Article 1-1 Effective CO2

Effective Use of CO2 in the Indoor Garden

The two main sources of supplemental C02 are: compressed CO2 in tanks and CO2 generated by burning propane and natural gas. Tanks of compressed C02 are the easiest and most straightforward for small hobby gardeners to use, while CO2 generators can add a lot of heat and humidity to the grow area, and are best left for larger operations. Heat can build up quickly in a small area, and without high ceilings, powerful and efficient exhaust fan capability, or multi-fan air circulation, heat from generators can actually slow crop growth.


Horizontal C02 application is a very effective means of delivering carbon dioxide to crop plants. Simply attach one end of the air hose to flowmeter-regulator equipment, the other end to the back of an oscillating fan. A low, continuous flow from the tank during light cycle is best.

Since most small growers use ‘straight-line’ air movement in their gardens, adding C02 to the flow of air through the plants, doesn’t interfere with the f cooling function of the air flow or the elimination of hot air by the exhaust system.

Fresh air enters the garden through an air intake source – intake vent or fan and is moved through the garden by an oscillating fan. Air moving through plants, supplies C02 and removes hot humid air, which is taken away by an exhaust fan and ducts. Note that air movement is in a straight line. The exhaust fan is at plant height or higher since heat rises in the air.


After 7 – 14 days, your crops will tell you:

  • how many plants are gaining from the extra C02
  • how much it is helping your crop plants. You can re-position oscillating fans, add C02 airlines to more oscillating fans, or increase C02 flow rate if growth rate is uneven or some plants need more C02. Usually growers become very enthusiastic about adding C02 at this point, since they can see how it’s helping their gardens. If little or no effect on growth is seen, check growing conditions for limiting factors: high garden temperatures, poor air movement, bugs, disease or incorrect nutrient mix all interfere with C02 uptake and growth.


The CO2 generators we use for carbon dioxide enrichment are very efficient burners of propane or natural gas. By completely oxidizing the fuel, the generator gives off pure carbon dioxide – and lots of heat and water vapor! Growers planning to install C02 generators in their gardens should anticipate having to deal with excess heat and humidity from their new equipment. We approach this problem a number of ways. One method involves placing the generator in a remote location and moving the C02 through ducting to the air intake, where it is delivered to the crop by oscillating fans. A fan attached to the duct draws the C02-rich air from the generator, helping to dissipate heat and causing some of the water vapor to condense inside the duct. Sloping the duct slightly and placing a tray or bucket at the end to catch condensation run-offs helps in removing condensation from the duct. Another method is to suspend the generator overhead – above the garden – and use timers or control systems to supply C02 for brief periods during the light hours. All fans are shut off, the C02 generator goes on, carbon dioxide drifts downward onto the garden, and when the generator shuts off (by a timer or thermostat) the fans are turned back on to cool the garden.

The disadvantages of this method are:

  • periods of high temperatures in the garden – with no air movement
  • limited amount of CO2 supplied to the garden
  • excess humidity levels in garden
  • heat builds up rapidly from even a small generator.

High temperatures inhibit CO2 uptake by the plants, since heat stress can cause breathing pores on the leaves to close, blocking out carbon dioxide and interfering with photosynthesis. Because excess heat can be so harmful, generators usually cannot be used for more than a few minutes at a time, limiting the total CO2 supplied to the crops.

High humidity levels raise even higher at ‘lights out’, when temperatures in the garden fall. Excessive humidity levels in the garden during the dark period create real dangers of disease problems for the garden.

There are questions as to how effective the ‘trickle-down’ method of applying CO2 really is. The breathing pores on the leaves are located on the underside; CO2 hitting the top surface of the leaves cannot enter the plant through the thick protective wax coating on the leaves’ upper skin. If CO2 (which is twice as heavy as air) pools around the roots of the crop, it can interfere with root gas exchange (oxygen into roots; CO2 and waste gases out) and affect the growth and health of the plants. If you can’t use a remote location for your CO2 generators, and you have to keep your generator in the grow room, here are some tips for effective use:

Add fans to help cool garden quickly after generator shuts off, and to drive CO2 through the crops. An extra oscillating fan (or two) and a larger exhaust fan with bigger duct diameter will help!

  • Add control systems to co-ordinate fans and generator. Very precise control systems can turn fans off and on when generator starts and stops; they can supply brief pulses of CO2 frequently during the light cycle.
  • Shut off generator two hours before lamps go off, and run oscillating and exhaust fans right through dark period to eliminate left over carbon dioxide.

Generators work best in:

  • very large gardens with excellent air movement ability
  • grow-rooms or greenhouses with high ceilings


use two or more small generators for a large garden. They will provide more even distribution of carbon dioxide – and less concentrated heat build-up than a single large generator.

Although the C02 generators require planning and careful use, they are a much less expensive C02 source than the compressed C02 tanks. Growers can take advantage of our naturally clean natural gas, which contains low levels of sulphur- growers in eastern North America have to contend with natural gas containing high sulphur levels, which can be harmful to the garden. If crops are responding to C02 applications well, consider using applications of “Growth Plus” and increasing nutrient strength gradually to take full advantage of excellent growing conditions.