Maximize your plants’ growth with supplemental CO2

Originally published in Issue 7

Whether you’re growing ornamentals or vegetables in a controlled environment, if you’re not providing supplemental carbon dioxide you could limit your crops from reaching their full potential.

If you are growing ornamental plants or vegetables in a controlled environment, including greenhouses, warehouses or growth chambers, carbon dioxide (CO₂) could be considered another nutrient like nitrogen. Plants require carbon dioxide in order for the process of photosynthesis to occur enabling the plants to grow, flower and in the case of vegetables produce fruit.

“Ninety-nine percent of the plants that are grown for food or ornamentals in controlled environments would benefit from supplemental carbon dioxide,” said DuPont Pioneer research scientist Jonathan Frantz. “The outside ambient level of carbon dioxide is just over 400 parts per million. Once inside a greenhouse or other closed structure that contain plants, the plants start to draw down the carbon dioxide level. Depending on how much plant material is in the greenhouse and how tight the greenhouse is, the carbon dioxide level will be drawn down a lot. Warehouses and growth chambers are going to be tighter than greenhouses so the draw down is going to be faster in those types of structures.”

Frantz said that some growers think a greenhouse has to be very tight with very few leaks to create a carbon dioxide deficit.

“It doesn’t take much for a carbon dioxide deficiency to occur and the level to drop well below 400 ppm,” he said. “How fast carbon dioxide will decrease depends on the light level and how many plants are in the greenhouse or growing structure. If there are plants in the greenhouse, then the carbon dioxide level is going to drop down quickly. In an hour the carbon dioxide level can drop 100 ppm.”

Working with an ornamental plant grower, Frantz said the effect of a carbon dioxide deficiency was most noticeable in the greenhouse during the winter months.

“From December through February when this grower was starting to propagate his geraniums for the upcoming spring season, the plants just sat there,” Frantz said. “The cuttings didn’t put on a lot of new roots. Growth was just slow.”

Frantz said if there is a carbon dioxide deficit in a vegetable greenhouse the plants would have low fruit yields and little growth.

“Growth is adding mass,” he said. “Development rate is important in the timing of flowering. I’m interested in how carbon dioxide might accelerate that development rate.”

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How much CO₂ is too much?

Frantz said a plant’s photosynthesis rate usually peaks at 1,000-1,200 ppm carbon dioxide.

“It really comes down to a matter of economics,” he said. “If the carbon dioxide level is doubled above 400 ppm, there is probably going to be a doubling of photosynthesis. But as the level of carbon dioxide gets higher, the photosynthesis response starts to flatten out. That’s where economics start to come in. If the level of carbon dioxide is tripled to 1,200 ppm, the rate of photosynthesis is not going to triple.”

Frantz said that the type of crop will influence how much supplemental carbon dioxide a grower wants to add to the production environment.

“In the case of low-value crops like lettuce, a grower is probably not going to want to inject a high level of carbon dioxide unless faster development rates are going to pay for themselves. For high value crops like herbs that are repeatedly harvested, it might be worthwhile to raise the level of carbon dioxide above 400 ppm.”

Benefits of supplemental CO₂

Frantz said there is a strong belief among some growers that their crops don’t need supplemental carbon dioxide or that their crops won’t respond to it.

“The culprit behind that belief isn’t the crop, it’s the way in which the crops are grown,” he said. “Greenhouse vegetable growers who use hydroponics to produce their crops have plants with essentially an unlimited root zone. And growers of tomatoes, bell peppers and cucumbers constantly harvest fruit off of those plants. In these cases, the supplemental carbon dioxide has some place to go and the plants continue to respond to it.

“In the case of ornamentals it’s a different situation. The plants are grown for their flowers. These plants are often produced in small containers, including cell packs, pots and even mixed containers. The root zone in these containers is finite, it’s defined. And no fruit is being harvested off of ornamental plants. For these plants there is a benefit from supplemental carbon dioxide for a while and then it is minimal because the carbon fixed by photosynthesis has fewer places to go.”

Frantz said ornamental plants grown with supplemental carbon dioxide can develop and finish faster. The benefits of supplemental carbon dioxide on ornamental plants have been seen in various container sizes including plug trays. Frantz also found in the case of vinca that plants also produce larger flowers.

“I wasn’t even looking for the effect on flower size,” he said. When I was harvesting the plants I noticed that there was such a huge difference in the flower size with the plants that received supplemental carbon dioxide that I started to document those differences.

“I was growing plants in flats and I wanted to see if I could shorten the production time. I was able to shave off a week of time, which is significant for flats. If a grower can the reduce crop time for flats from six weeks down to five weeks, that’s a major reduction.”

Although ornamental plants develop faster using supplemental carbon dioxide, Frantz did not do any studies to determine if vegetative cuttings would root faster.

“The ornamental grower that I worked with was growing vegetative geraniums. Anecdotally he noticed that the rooting of cuttings was better,” Frantz said. “And with his stock plants he was able to take more cuttings off of the plants. In the case of stock plants, it’s similar to greenhouse vegetables because cuttings are constantly being harvested, so the plants can continue to respond to the carbon dioxide. The grower also indicated that the stem diameter of the cuttings was broader, but we did not measure that. That would be something that I would expect to occur.”

In the case of greenhouse vegetables like tomatoes, Frantz said supplemental carbon dioxide could possibly increase yields per area and yields per day.

“If the production cycle enables a grower to harvest every 14 days, he may find that he is able to produce larger fruit using supplemental carbon dioxide. However, he may be able to produce more fruit and not necessarily larger fruit. Cherry tomatoes would be a good example of producing more fruit. With some trialing of supplemental carbon dioxide, a grower may be able to train his plants to produce larger or more fruit depending on market demand.”

Keep the air moving

Frantz said the key to adding supplemental carbon dioxide is to ensure there is good mixing of the air regardless of whether production is in a greenhouse or other type of closed environment.

“Carbon dioxide is heavier than air,” he said. “If there is insufficient turbulence when the carbon dioxide is added that could result in a layer of carbon dioxide that is quite high at the bottom of the greenhouse or controlled environment structure. If the carbon dioxide is stirred up, it’s not going to settle out again. Once it’s mixed, it’s mixed.”

For more: Jonathan Frantz, DuPont Pioneer, jonathan.frantz@pioneer.com; (207) 745-3403.

Editor’s note: In the next issue (Issue 8) of Urban Ag Products learn about the economics of using supplemental carbon dioxide.

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