The big four in plant growth are light, temperature, water and CO2.

September 27, 2022  By Tineke Goebertus

We always complain that there is not enough light in the winter. We are aware that the temperature in the greenhouse is too warm in the summer, and when the drip irrigation is plugged, the plants show water deficiency and we are worried. But when there is something wrong with the CO2, it is not that obvious. Yet, it can cause significant yield loss while a lot of money is being spent on CO2 at the same time!

So, when you have decided to supplement CO2 in your greenhouse, make sure you do it well or you will be disappointed.

The dimensions of your CO2 distribution system could and should be properly calculated in relation to the area, the size of the CO2 fan, the diameter of the pipes and where to add the different sized washers. The goal is to have the CO2 evenly distributed over the entire area such that all of the plants can benefit from it. 

The inflatable CO2 tubes come in different diameters and have the option of different distances between perforations. These CO2 tubes are also a function of the total installation and should not be different from one year to the next based on factors such as availability or price. 

There are specialist companies who can design a proper distribution system from beginning to end for your specific situation.

The quality of CO2 is not a given.

The quality of the CO2 itself is not a given. I am still very surprised how lackadaisical this important aspect is treated by many! The quality of the CO2 should be monitored closely and diligently; carbon monoxide (CO), ethylene (C2H4) and nitrous oxides (NOx) are real and true enemies of your crop. 

The damage thresholds are different for a calamity or a residual dosing situation. In most cases there is a CO sensor installed after the fan in the supply line into the greenhouse. If it works well, it will only act in case of a calamity, usually set at 15 ppm. However, much lower levels of these toxic gases could still result in substantial residual damage, and the sensor will not pick it up! 

For example, the damage threshold for NOx is 11 ppb over a period of 8 hours, while the damage threshold over a period of four weeks becomes 5 ppb. Similarly, the C2H4 damage threshold is 40 ppb over a period of one day, but this threshold drops to 16 ppb over a period of one year.

The most utilized CO2 sources are flue gas from a natural gas boiler, liquid CO2, or scrubbed flue gas from cogeneration. Make sure that all systems work well and have the quality of the flue gas checked on a very regular basis. 

It follows that when the greenhouse air is not exchanged that often, and/or when you are dosing CO2 at higher levels, the risks are much greater.

More and more, greenhouse gas analyzers that measure harmful gases in the greenhouse are being installed. 

CO2 from the flue gas can only be dosed when there is a condenser present to reduce the temperature of the gas. There should be a temperature check placed after the CO2 fan to make sure that the gases dosed into the greenhouse are no warmer than 56C or you risk melting the PVC lines and generating ethylene.

When the CO2 is being dosed into the greenhouse, it is mixed with a lot of ambient air from around the CO2 fan. Make sure that this room is well ventilated and that no machines, e.g. forklifts, add their exhaust to it.

Both the quality and location of the CO2 sensor in the greenhouse are critical. The amount of CO2 dosed into your greenhouse, and thus, the money spent, is often decided on by one CO2 sensor. Please make sure that it is in the right location (inside the crop), has a short intake tube (which will improve reaction time), has a clean filter (obvious) and that, in the case of one with an internal pump, it works.

Be sure to calibrate the sensor very regularly; everything rides on this machine! 

Finally, make sure that there is no water in the CO2 supply lines and that the inflatable CO2 tubes themselves are not in the water. 

Tineke Goebertus is a greenhouse consultant in BC with Vortus Greenhouse Consultants Inc. She can be reached at vortus@shaw.ca.

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