.

Also, in my opinion, growing in a closet grow indoors, that has a very good air turn-over rate, the natural air has plenty of CO2 in it, and so if you are pushing the hot air out, that has higher oxygen(O2) in it due to the plant’s actions, and that air is being replaced with fresh air from the inside of the house, which might have slightly higher levels of CO2 due you, your family, and or pets living in there, and your plant will have access to plenty of free co2. If you supplement with CO2, you do need a way to prevent that CO2 from being pulled out of the grow room/closet.

Only suppliment CO2 when the lights are on, at night it is a waste.

Hope this helps,



I do admit that the canopy was dense in the 1st grow, but I’ve seen guys growing scrogs with foot long colas with a 250w hps (also with dense canopies) and basically the whole length of the bud gets filled up. Below is an example of a 250w scrog - 6.5’ x 3.2’. The light isn’t penetrating deep into the canopy but it is still much more than mine did at less than half then length of these.

About the closet. I know that the air in the house will replace C02, but what if the room door is closed? The only place air is able to get into the room is the thin gap under the closed door.

I smoked a bong in the room (bad for the plants, I know) but what I saw was that the smoke seemed to circle the room and not go outside into the passage and this was with the room door fully open. So basically the smoke from the bong allowed me to see the air current within the room and it seems that the air within the room is being circulated because of the 6inch exhaust. It basically circles the room and re-enters the passive intake. So with the room door closed, it seems that most of the intake will be bringing in air that has been re-used.

Nope, I’m not convinced, lol. All the untrimmed undergrowth probably took away from your tops developing better, much more than any worries about not enough CO2.

Again, you probably really needed to undershuck, and remove all those under shoots that didn’t develop anything. And the development in a ScrOG of the tops will still depend a lot on the strain, so you cant compare one picture to another unless it is the exact same strain. Not all strains respond as well to ScrOGing.


Also, check this grow out! It is only under T5s, just to show you it isn’t the “250 watt HPS” penetration, he seems to be matching with his T5s what is showing in your shared photos from (site).

It’s not necessarily the light, it’s not the lack of CO2, don’t worry, the knowledge of training and pruning comes with experience, you’ll get there.


I just watched the rest of this documented grow on his you-tube. It was really pretty well documented and he ended up with just over a half pound dry weight. This sounds about right under these T5s, he got well over 230 grams.

Thanks for the reply.

Yeah the training and pruning thing is really something that may take a while to learn. I removed the scrog from my current grow and tied down some branches to let more light penetrate the sides of the buds, but I didn’t really want to use these cotton wool string for fear they might cause some bud rot where they meet the stem as the bud seems to be growing around them and I assume they will block air and allow spores to germinate. Keeping a close eye though.

I also pruned the undergrowth and undeveloped bud sites, and probably took off way to much as my canopy is very shallow now (probably less than 1 foot deep this time).

I remember in the 1st grow that I had a Jack Harer in there as well. Grown from seed and did very minimal to no pruning on it but the buds shape were basically the same as those in the video you posted so maybe it is genetics but like I also said before, I get these clones, Cheese,LSD and Kush clones from a friend and his grow nice tall colas that swell up nicely throughout the bud and not just on top like mine did…good to know that experience will eventually fix this.

I forgot to mention something about my nutes.

I recently got a PPM and PH meter and found that my PPM are at around 600 to 700 at this stage and PH-5.6 (week-5 of flower). It was probably the same in the 1st grow because I tend to use less nutes as is commonly taught. I also read that others are going up to 1500 PPM at this stage of flowering.

Do you think that my low PPM could also contribute to smaller, airy buds?

I was under the impression that bigger buds come from light + CO2, and nutes only help the plant stay healthy but doesn’t contribute to bigger buds though. Been watching a few Youtube videos from “Growboss” and he often states over and over that nutrients has nothing to do with bud size. More nutes don’t mean bigger or better buds,etc, etc.

I’m a general plant lover and learned years ago how plants in packs on weight for size, and it comes down to CO2, because CO2 is oxygen combined with a solid particle of carbon (1 carbon + 2 oxygen molecules). The plants split this by photosynthesis made possible by the light (which acts as a catalyst only) and expels the oxygen, and then use the solid carbon as part of a building block for tissue (eg: when we burn some bud, the ash left in the bowl is the carbon we breathed out)…so when I see PPM of 1500, it seems unnecessary, but then again, theory always seems a bit different from actual practice.

So I basically need to know if everyone goes between 1200ppm - 1500ppm in late bloom. I’ll be flushing and feeding plain water 1.5 weeks before chop and my again, my ppm are 600 - 700 at the moment. I’m in week-5, but only started counting 1 week after I flipped so I have about 3 to 4 weeks before chop.

I agree, experience will play a big part.

I also agree super high PPMs does not make for bigger denser buds, and 600-700 PPMs availiable to the roots, in hydro especially and with a good properly balanced nutrient, is plenty in most cases to get your plants to develop to the fullest. It is what we recommend running our “Marijuana Booster” brand nutrient line at, an EC of 1.2 or 600 PPM in the standard USA common conversion.

CO2 will help, don’t get me wrong. It is just a lot of work to set up your grow area so all the extra CO2 isn’t wasted and in my humble opinion – if you get a good turn over in the small grow room, you will usually be fine as the normal air in a house or even outside has plenty of CO2 (it is a sad and unfortunate fact that atmospheric CO2 levels are at all time highs, and it hasn’t been this high since about 800,000 years or more ago, humans have only been around about 200,000 years and cannabis was only domesticated about 10,000 years ago [maybe 15-20 thousand years ago at the most according to some new archaeological findings]).

The fact that the small grow room is in and attached to a larger room that you think doesn’t get much turn over of fresh air could be a concern, you could think of ways to have fresh air introduced regularly to the larger room as an option. But I bet CO2 is getting in that bigger room nonetheless, and again a lack of CO2 should probably be the least of your worries or not a worry at all.

Also as you noted, strain will play a part, especially when growing from seed. When growing from clone you should be able to expect the clone to preform the same as the parent it was cut from, but training and proper treatment during all phases of growth will make a huge difference in the end result. Also, is your friend using the exact same light set-up you are using? If your friend is using a 600 watt HPS in a bigger room, again, it can make a really notable difference.


Thanks again for your reply’s.

Good to know that 600 - 700ppm are sufficient. I decided to add 2 DIY C02 bottles in the room as it is very little work to make and maintain it. This way I at least have some piece of mind that the closed room has some CO2 circulating within it, even if it doesn’t make a difference to the grow - I’ll just feel better about adding it. Will take my about 10mins to make the 2 bottles anyway and then I can stop thinking - what if.

Yes my friend is using the exact lighting that I am but he isn’t using a scrog. He tops, trains and lolli-pops so that there is 1 center cola and about 4 or 5 colas evenly spaced around it and he has about 4 plants under one 400w bulb with this same shape (1 large center cola, with about 4 slightly smaller ones evenly spaced around it)…but he has a MUCH larger grow room and he NEVER weighs his bud ever. He has so many in rotation that he doesn’t care to increase his yield any more than what his getting. He even harvests early at times to make space for new clones. He is more worried about what the bud looks like, rather than the weight or ripeness…and his bud looks very beautiful. Light green with orange hairs and full of crystals.

I’m not in the position to do that as I need to optimize my small grow to get the most weight in the least amount of space. My bud is noticeably stronger than his but mine isn’t as light green and good looking. I’m considering making the closet slightly bigger to accommodate some side lighting and will probably do that in the next grow.

Thanks for your help again.

If using LED for main lighting is safe and/or beneficial to use CFL lights on the sides and under leaves during veg and flower?