Grow room and indoor greenhouse growers online agree that for optimum plant growth, you should enrich the air around the plants with carbon dioxide (CO2) during the daylight growth cycle. Photosynthesis (plant growth) requires light, nutrients, water and CO2.
If you give your plants the perfect amount of light, water, and nutrients, the limiting factor in their growth will be the amount of CO2 in the air.
While normal outdoor CO2 levels (about 400ppm) will achieve normal plant growth, doubling or tripling the CO2 levels can increase the growth rate. However, above 2,000ppm CO2 becomes toxic to plants, and above 5,000ppm CO2 becomes potentially harmful to people. Most experts agree that 1,500 ppm is the maximum CO2 level for maximum plant growth, although any CO2 level between 1,000ppm and 1,500ppm will produce greatly improved results.
CO2 Controller Setup
CO2 enrichment can easily be accomplished with CO2 controllers and digital tank regulators. In this example, we will use the Day/Night CO2 Monitor and Controller for Greenhouses combined with our CO2 Tank Regulator.
The Day/Night CO2 Monitor and Controller is set by default to 1,000ppm CO2. You can change this in the setup. CO2 controllers will automatically stop powering the tank regulator during the dark period. Mount the controller 2-3 feet above the floor of the grow room, or just above the canopy of plant tops as they grow taller.
Tank Regulator Setup
Connect one end of the CO2 tank regulator to a 20lb. or 50lb. CO2 tank, and the other end to several feet of 1/8” flexible tubing with holes drilled in the length of it that is suspended above the plants. You can purchase pre-drilled tubing at many grow-room suppliers online. Alternatively, you can put the end of the tubing behind a fan high above the plants. CO2 is heavier than air, so it will flow down to the plants.
Turn on the gas at the tank. Plug the regulator into the Day/Night CO2 Monitor and Controller, and plug the controller into a grounded 120VAC wall outlet. Because the CO2 level in your grow room will be less than 1,000ppm, the regulator will turn on instantly.
Set the CO2 flow control to approximately 5 SCFH (Standard Cubic Feet per Hour), which is 0.08 cubic feet per minute (5/60).
CO2 Calculator
To calculate the amount of CO2 you need, you’ll need to calculate the size of the grow room. For example, if you have a 8x8x7 foot grow room, you would need to cover 448 cubic feet (8x8x7=448). To compute the amount of CO2 required to raise the level to 1,000ppm use this equation:
Width x length x height x desired CO2 level in parts per million.
So in our example, we have 8x8x7x0.001 = 0.448 cubic feet of CO2 needed to raise the level inside the grow room to 1,000ppm CO2.
We can roughly calculate how long the regulator will be turned on with this formula:
CO2 needed in cu.ft. / flow rate per minute
Since we need 0.448 cu.ft and the regulator is set at 0.08 cu.ft. per minute, the CO2 Controller will activate the solenoid on the regulator for approximately 5.6 minutes to raise the CO2 level to 1,000ppm (0.448/0.08).
How long will a CO2 Tank Last?
A pound of liquid CO2 in a tank contains 8.741 cubic feet of gas, so a 20lb. cylinder would hold 175 cubic feet CO2, and a 5lb. cylinder would hold approximately 44 cubic feet CO2.
A 100% air exchange every 2 hours (air leakage through cracks and doors) is normal in most grow rooms and tight greenhouses, so you will use .448 cu.ft. of CO2 every 2 hours, or about .224 cu.ft per hour (.448/2). Using .224 cubic feet per hour and 18 hours of daylight, you would expect to use about 4 cubic feet per day (.224x18). This means the 20lb. tank should theoretically last 44 days (175/4), while the 5lb. tank will last about 11 days (44/4). These estimates do not take into account plant uptake of CO2, opening doors or curtains, or other opportunities for CO2 to escape and require replenishment.
CO2 Calculation Tips
1. Experts disagree about the optimum level of CO2 for plant growth. You can find arguments for 800ppm, 1,000ppm, 1,200ppm and 1,500ppm. We recommend starting at 1,000ppm, then adjusting your CO2 levels over time with experience.CO2 levels below 400ppm and above 1,500ppm will reduce plant growth.
2. Experts do agree that raising CO2 alone will not guarantee plant growth. Proper soil, nutrients, watering, light and cleanliness must be achieved in combination with enhanced CO2 levels for optimum results.
3. A fan inside the grow area pointed at the end of a length of tubing from the regulator will circulate the CO2 in the air for maximum benefit. Otherwise, you can connect the regulator to a length of tubing with pin-holes strung around the top of the grow room so that the CO2 will "drop" on the plants (CO2 is heavier than air).
4. If you are growing indoors, purchase an inexpensive timer to control your day/night light period. Never run CO2 during the dark period.
5. Note the temperature on the Day/Night CO2 Monitor and Controller. Temperatures above 27°C (80°F) over time can result in elongated stems and lower yields of mature plants.