CO2 Enrichment for Indoor Gardening



Biologists and plant physiologists have long recognized the benefits of higher CO2 content in the air for plant growth. Horticulturists and greenhouse growers have used CO generators to enhance growth rates on plants for many years with good results.

With the advent of home greenhouses and indoor growing under artificial lights and the developments in hydroponics in recent years, the need for CO2 generation has drastically increased. Plants growing in a sealed greenhouse or indoor grow room will often deplete the available CO2 and stop growing. The following graph will show what depletion and enrichment does to plant growth:

CO2 Diagram

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Below 200 PPM, plants do not have enough CO2 to carry on the photosynthesis process and essentially stop growing. Because 300 PPM is the atmospheric CO content, this amount is chosen as the 100% growth point. You can see from the chart that increased CO can double or more the growth rate on most normal plants. Above 2,000 PPM, CO2 starts to become toxic to plants and above 4,000 PPM it becomes toxic to people.

With the advent of ideal growing conditions conditions provided by metal high-intensity discharge (H.I.D.) lighting systems, hydroponics, environmental controls such as temp., humidity, etc. and complete, balanced plant nutrients such as Ecogrow, the limiting factor on plant growth rate, quality, size and time to maturity becomes the amount of carbon dioxide available to the plants.


This has been the most common method of CO2 enrichment for many years. A number of commercial growers and greenhouses use it in their larger structures. The most common fuels are propane, butane, alcohol and natural gas. Any of these fuels that burn with a blue, white or colorless flame will produce carbon dioxide, which is beneficial. If a red, orange or yellow flame is present, carbon monoxide is being generated due to incomplete combustion. Carbon monoxide is deadly to both plants and people in any but the smallest quantities. Fuels containing sulfur or sulfur compounds should not be used, as they produce by-products which are harmful.

Most commercial CO2 generators that burn these fuels are too large for small greenhouse or indoor grow room applications. Some small ones are avai fable or a Coleman lantern, bunsen burner or small gas stove can be used. All of these CO2 generators produce heat as a by-product of CO2 generation, which is rarely needed in a controlled environment grow room but may prove beneficial in winter growing and cool area greenhouses.

The rate of CO2 production is controlled by the rate at which fuel is being burned. In a gas burning CO2 generator using propane, butane or natural gas, one pound of fuel produces approximately 3 pounds of carbon dioxide gas and about 1.5 pounds of water vapor. Approximately 22,000 BTUs of heat is also added. These figures can vary if other fuels are used.

To relate this to our standard example in an 8′ X 8′ X 8′ growing area, if you used ethyl or methyl alcohol in a gas lamp or burner at the rate of 1.3 oz. per day, we would enhance the atmospheric concentration of CO2 to 1300 PPM if the room was completely sealed.

An enrichment standard of 1300 PPM was chosen as it is assumed that 1500 PPM is ideal, and that the plants will deplete the available CO2 supply by 100 PPM per hour. Remember, the normal atmosphere contains 300 PPM of CO2. A 100% air exchange (leakage) every two hours is assumed to be the average air exchange rate in most grow rooms and tight greenhouses. If many cracks and leaks are present, this exchange rate will increase significantly, but added CO2 (above 300 PPM) will also be lost. If a vent fan is in use, disregard CO enrichment, as it will be blown out as fast as it is generated.

A circulation fan is beneficial, as it moves the air about in the greenhouse or grow room. If the air is still, it can cause a “depletion layer effect”. This effect causes the CO2 right next to the plant leaf to be quickly depleted. If fresh air carrying additional CO is not brought to this surface, photosynthesis and growth will diminish and eventually cease.

There are a number of factors involved in keeping the CO2 content at the desired concentration level. 1. If the greenhouse or grow room is not tightly sealed up, add up to 50% to the CO2 generator production volume. 2. If temperature is increased fiom 70 F to 90 F, add 20% to the volume generated, and vice-versa. 3. If the grow area contains large or tightly spaced plants, add 20% to 30% to the CO2 volume generated.

If more light is used, more CO2 can be utilized and should be produced proportionately up to the practical limit of 5,000 footcandles per square yard and 1500 PPM CO2 atm. content. When more CO is generated, more water and plant nutrients should be used, again to a practical limit of 2X normal. lf your plants are going to grow faster because of CO2 enrichment, they will need more nutrient and water.

NOTE: One pound of CO is equivalent to approximately 8.7 cu. ft. of gas at standard temperature and pressure.

If different hydrocarbon fuels are used, the heat content, in terms of B.T.U. should be taken into account. If the BTU per hour rate is half that of ethyl alcohol, twice as much must be burned to generate the same approximate amount of CO2 desired. The amount of CO2 generated depends on the carbon content of the fuel being used. The BTU per hour heat content can be obtained from literature or suppliers.



This is the second most popular method of CO2 enrichment and provides fairly accurate, controlled results. Compressed CO2 comes in metal containers under high pressure. Small cylinders contain 20 lbs. of compressed CO and large tanks hold 50 lbs. Pressure ranges from 1600 pounds per square inch to 2200 PSI.

To enrich available CO with compressed gas, the following equipment is needed:

1. Tank of compressed CO2

2. Pressure regulator

3. Flow meter

4. Solenoid valve, (plastic or metal)

5. Short-interval 24 hr. timer capable of having an “on time” variable from one to 20 minutes.

6. Connecting tubing, fittings and adapters


This method allows for the injection of a controlled amount of CO2 into the growing area at a given interval of time. The pressure regulator reduces the compressed gas pressure from 2200 lbs./square inch to a more controllable amount (100 to 200 PSI) which the flow meter can handle. The flow meter will deliver so many cubic feet per minute of CO2 to the plants for the duration of time that the solenoid valve is opened. The timer controls the time of day and length of time that the solenoid valve is open.

To operate this CO enrichment system in our standard 8′ X 8′ X 8′ grow room area, we want to add enough CO to increase the near depleted level of 200 PPM to 1500 PPM. We must then add 1300 PPM of CO2 to a volume of 512 cu. ft. We would like to do this in intervals of time relative to the natural air exchange rate (leakage rate) to keep the CO level near the 1500 PPM range.

Let’s select an injection time interval (CO2 enrichment time) of every two hours. First, we must determine how many cubic feet of CO2 must be added to 512 cu. ft. of volume to increase our 200 PPM to 1500 PPM. To do this, multiply the room volume of 512 cu. ft. by .0013 (1300 PPM) to obtain 0.66 cu. ft. of CO2 that is needed. Set the regulator at 100 PSI and the flow meter at 20 CFH (Cubic Feet per Hour) or 0.33 cubic feet per minute. If we set our timer to stay on for two minutes every two hours, we will get the 0.66 cubic feet of additional CO we need to bring the CO level to the 1500 PPM optimum level needed.

Each pound of CO compressed gas contains approximately 8.7 cubic feet of CO gas at atmospheric pressure. Compressed CO2 costs around 50 cents/lb. at most supply houses. At that rate of 0.66 cu. ft. every two hours for 18 hours per day, this method will cost around 30 cents per day to operate. The timer should be set to deliver CO2 during the “on time” (daylight time) for which the lights are set. This is the only time the plants can use CO2; they do not use it when it’s dark.

The compressed gas method of CO enrichment has the advantages of fairly precise control, readily available equipment ($150.00 to $300.00 average cost for an installation) and it does not add extra heat to the growing area. It also works well for small growing spaces and after initial equipment costs, is not expensive to operate.