Passive CO2 is So Cool

By Casey Jones Fraser
Published: December 27, 2016 | Last updated: April 21, 2021 10:08:27
Key Takeaways

There are many ways to supplement CO2 in your indoor garden. Passive CO2 products in particular can help you balance your budget, and help your plants deal with the heat.

Summer heat is hitting growers just as hard as ever this year. You may be hoping for 75 to 80°F with your lights on, but outdoor temperatures are trying to heat up your indoor operation.


Air-conditioning is the first line of defense. CO2 is another helpful addition to a hot room. Your plants can tolerate slightly higher temperatures in an environment with elevated levels of CO2. “Slightly higher” is an important phrase. If your thermometer reads higher than 85°F, don’t be surprised if your yields suffer. CO2 can increase yield and heat tolerance, but it cannot work miracles.

In many large gardens, CO2 generators are the best way to supplement carbon dioxide. However, other gardeners are working in smaller situations or perhaps with tighter budgets. Initially, CO2 tanks and a regulator are cheaper than a generator, but tank refills will quickly bring the cost right back up.


Unfortunately, CO2 generators create more heat. These metal boxes have flames that burn either natural gas or propane. If your growroom is too hot already, generators may be a bad idea.

When you need to keep a tight budget and avoid additional heat, while still adding CO2, consider passive CO2 products. Your local hydro shop probably carries some sort of fungus in a bag, or sheets of fabric that you hang in the growroom.

The fabric CO2 pads are laced with powdered compounds that can release carbon dioxide when left in a moist environment. If the relative humidity in your growroom is somewhere around 50%, these pads will steadily release gas with a gradual degradation. Always check packaging for the manufacturer’s suggested replacement schedule.


The bags of fungus are another option for a low start-up cost and instant CO2 without heat. Just like the CO2 pads, these bags slowly release carbon dioxide for your plants. Instead of powdered compounds, they are full of organic matter and living fungus. The fungus releases CO2 as part of its normal life functions. A small hole in the bag or bucket allows the gas to escape.

To ensure your passive CO2 purchases are working properly, seal the room and use a digital CO2 monitor. Many growers have air blowing into and out of their grow space. This is a great idea when temperatures are cool outside and your garden is running without carbon dioxide supplementation. However, air-conditioning and CO2 work more efficiently when they stay in the garden. Exhaust fans will suck the air out of your growroom, CO2 and all. Turn up the AC and shut the vents.


Digital CO2 controllers can be very expensive, but CO2 monitors are more affordable. Add the passive CO2 source to your garden and watch the numbers move up on your digital meter. After a period of days or weeks, levels may drop below your ideal range of 1,200 to 1,500 ppm. At that point you will know to bring in fresh CO2 pads or fungus bags. In my experience, passive CO2 is budget friendly and easy to maintain. Even with levels as low as 750 ppm, plants will metabolize nutrients more quickly.

There are also a few inefficient and ineffective passive CO2 methods you might want to avoid, such as baking soda and vinegar, dry ice, brewer’s yeast and sugar water. These methods are unreliable and disappointing when tested with a digital meter. Dry ice is the only one of those methods that will make a significant difference in carbon dioxide levels, but you would need to replace it multiple times every day. No thanks.

Two ways of adding a mild boost of CO2 are mycorrhizae and compost tea. Various species of fungi, such as beneficial root mycorrhizae, give off CO2. Soil growers should use plenty of fungal sources to boost root health and CO2 levels. When compost tea is properly brewed, it contains millions of living fungal cells. Spray those fungi onto plants leaves where they can give off carbon dioxide near the stomata.

Keep daytime temperatures as close to 77°F as possible, but consider adding CO2 if it gets much hotter. For good results, follow the directions on the package. For great results, add a digital monitor. Stay cool and garden often. Bless.


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Written by Casey Jones Fraser

Profile Picture of Casey Jones Fraser
Casey Jones Fraser owns Garden Grove Organics in northern Kentucky/Greater Cincinnati. He has a degree in communications and electronic media. He believes that indoor gardeners can achieve the highest-quality crops and maximum yields when proper science is applied. Since 1998, Casey has been testing various nutrients and supplements in search of outstanding harvests.

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