CO2 in Your Indoor Garden

Plants need 17 essential elements to grow and reproduce. But just three elements—carbon, hydrogen and oxygen—account for more than 95% of a plant’s dry weight. If you want to dramatically increase yields in an enclosed growroom, providing your plants with extra carbon dioxide is an excellent investment.

In indoor gardens, light is usually the limiting factor for optimal plant growth. Once light is no longer the limiting factor, carbon dioxide becomes the limiting factor, so make sure your plants are getting all the fresh air and carbon dioxide they need.

How Much CO2 do Plants Need?

Fresh air contains carbon dioxide at about 370 ppm, so providing adequate ventilation and air movement is a good first step. Unfortunately, in a tightly enclosed greenhouse or growroom, ambient CO2 can get used up quickly. For example, in a plastic greenhouse, CO2 levels can be reduced to less than 200 ppm just 1-2 hours after sunrise. At levels below 200 ppm, plant growth will be greatly limited, and at levels below 100 ppm, plants will stop growing altogether.

On the other hand, carbon dioxide levels above ambient levels can have a dramatically positive effect on plant growth. By increasing carbon dioxide levels to 750-1,500 ppm, growers can expect a 30-50% increase in yields over ambient CO2 levels, and time to fruiting and flowering can be reduced by 7-10 days.

Just as excessive levels of fertilizer salts can harm plants, excessive levels of CO2 enrichment can have adverse effects. In a growroom, levels above 1,500 ppm are considered wasteful, and levels above 5,000 ppm are harmful. Excessive levels will cause the stomata on plant leaves to close, temporarily stopping photosynthesis. And since plants will no longer be able to transpire water vapor adequately when the stomata are closed, leaves can become scorched.

Adding More CO2 to Plants


Dosers are a relatively easy way to add CO2 to a growroom. Compressed carbon dioxide is stored in aluminum or steel tanks under pressure. A doser includes a pressure gauge, a solenoid valve, a timer, a flow meter and a length of tubing. Simply attach the doser to the tank and open the valve on the top of the tank. Set the flow meter to the desired flow rate, and then set the timer to open the solenoid valve for the required amount of time. Calculators are available online to help determine the right settings for any-sized room.

Since photosynthesis only takes place when lights are on, the timer should be set to turn off at night. In fact, since CO2 is released by plants through respiration at night, it is not uncommon for levels to build up to 450-700 ppm in the growroom by morning. Setting the timer to begin dosing CO2 one hour after the lights come on, with the last dose one hour before the lights go off, is the most economical way to provide your garden with supplemental CO2.

Even in a relatively sealed room, most of the CO2 will be taken up by plants or leaked from the room in just a few hours. To keep CO2 at optimal levels, it is best to dose for short periods of time at higher volumes than dose for longer periods of time at low volumes. Your plants will benefit the most if you bring the room to your target ppm in 5-10 minutes.

Some growers like to hang CO2 tubing with pin holes drilled along its length over the crop. Since carbon dioxide is heavier than air, it will drift down through the canopy to feed the leaves. Other growers like to place the tubing outlet next to an oscillating fan. The CO2-enriched air will circulate throughout the crop. Either way, good air circulation in the room will eventually bring the carbon dioxide-rich air in contact with plant leaves, as long as the air is not exhausted from the room before plants can absorb the CO2.


Carbon dioxide generators are another cost-effective way of adding carbon dioxide to a growroom, especially larger rooms. When natural gas or propane is burned completely, the exhaust fumes are made up almost entirely of carbon dioxide and water vapor. A CO2 generator using just a single burner should produce enough carbon dioxide to adequately supplement an 800 cu. ft. growroom.

One of the drawbacks of open-flame generators is they also produce heat and water vapor as by-products. In a 10- by 10-ft. growroom, a generator will raise the room temperature by about 5˚F on average, while simultaneously raising the relative humidity by about 7%. If heat and humidity are already a problem, the CO2 generator could make it worse. Therefore, sometimes growers use bottled CO2 and dosers in the summer, since they produce no extra heat or humidity, and CO2 generators in the winter, when the extra heat production can actually be a benefit.

Only use generators rated for indoor, horticultural use. They are safe and efficient, and they provide complete combustion of natural gas or propane with a pure, blue flame. Incomplete combustion from poorly designed systems can produce dangerous waste gases that can potentially damage the crop. For example, only .05 ppm of ethylene gas, a by-product of incomplete natural gas combustion, can cause premature senescence, causing the leaves to shed prematurely.


To conserve carbon dioxide, a controller is often used in conjunction with dosers or generators. A photocell in the controller senses when the lights are off, turning off the CO2 at night when plants don’t use carbon dioxide. Controllers also coordinate CO2 supplementation with temperature and humidity control. If temperature or humidity exceeds predetermined set points, exhaust fans are turned on to purge the room. To conserve carbon dioxide, the controller automatically turns off the doser when the exhaust fans are on.

Some carbon dioxide controllers also have inputs for monitors. A monitor uses infrared light to read the ppm levels of CO2 in the air. If levels fall below a minimum set point, the monitor sends a signal to the controller to turn the CO2 on. When the target is reached, the monitor sends another signal to turn it off. In this way, precise control of CO2 levels can be achieved, with little wastage.

Maximizing CO2 in the Growroom

Here are some tips for maximizing the benefits of CO2 in a closed growroom:

* Use air-cooled lights with glass inserts. Half the heat will be removed from the lights before it even enters the room, and the sealed glass will minimize CO2 loss.

* Use full-spectrum lighting with an enhanced blue spectrum. The blue end of the spectrum stimulates chlorophyll production and stimulates the stomata on the leaves to open.

* Use oscillating fans for good air movement. Dead zones of stagnant air can form a vapor barrier on the bottom surface of leaves, preventing CO2 intake.

* Use recirculating air conditioners and dehumidifiers. If exhaust fans have to cycle too often, much of the CO2 will be wasted.

* Keep air temperatures at optimal levels. Warm air temperatures speed up photosynthesis and CO2 intake, but if temperatures get too high, leaf stomata will close to conserve water. CO2-enriched plants generally thrive in temperatures 3-5 degrees higher than non-enriched rooms.

* Keep relative humidity between 40-60%. Under low humidity conditions, leaf stomata close, reducing CO2 intake.

* Increase the ammonium-to-nitrate ratio in your fertilizer. Under high CO2 levels, plants won’t assimilate as much nitrate-nitrogen, whereas the ammonium form of nitrogen will be used more efficiently.

* Use additives such as humic acid. Humic and fulvic acids improve the uptake of iron and other trace elements. Iron is a catalyst for chlorophyll production, and it contributes to more efficient photosynthesis under high CO2 conditions.

* Use organic CO2 boosters, such as straw inoculated with mushroom spawn. More than 50% of the mass of the straw will be converted to CO2.

If you have a small garden in a large room, an oscillating fan may be all you need to grow good plants, but the more plants you pack into a room, or the harder you push the yields of your plants, the greater the need for carbon dioxide supplementation. For intensive production, CO2 might be the best investment you could ever make.

By Harley Smith