Passive CO2 is So Cool

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.