When discussing food for our plants, we often consider the nutrients we put into the soil. Still, we often forget that photosynthesis is also required for plant growth. Without light, water, and CO₂, we wouldn’t have the catalysts for photosynthesis. Just as increased nutrients can lead to larger yields, higher amounts of ambient CO₂ in your grow area can also lead to higher yields.
Supplementing CO₂ is a common practice in commercial cultivation but not widely used in home grows, and we’ll cover some potential reasons for that in this article. Together we will explore the conditions that need to be met, the best time to use it, and how to calculate the amount of additional CO₂ needed for your grow space. We’ll also cover monitoring options, desired levels for different growth stages, and some DIY methods for boosting CO₂ in a grow tent.
TL/DR | Key Takeaways:
- Works best in sealed environments
- Requires high-intensity lighting
- Humidity will increase
- Nutrient levels will need adjusting
- Monitoring is essential to plant and human health
Why do Cannabis Plants Need CO₂?
The need for CO₂ is not exclusive to the cannabis plant. All plants need a minimum of 200 ppm CO₂ in the air to facilitate growth through photosynthesis. Without it, we would have no photosynthesis and no growth. Ambient atmospheric levels of CO₂ are around 400 ppm.
Studies have shown that increased levels of CO₂ can also increase plant growth. This is in part related to the Calvin Cycle (C3), a series of biochemical reactions that occur during the lights-on phase as a direct result of photosynthesis.
What is the Role of CO₂ in Photosynthesis?
Light energy converts water and other compounds into gases such as oxygen and chemicals like ATP and NADPH. Chemical energy produced from the light reaction is used by the enzyme Rubisco for carbon fixation. Chemical energy produced from this reaction converts carbon dioxide and water into carbon compounds that eventually form glucose/sugars. These invisible actions form the Calvin Cycle, of which CO₂ is essential.
To sum up, it’s complicated, but we can easily understand that without CO₂, we would have no photosynthesis and, therefore, no growth. We know that increased levels can increase photosynthesis, promoting increased plant growth and larger yields. The role that CO2 plays is important, but is it something that home growers need to supplement?
Environmental Factors to Take Into Account When Adding CO₂
The benefits of increased CO₂ in the grow tent would be wasted if a few other factors weren’t met. To take full advantage of supplemental CO₂, light intensity must be increased beyond normal conditions. A byproduct of increasing light intensity is additional heat in the grow tent. Although the additional heat benefits CO₂ absorption, cooling equipment may still be required to balance temperatures with the higher light output.
Cannabis plants exchange gases and water from tiny pores on the underside of the leaves called stomata. Although CO₂ encourages the “guard cells” that regulate the opening and closing of the stomata to opt for the closed position, the increased heat in the tent and the increased leaf surface temperature (LST) cause the stomata to remain open so they can take in more CO₂ and perform photosynthesis at a higher rate. This will increase humidity.
Because the stomata remain open longer, transpiration increases humidity in the grow tent. A dehumidifier may be needed to maintain this environmental factor in the desired range. Supplemental CO₂ for cannabis is commonly used throughout the flowering stage when keeping humidity low is most important.
Do Autoflowering Plants Benefit from Increased CO₂?
Autoflowering varieties are cannabis strains that do not need a specific light schedule to initiate the flowering process. Autoflowers are independent of the photoperiod. Because they have a limited amount of vegetative time, supplemental CO₂ can be beneficial. The increased photosynthesis will help plants grow faster in a shorter period.
Larger plants will have longer branches and more bud sites, leading to larger yields. However, the size of the root zone does have a direct correlation to a plant’s ability to use additional CO₂. This may reduce the overall impacts, but additional growth is still additional growth when adding up your harvest. We’ll cover the importance of root zone size in a few moments.
How to Calculate the Amount of CO₂ Needed in Your Grow Space?
The measurement used to calculate the concentration of CO₂ in the air is parts per million (ppm). It is extremely important to know that high concentrations of CO₂ can be lethal. CO₂ is heavier than the air we breathe and can be found in higher concentrations lower to the floor. Monitoring grow rooms and other grow spaces is important, and we will definitely cover the monitoring options later in this article.
To calculate the amount of CO₂ needed to raise the air’s ppm to 1,000, you need to do some math. To reach 1,000ppm in a specific area, multiply the LxWxHx.001. The answer will provide you with the cubic feet of CO₂ needed to raise the levels in your grow room to 1,000ppm. CO₂ is often sold by the pound in tanks; one pound of CO2 equals 8.7 cubic feet.
If that sounds confusing, and you are using a professional, controllable CO₂ dispersion source, you can use a calculator like this one to figure out the flow rate of CO₂ levels to bring the grow space to the desired ppm.
Cubic volume x
.0006 = 600ppm
.0008 = 800ppm
.001 = 1,000ppm
.0015 = 1,500ppm
.002 = 2,000ppm
How to Enrich Your Cannabis Grow Environment with CO₂?
A singular factor that separates many commercial grows from home cannabis cultivation is whether the growing environment is sealed or not. Sealed rooms do not exhaust or intake air. Environmental conditions are controlled within the room, and CO₂ is supplemented due to the lack of fresh air coming into the room.
Grow tents are not considered a “sealed” growing environment; I’ll explain why this has a big impact in just a moment. The use of CO₂ can be performed in sealed or non-sealed environments, but the approaches will vary.
One constant is that CO₂ supplementation should take place during the lights-on phase. The primary benefit of CO₂ is increased photosynthesis, which only happens with the lights on.
The distribution point of CO₂ should be placed above the canopy. CO₂ is 1.5 times heavier than air, so as CO₂ is released above the canopy, it will fall downwards onto the plants. Using fans to circulate the air within the grow space helps keep CO₂ from collecting at the bottom of the tent. Regardless of how you supplement CO₂, you will need a way to monitor the levels accurately.
Monitoring Tools Needed
Keeping tabs on the CO₂ levels is an important part of supplementation. It ensures you are hitting target levels without wasting resources. Most CO₂ sensors measure infrared light in an air sample using a non-dispersive infrared (NDIR) sensor to calculate ppm. Sensors come in a variety of shapes and price points.
Growers should monitor CO₂ levels at the canopy level, where they matter most for photosynthesis. Many of the sensors can be customized to set alarms or triggers at certain ppm levels. In greenhouses, if CO₂ levels are too high, sensors can trigger the use of extraction fans to bring them back within the target range.
Popular sensors in the tents of home growers include AC Infinity and Pulse Pro. These devices offer the ability to send real-time data about CO₂ levels directly to your phone. Less sophisticated devices like the Autopilot Desktop monitor can also be used. Choosing a device that logs the highs and lows for the day is best.
In the Vegetative Stage
Claims that seedlings need CO₂ are slightly misguided. CO₂ can indeed promote faster growth in vegetable seedlings, but for cannabis to take advantage of that additional CO₂, growers need a high-intensity grow light. The problem is cannabis seedlings don’t do well under very strong light.
We also know that seedlings and young vegetative plants have small root systems. The size of the root zone is also an important factor in the impact of supplemental CO₂. As the root zone grows, the cannabis plant can capture more CO₂. The root zone is a major storage point of carbohydrates (glucose/sugars) and respiratory activity. If the root zone is too small and the CO₂ is too abundant, it can cause a down-regulation of photosynthesis. This negatively affects the plant.
Down-regulation of photosynthesis represents a decreased function and production at a cellular level. It can occur in the abundance of CO₂ if the root zone is not large enough to store the carbohydrates and sugars produced by photosynthesis. A lack of nitrogen availability in the soil can also trigger down-regulation in the presence of elevated CO₂. Essentially, you are giving the plant too much of a good thing, and it can’t process all the CO₂.
See our chart below for ideal ppm levels during the various stages of growth.
In the Flowering Stage
This is the time to really go for it when using supplemental CO₂ in the cannabis grow room. By this stage of growth, the root system is well-developed. It can handle the additional transpiration and carbohydrate storage demands from higher ambient CO₂. The plants are mature and hardy. They can withstand the added light intensity and higher leaf surface temperatures required to maximize the benefits of adding CO₂ to your grow.
As your CO₂ levels increase and the plant creates more energy from photosynthesis, growers will need to adjust their feeding levels. The elevated levels of photosynthesis will require more nutrients in the soil for the marijuana plants to consume, primarily nitrogen. A lack of plant-available nitrogen can negate the benefits of adding CO₂. Most growers prefer to reduce CO₂ levels back to a natural level in the final week or two before harvest.
| Stage | CO₂ levels |
| No growth | 0-200 |
| Ambient atmospheric levels*** | 400 |
| Seedling stage | 400 |
| Vegetative stage | 600 – 700 |
| Late vegetative stage | 800 – 1200 |
| Flowering stage | 1200-1500 |
| Pre-harvesting stage | 800-400 |
| Harvesting stage / maximum | 1800 |
| Health hazard* | 2000-5000 |
| Serious risk** | 5000 + |
* Headaches, drowsiness, loss of focus, increased heart rate
** Carbon monoxide detectors often don’t detect CO2, but there are many specific detectors available.
*** Signs of too much CO₂ for the plant include looking weak and yellow, including necrotic and chlorotic leaf spots.
Are there any DIY options for CO₂ Enrichment at Home?
The internet is full of crafty ideas and ways to boost ambient CO₂ levels in your grow tent. We will cover some of them here, but they all have similar drawbacks. Many DIY sources will take time to maintain and require frequent attention. While these techniques have their “pros,” I’ll get to the “cons” later in this article.
One of the most undervalued DIY grow hacks for adding CO₂ into your grow environment is your breath. The simple act of talking can raise the ambient CO₂ levels in your lung room and grow tent. After a two-hour podcast in the garden, the levels are often a few hundred ppm higher. So, read to your cannabis plants or sing them a song; they’ll never tell you your breath smells funny.
Mushroom bags
This is the most common DIY source for supplementing CO₂ in the grow tent. Mushrooms naturally produce CO₂ as part of their metabolic process. By using special kits like the exhale bag, growers can easily add a continuous stream of supplemental CO₂. Growers hang the bag above the canopy and let the CO₂ fall onto it. This low-maintenance option can reportedly supplement CO₂ for six months in small grow tents.
Chemical reactions
Baking soda and vinegar are a popular combination for DIY CO₂ generation. Baking soda is a base; vinegar is an acid. When a base and acid are combined, they react, sometimes explosively. In this case, there is a two-part chemical reaction. The vinegar reacts with the sodium carbonate in baking soda and forms sodium acetate and carbonic acid. Instantaneously, the unstable carbonic acid converts into carbon dioxide and water.
Fermentation is another chemical reaction method that generates DIY CO₂. Growers need only three simple ingredients: sugar, water, and yeast. Taking that knowledge one step further, this study concludes that glucose and sucrose generate higher rates of CO₂ than lactose.
I have seen the suggestion of burning tea candles in the grow tent. Heat vaporizes the liquid wax into molecules of hydrogen and carbon, which are then drawn into the flame, creating the familiar heat and light, as well as water vapor and CO₂. Although the science absolutely makes sense, leaving an open flame in your grow tent is absurd and HIGHLY advised against.
Commercially Available CO₂ Enrichment Options for Cannabis
Large-scale cannabis cultivation is an industry measured by the gram. Their programs are specifically tuned to maximize yield, which in turn can maximize profits. The use of supplemental CO₂ is standard, and there are a variety of professional delivery methods to choose from. Instead of hitting a general range, precision instruments are used to maximize benefits and minimize waste. Sealed room environments are critical to meet those standards.
CO₂ injection systems
Injection systems involve tanks, pressure regulators, solenoid valves, timers, and a monitoring system. They detect atmospheric CO₂ levels and, based on preset parameters, keep the concentrations at a specific level. Valves open and close without the need for human intervention. These systems can also be turned off during the dark period, which saves resources and money.
Commercial cannabis cultivators that rely solely on artificial light in windowless, sealed rooms tend to use this option instead of the others. It is cleaner than the next suggestion, and the placement of the emitters can be custom-tailored to the room and the location of the plants within it. Home growers with a big budget and a desire to maximize their yields will often choose a smaller version of this system.
CO₂ generators/burners
Supplementing CO₂ using a generator/burner is a method that involves burning liquid propane, butane, or natural gas, which releases CO₂, water, and heat. Generators are positioned above the canopy, allowing the generated CO₂ to fall into the plants. Industrial units can effectively cover thousands of square feet at a time. These systems are widely used in various styles of greenhouse cultivation facilities.
It is important to note that the type and purity of the fuel matters. Incomplete combustion can result in impurities that can be detrimental to plant health. Incomplete combustion can also produce carbon monoxide, which is dangerous to human health. Growers can tell by the color of the flame if there are impurities in the fuel. Greenhouses have an advantage over sealed rooms because they can open portions to exchange the air should there be any concern.
Dry ice
We all know CO₂ is a gas, but did you know that the solid form is dry ice? Unlike ice, dry ice doesn’t melt into a liquid; it sublimates directly into a gas. Growers can boost CO₂ levels using dry ice. Professional operations use special cylinders to house the dry ice and a gas flow meter to regulate the dispersal. By controlling the release to the canopy, professionals can maintain a specific ppm.
Smaller operations have relied on math to calculate the amount of dry ice needed to reach a target ppm. In general, it takes roughly 1 lb of dry ice to produce CO₂ levels of 1,300ppm in a 100sqft space for one day. Growers could replace that amount of dry ice each morning. Other cultivators will use smaller chunks of dry ice and replace them more regularly throughout the day.
Should You Add CO₂ to Your Weed Grow?
Here is where I feel like I may be a bit of a Debbie Downer. Supplementing a home cannabis grow with CO₂ is not going to make a bad grower into a good grower. Plants need to be at optimal health levels, meaning their hydration and nutrition must be at peak performance. Lighting needs to be specifically tuned in relation to the elevated levels. Without tools to measure light intensity or ppm of CO₂, home growers are just guessing.
As for the “cons” to the DIY chemical reaction methods mentioned earlier, there are a few. Most of these DIY solutions for CO₂ generation require daily or hourly maintenance to facilitate constant CO₂ output. The levels created by these methods are often not significant enough to reach target levels in a sustained manner, but again, you’d need a measuring tool to confirm that. These methods are cheap attempts and often fall short, although they are sound in principle.
More importantly, if we as home growers are exhausting the entire volume of air in our tents every 1-3 minutes, we would be adding CO₂ just to pump it immediately out. Sealed room environments are superior because none of the air is removed from the room. All of these reasons are why we see CO₂ supplementation used professionally but not very often in home cannabis cultivation.
Considerations When Adding CO₂ to a Cannabis Grow Tent
First and foremost, ensure that you have an accurate and reliable way to measure ppm levels of CO₂. These devices not only protect the health of your cannabis plants, but they can also protect the health and wellness of pets and humans inside the building. Other considerations include:
- How good of a grower are you? Unhealthy plants won’t benefit from added CO₂
- What is your budget? Supplementing CO₂ will increase your electricity bill through increased lighting and environmental controls to create the ideal conditions for CO₂ uptake. It will also increase your materials costs.
- Do you have the tools to measure ppm and PPFD (light intensity usable by plants)?
- Are you in a sealed room?
Conclusion
Supplementing CO₂ has been shown to positively affect cannabis growth. By increasing their ability to photosynthesize, cannabis plants can grow quicker, stronger, and bigger. Many cannabis growers, whether they grow at scale or in a small bedroom closet, share the goal of increased yields and faster harvests. As a result, growers will try many techniques to gain these advantages.
Boosting the growth rate through supplemental CO₂ depends on the light intensity, leaf surface temperature, size of the container, root zone maturity, and the soil’s available nutrition, namely nitrogen. Without them, growth response is limited. I tend to recommend CO₂ usage once all of the other variables are dialed in and growers are comfortable with their skills.
If you were to read this article to your cannabis plants and you had a way to monitor CO₂ levels, I would bet your ppm levels have just increased. There are little things we can do as home growers that add up to create better flowers at the end of the day. Step one: start with authentic ILGM genetics. Step two: grab yourself a free copy of Roberts Grow Bible so you are prepared with all the necessary knowledge. Third, consider if CO₂ is right for you.
What has been your experience with CO₂ supplementation? Have you tried any of the DIY methods mentioned above? How did it go? Let us know in the comments and help spread the knowledge to the ever-expanding global cannabis-growing community. Together, we learn; together, we share better herbs. One love!
