Increasing Cannabis Growth With CO2

Using carbon dioxide (CO₂) for your cannabis grow is an advanced topic, but if you’re reading this it is safe to say that you achieved this stage of expertise in your growing journey. Hats off to you!

We care a lot about grow lighting but when it comes to high rates of photosynthesis, measuring and controlling your lighting is not enough. Photosynthesis mainly requires photons (light), water and CO₂, so keeping control of your CO₂ may be just as important as watering your plant or ensuring adequate DLI values. If you want to learn more about those topics, we recommed our article on PAR levels for cannabis and DLI levels for cannabis.

Should You Add CO2 to Your Grow?

Adding additional CO₂ might not make sense for any indoor grow as it also comes with its cost and complexity. There are two main prerequisites to get the most out of additional CO₂: Lighting and temperature.

Grow Lighting

To maximize photosynthesis, you need a lot of light. But not just any light, but light that is usable for plant photosynthesis (PAR). Therefore, a high PPFD is what you’re aiming for. If PAR and PPFD is new to you or you want a quick brush-up, we recommend our other article explaining these values in more detail.

Not all grow lights are able to provide such a high PPFD. Especially lower-powered grow lights such as CFLs, fluorescents (e.g. T5, T8, T12), or small LEDs may not be able to produce high enough PAR levels – so light becomes the limiting factor and not CO₂.

High-powered grow lights such as HPS and large LEDs usually supply enough PPFD to make additional CO₂ a viable option. If your CO₂ levels are too low, some of the light even goes to waste (and therefore your electricity bill) as your plants won’t be able to “digest” it due to lack of CO₂.

If you want to know for sure how much PPFD your grow lights are able to supply, use a PPFD meter such as our PPFD meter app to measure it. If you’re considering new grow lights, our article for choosing the right grow light is worth checking out as well.

Grow Room Temperature

For a high CO₂ to be effective, a higher environmental temperature is required. If your grow is kept cool, the effect is not as drastic and adding CO₂ may not be worth it. If you’re able to keep the temperature in your grow room above 85°F (30°C) during its lighting period, then CO₂ can be used by your plants effectively.

Increasing Yields with CO2

CO₂ is the main influential factor when considering optimal PAR and DLI levels. As for really maxing out the rate of photosynthesis, all plants do have a point where more light just does not equal more growth. But more often than not, inadequate CO₂ supplementation is the primary limiting factor.

CO₂ is measured in parts per million (ppm). General indoor CO₂ levels of about 400 to 600 ppm quickly restrict photosynthetic activity to less than half of what would be possible. Supplementing higher CO₂ to reach at least 800 ppm greatly boosts photosynthetic activity and allows for higher PAR levels and thus a higher PPFD. If you really want to max out your cannabis plant’s growth, we recommend the following CO₂ levels:

Growth Phase	CO2 Level (ppm)
Seedling / Clone	400 – 600
Vegetative	400 – 1200
Bloom / Flowering	800 – 1500

How to Add CO2 to Your Cannabis Grow

There are many ways to add CO₂ to your cannabis grow ranging from cheap and easy to expensive and complex. In general, you need something that produces CO₂ and allows it to “fall” onto the plants from above. As CO₂ is heavier than air, it will sink to the bottom of your grow room where it doesn’t have much of an effect. Using a fan to circulate the air at the bottom of your grow room is an easy way to re-introduce more CO₂ to the air around your plants’ leaves.

The following list describes your options and our recommendations in more detail.

Hobbyist Recommendation: Yeast Fermenter

For the hobbyist, we recommend installing what is called a yeast fermenter or CO₂ fermenter. As the name implies, it is a combination of yeast, sugar, and water that produces CO₂ through fermentation and can be done at home pretty easily and safely. Even though the CO₂ production won’t be as high and as controllable as with other systems, fermentation provides a fairly quick and easy way to get started with adding CO₂ to your cannabis grow.

We put together a shopping list (with links to Amazon) of all the individual items that you'll need:

• Fermenting Jug with Airlock
• Turbo Yeast
• Tubing
• Plain white sugar from your local grocery market

To set up your mixture, use about 60% of regular tap water, then fill the remaining 40% with sugar, and add the yeast (Turbo Yeast recommended). Let it sit at room temperature of about 70°F / 21°C for the fermentation to happen. This initial mixture should last you about 3 to 10 days. In order to keep it going, pour out about a third of it every week and refill a fresh mixture.

A video of a yeast fermenter system in action can be found here:

Advanced Recommendation: CO2 Injection System

If you’re really serious about controlling your grow room’s CO₂, the most effective is a CO₂ injection system that uses compressed carbon dioxide and a controlled injector. The CO₂ usually comes in bottles or tanks that are then hooked up to a control valve that allows you to adjust the amount of CO₂ you’re injecting into the air in a very controlled manner. We recommend using an automated system so you don't have to worry about valve settings and pressure levels.

As you can imagine, such systems pose a considerable financial investment into your grow as the equipment itself is costly and you periodically need to refill your CO₂ tank. However, if you’re looking for the most advanced and effective way to enrich your CO₂, this is the way to go.

We put together a shopping list (with links to Amazon) of all the individual parts that we recommend:

• CO2 Sensor & Controller
• Controllable Solenoid Valve
• Rain System

Not Recommended: CO2 Generator

CO₂ generators burn some kind of fossil fuel or alcohol to produce CO₂. Apart from the obvious fire hazard, this method of generating CO₂ is not only harmful to the environment but also to your health as byproducts such as dangerous carbon monoxide are produced as well. Even though there are specialized CO₂ generators for gardening applications available, we recommend never using such a device due to the mentioned downsides and risks.

Other Options to Generate CO2

Placing CO₂ bags or compost in your grow room will also add CO₂ to its air. CO₂ bags are usually filled with fungi that grow easily and produce CO₂ while doing so. However, even small grow tents already require multiple CO₂ bags to reach a significant enhancement of CO₂ levels, so it won’t be particularly effective. Using compost comes with its downsides as well as it might produce an unpleasant smell and not that much CO₂ as well.

Another option would be using dry ice. Dry ice is pure, concentrated CO₂ that is released in the air as it melts. However, dry ice is dangerous to our skin, expensive, and won’t last long.

Prepare Your Grow For CO2

After you’ve decided on a method to add the additional CO₂, there are other measures that you need to take to use it effectively:

Turn It Off In The Dark

It makes no sense to add CO₂ in the dark period, as there won’t be any photosynthetic activity in the lack of light. So if you’re using a controllable system, you can even save more money by having your CO₂ enrichment turned off about an hour longer than your lights without any downsides on the effectiveness of additional CO₂.

Seal Your Grow Room

As the surrounding air has a lower CO₂, you should seal your grow space adequately so the added CO₂ won’t leak out immediately – especially with very high CO₂ levels above 1000 ppm. Doing so also requires you to keep track of your grow room’s humidity and temperature as the climate in your newly-sealed grow space will be different.

Keep Yourself Safe

CO₂ is to plants what oxygen is to us humans. Your plants flourish under high CO₂ levels whereas we need to keep an eye on our safety. Concentrated CO₂ is dangerous and its effects can range from slight drowsiness up to death. Even though very high CO₂ levels of 1500 ppm are still considered a safe amount, you should still avoid breathing such air for too long of a time. This implies minimizing time in your grow room when your CO₂ enrichment is active as well as making sure that other areas of your location are sealed adequately from the grow space. If you’re using an injector system, you must also make sure that CO₂ levels can’t go beyond what is necessary and safe.

Measure Your CO2 Levels

It is obvious that you can’t control what you don’t measure. We would love to tell you that our PAR meter app measures CO₂, but this remains a grower’s dream for now. Anyway, CO₂ can be measured with fairly low-cost equipment and investing in a CO₂ meter or CO₂ monitor is a must when altering the air in your grow room.

If you haven't got one in your grow room already, we recommend getting a combined CO₂, temperature, and humidity sensor that allows you to monitor all relevant environmental factors. We can recommend the following CO₂ monitor that is using an accurate NDIR sensor and is rechargeable via USB off Amazon or AliExpress:

Measure Your Grow Lighting

Our app allows you to accurately measure PAR in PPFD without any external sensor by utilizing the smartphone's camera. We use lab-grade equipment for our development and testing. We can confidently state that our app is achieving the desired accuracy at ease. We've even written a white paper on how to achieve industrial-grade accuracy for your own grow as well.

Sources

• Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2
• Fluence Photobiology Guide

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