Carbon dioxide is a colorless, typically odorless gas made up of one carbon atom attached to two oxygen atoms. It is an important aspect of photosynthesis and must be present in some form for plants to grow, which means a well-run indoor garden generally benefits from some additional carbon dioxide, provided the garden has no other issues that must be corrected first.

Plants absorb carbon dioxide through their stomata, small openings in the epidermal layer of the undersides of their leaves. These openings allow CO2 in for use during photosynthesis and allow the plant to vent out oxygen and water vapor. During the reaction, plants take in six water and six carbon dioxide molecules, and light, to form one sugar and six oxygen molecules. The plant vents off the oxygen, and uses the sugar for growth.

A shortage of carbon dioxide hinders or even prevents plant growth, while a moderate boost allows many plants to thrive, especially in warmer-than-usual environments, which are often the case with indoor gardens.

When supplementing CO2, the goal is to provide the ideal range of additional CO2, which should be higher than atmospheric conditions, most commonly in the range of 1,000-1,300 ppm, depending on the specific needs of the crops. CO2 levels exceeding this range have little additional positive effects, and levels exceeding 1,500 ppm may hinder plant growth.

Light levels and other atmospheric conditions also play a role in how much extra CO2 is ideal for a garden. It’s always best to research the needs of what you’re growing.

Surrounding the plant’s stomata are guard cells that allow the stomata to open when light and moisture are conducive to growth, and to close in times of drought, high heat and darkness. This is why supplemental CO2 is recommended only during the lit hours of a garden, whenever possible, unless the source of supplemental CO2 is continuous and there is no way to turn it off.

Given that CO2 is an essential piece of the photosynthesis puzzle, most indoor gardeners will want to provide their plants with carbon dioxide in one form or other. Luckily, there are many options. Here are nine ways to add CO2 to your growroom.

Fresh Air

Fresh air contains about 400 ppm of CO2, and plants grown outdoors only receive CO2 from what is available in this air, which is an adequate amount for normal plant growth. For indoor growers looking to constantly provide this amount of CO2 to their gardens, it is as simple as providing fresh air—continually removing the air that is depleted of CO2 from inside the garden and replacing it with fresh air from outside.

Fans are used for this purpose, with higher cfm-rated fans moving more air per minute. Using this method, a grower can only ever supply atmospheric levels of CO2 to their plants and nothing more, and run the risk of altering their growing environment in other ways, such as increased humidity levels.

Natural Gas

When they are burned, natural gas, propane and liquid fuels give off carbon dioxide, and there are specialized carbon dioxide-generating burners designed for this purpose. These are most commonly used in large greenhouses and are less suitable for small indoor gardens. These burners are a heat source as well, which must be taken into account when planning your garden’s cooling requirements. Since there are inherent dangers in their use, such as flames and fuel in an enclosed space, I do not recommend their use in home gardens.

Compressed Tanks

Compressed CO2 tanks come with a regulator that allows the gas to be slowly released over an extended period of time. Tanks of compressed carbon dioxide are available at many hydroponics stores and traditional commercial outlets. They are a popular in home indoor gardens, but tanks must be routinely brought in and out of the garden, which may be cumbersome compared to other options.

Dry Ice

Dry ice is the frozen, solid form of carbon dioxide. When it is left at room temperatures it melts, releasing carbon dioxide. Since dry ice melts relatively quickly, it would need to be replaced on a regular basis—think several times a day. This alone makes dry ice an extremely labor-intensive and potentially cost-prohibitive method of CO2 supplementation.

Pads & Buckets

Prepackaged pads or buckets containing sodium bicarbonate (baking soda) and citric acid are easy ways to add more CO2 to an indoor garden. When sodium bicarbonate and citric acid come in contact with water, they rearrange to produce sodium citrate and carbon dioxide. This method is a low-mess option, as the pads or buckets are simply swapped out as needed.

Yeast

When grown in carbohydrate solutions, yeast will produce carbon dioxide along with alcohol. Yeasts are small fungi commonly used to produce carbon dioxide to make bread rise and to carbonate old-fashioned sodas, beers and sparkling wines. Yeast can be mixed with sugar in a container, and once it is started it will bubble carbon dioxide until either it runs out of sugar, or kills itself via alcohol poisoning.

Fungi

Larger fungi such as mushrooms can also be used to naturally supplement CO2 in the grow space. There are bags on the market that contain mushroom spores in a medium expressly designed for the purpose of slow-releasing CO2 in the garden. As the fungi grow, they give off carbon dioxide. When selecting bags for purchase, make sure the medium has not already been fully colonized, as this is a sign it is almost used up.

Bottles

Another emerging product is the CO2 canister, a user-activated, all-natural CO2 generator that begins to emit CO2 once the consumer adds warm water and shakes the bottle. These canisters have become popular with gardeners looking for an easy and effective way to supplement CO2 levels without adding any heat, open flames or additional equipment.

Sourcing CO2 for Your Indoor Garden

The Gardener

Finally, one of my favorite sources of carbon dioxide is the gardener. As a large mammal, humans exhale carbon dioxide as part of our respiration. A gardener in a garden is a source of CO2 that is already contributing to the health of the garden in other ways. A bit of healthy, heavy breathing can be good for both the garden and the gardener.