CO2 Supplementation, Simplified.

By Garrett Cropsey
Published: January 1, 2017 | Last updated: April 21, 2021 10:12:03
Key Takeaways

One of the easiest ways to provide indoor plants with extra CO2 is by supplementing it with mycelial CO2—a byproduct of growing fungi. Maximize your efforts using this method of supplementation with these tips.

Plants grown indoors often consume more CO2 than what enters the grow space naturally. These same plants become more productive when CO2 levels are maintained at higher levels than atmospheric concentrations.


For these reasons, indoor gardeners provide supplemental CO2 to their growrooms using a variety of methods. Compressed tanks, burners and the gardener’s own breath are all effective at providing the CO2 necessary for growth, but mycelial CO2 production is the easiest and most cost-effective means of maintaining beneficial CO2 levels in gardens up to a certain size.

Mycelia are the fibrous, root-like networks formed by fungi throughout a substrate. Fungi are more closely related to animals than to plants; like animals, they exhale CO2 as they grow. This CO2 mixes throughout the grow space for use by plants. Mycelia come in small, pre-inoculated bags, buckets, pads or bottles already producing CO2, without giving off any excess heat or requiring complicated set-ups.


While mycelial CO2 is low maintenance, it is important to place it in the correct area in a grow space to get optimal results. It should be placed high enough in the room so the CO2 cascades over the plants instead of pooling at the bottom of the grow space where plants cannot reach it. Some forms of supplemental CO2 can be placed on or near the grow lights, which also helps optimize mycelia productivity, as this location increases heat and oxygen flow to the bags.

For any supplemental CO2 to be effective, it’s important to ensure the growth of your plants is not significantly limited by other variables. Considering mycelial CO2 doesn’t provide as much of the good stuff as compressed gas or burners, it is important to set your ventilation fan to run only when the grow space thermometer reaches the highest temperature tolerable by the plants.

This ensures the CO2 produced by the mycelia remains in the grow space long enough to be taken up by the plants. If the grow space contains multiple, high-power lights, they should be water-cooled rather than fan-cooled to ensure the CO2 produced by the mycelia can be used up, while keeping the lights and the grow space from overheating.


Eventually, the size of a grow space limits the effectiveness of mycelial CO2 supplementation. With large growrooms containing many lights, compressed gas or burners remain the most cost-effective means of providing the extra CO2 plants need.

However, there are ways to maintain or even improve the cost-effectiveness of mycelial CO2 supplementation. Try adding them to a new substrate or experimenting with different species of fungi, for example.


The Extra Mile

Mycelia eventually stop generating CO2 once they have consumed all of the usable material within their package, at which point they become inactive, but not “dead.” By providing mycelia with a new food source, they become active again and are happy to continue growing and producing CO2 while they decompose the new material.

Hay, manure, coffee grounds, rye pellets and many other materials are suitable for consumption by mycelia and can be prepared/sterilized at home or bought prepackaged online. Older mycelia can be broken down and mixed into the new media manually, or via syringe if adding liquid media to pre-sterilized bags.

Gardeners using soil substrates can also mix mycelia directly into the dirt before adding plants. These mycelia will produce more and more CO2 as they grow throughout the soil during the entire lifetime of plants, and will remain active even when new crops are added into the mix.

For those looking for more than just the additional CO2 in their growrooms, mushrooms like oysters, shiitake, wine caps and lion’s mane not only produce CO2 while they grow, but can also be eaten. All of these edible mushrooms enjoy warm indoor environments and provide delicious rewards for the oxygen they get from the plants. These fungi can produce several flushes of mushrooms before they finish decomposing their substrate, if they are given more water after each flush.

To recap, mycelia CO2 can be of great benefit to indoor gardeners if used correctly. Mycelia placed high in the grow space with minimal ventilation will generate a helpful boost in CO2 levels for extended periods of time for smaller grow spaces without the need for complicated, expensive equipment.

Mycelia can also be reused by adding them to new grow media and, with some extra effort, can even grow edible mushrooms. Fungi and plants have an ancient partnership and the gases they exchange throughout the grow space greatly benefit the plants within.


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Written by Garrett Cropsey

Profile Picture of Garrett Cropsey
Garrett Cropsey is a biologist currently living in Boulder, Colorado. His love of nature began in Longmeadow, Massachusetts, where his first job was with the local parks department. He has a degree in ecology and evolutionary biology from the University of Colorado-Boulder, where he graduated Magna Cum Laude in 2010.

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