The benefits of supplemental CO2 are worth the cost

Originally published in Issue 8

The benefits of supplemental carbon dioxide (CO2) on greenhouse crops are easy to quantify. Faster production times, higher yields and larger fruit are some of the results of incorporating CO2 into the greenhouse atmosphere to optimize plant growth.

Greenhouse crop consultant Marco de Bruin at de Bruin Greenhouse Consulting never worked in a greenhouse operation that didn’t use supplemental carbon dioxide (CO2).

“I have grown a variety of tomatoes including, tomato on the vine, grape tomato, cluster tomato, Roma tomato, as well as cucumber, sweet pepper, lettuce and basil,” de Bruin said. “Supplemental CO2 was used in all of the facilities I have worked in. e modern glass and plastic greenhouse operations being built today all have CO2 systems in them.

“In traditional glass greenhouses there can be a lot of air exchanges that occur through the roof. at can help to reduce the temperature and humidity in the greenhouse, but it can remove CO2 from the greenhouse as well. e roof vents start to function like a Venturi drawing the air and CO2 out of the greenhouse.”

de Bruin said it is best to distribute supplemental CO2 between the leaves of the plants.

“Growers will install lay-flat tubing in the greenhouses, particularly with row crops like tomatoes, peppers and cucumbers,” he said. “CO2 is then injected into the greenhouse environment, particularly into the microclimates around the plants.”

de Bruin said the growth response to supplemental CO2 is similar regardless of the crop being grown.

“CO2 is one of the building blocks for photosynthesis,” he said. “A grower always has to have the supplemental CO2 in combination with light, whether that is sunlight or supplemental light. If there is no light then there is no point in using supplemental CO2. So at night no supplemental CO2 is dosed into the greenhouse.”

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Avoiding Plant Stress

de Bruin said the ideal levels of CO2 in the greenhouse vary with the crop, but they vary more due to the growing environment.

“During the summer when plants are actively growing, the CO2 level in a greenhouse can drop from ambient 380-400 parts per million to 50-100 ppm in a very short period of time,” he said. “ The sunlight intensity, the temperature and the humidity in the greenhouse are factors that are extremely important. If plants are stressed and their stomata, which take up CO2, are closed, then CO2 dosing should be greatly reduced or even stopped since there is no bene t to the plants under these conditions. A grower needs to make sure that the temperature and humidity levels are appropriate for dosing CO2 to maximize the rate of photosynthesis. e dosing level is very dependent on stress factors.”

de Bruin said there are certain times of the day and certain times of the year when a grower should actually reduce the dosing level of CO2 due to the stress level on the plants.

“Heat stress is a combination of radiation and a grower’s ability to maintain the climate in the greenhouse,” he said. “Once the greenhouse starts to heat up and the grower starts to ventilate, some of the CO2 and a lot of the humidity is being pushed out. Humidity is the major driver in transferring the heat out of the greenhouse. As the humidity is lost, the plants start to transpire faster which increases the humidity. When the evaporation level gets too high for the plants to maintain, the stomata start to close and CO2 is no longer taken up by the plants. At that point, the CO2 is going straight out of the greenhouse. If a grower can keep his plants from undergoing stress, then CO2 can be dosed up to the most efficient level for both the plants as well as for the grower’s pocketbook. There are some really high tech greenhouses that are able to go to an extremely high level of CO2 before they encounter this stress situation.”

de Bruin said the level of CO2 can go as high as 1,000 ppm, but the rate of payback starts to flatten off after a grower achieves CO2 levels of 700-800 ppm.

“It is possible to achieve additional production above this rate, but it becomes a question of whether or not it is feasible,” he said. “Dosing capacity requirements go up, investment requirements go up and at the end of the day there is a certain price limit as to what a grower can receive for the product sold, whether his crop is tomatoes, peppers or cucumbers. How much a grower doses CO2 is determined by the cost of the CO2 and the selling price of his product.”

Sources of Supplemental CO2

de Bruin said the most common way for introducing supplemental CO2 into greenhouse vegetable and ornamental crops is to dose the flue gases from the boiler, which contain CO2.

“Basically the flue gases from the boiler are rerouted through the greenhouse so that the plants can filter out the CO2,” he said. “Some of the CO2 will still leave the greenhouses, but 100 percent of the CO2 would have been lost into the atmosphere if the flue gases would have been allowed to go out the chimney.

“What growers have done to use the CO2 is to install a heat storage tank. While they are producing heat they store it in an insulated water tank. They then can use the flue gases that come o of the boiler during the day when the sun is shining and the plants are actively using CO2. A computer system regulates the heat from the storage tanks to go into the greenhouses as needed. is can allow the boilers to turn o at night if no extra heat is needed. ese boiler systems are very efficient.”

de Bruin said growers can also use unit heaters, some of which are combined heaters that allow a grower to dose CO2 in the greenhouse as well as using the flue gases that the heaters produce.

“In regards to noxious gases, the unit heaters and boilers that I’ve seen in most growing operations are designed to have extremely low NOx (nitrogen oxides) and burn very clean,” he said. “These are monitored very carefully.” de Bruin said for growers who are going to have to heat their greenhouses regularly with either unit heaters or boilers, it makes sense to use the CO2 in the flue gases. He said since propane costs three times as much as natural gas, a grower could quickly make a case for using liquid CO2 instead.

“If a grower is located in the South or on the coastlines the heating requirements may be considerably less,” he said. “In these locations, using liquid CO2 or a combination of flue gases and liquid CO2 could be more economical as well. With the liquid CO2 set ups, growers lease the tanks and the evaporator units. There are several companies that specialize in greenhouse applications that deliver a complete package.”

de Bruin said growers can figure they will harvest 20-30 percent higher yields when they dose CO2 versus relying on ambient CO2. “If a grower is going to produce 20-30 percent higher yields, installing the equipment to dose CO2 pays o quickly,” he said. “Going from ambient CO2 levels to 400-500 ppm is a very quick payback.”

For more: de Bruin Greenhouse Consulting, (507) 451- 0701; Marco@deBruinGreenhouseConsulting.com; http://www.deBruinGreenhouseConsulting.com. Editor’s note: For more information on the use, bene ts and costs of supplemental carbon dioxide see “Carbon Dioxide in Greenhouses” Factsheet from the Ontario Ministry of Agriculture, Food and Rural Affairs.

David Kuack is a freelance technical writer in Fort Worth, Texas; dkuack@gmail.com.