Growing in a “perfect” greenhouse climate

Originally published in Issue 16

Although it may be difficult to create the perfect greenhouse climate for growing plants, there are variables growers can control to maximize plant growth.

Regardless of the type of crop being grown in a greenhouse, the climate a grower is trying to achieve requires controlling the same variables.

“Greenhouse growers are trying to control temperature, humidity, light level, carbon dioxide, and in some instances, airflow and air distribution,” said mechanical and agricultural engineer Dr. Nadia Sabeh, founder of Dr. Greenhouse. “Depending on the crop, these variables have different set points. They also might have different acceptable maximum and minimum ranges or levels.

“Depending on the crop, these variables can be changed during different times of the day. For instance, a tomato crop wants a daily average temperature around 72ºF. If the plants experience high temperatures during the day, if the temperature is able to be cooled down during the night, as long as the average temperature is 72ºF, the tomato plants are happy. For lettuce, a grower may not be able to manipulate the day or night temperature to make up for exceeding the maximum temperature that occurs during the day or night. That’s one way these crops differ.”

Sabeh said when it comes to controlling the greenhouse environment growers usually focus first on temperature.

“The first line of defense against warm temperatures is not shading,” she said. “Growers are trying to maximize as much light into the greenhouse as possible. As soon as a shade curtain is closed the solar input is reduced. The first line of defense for cooling a greenhouse is ventilation, either natural or mechanical.

“If ventilation can’t achieve the temperature a growers needs, then some form of cooling is added. Typically cooling is done through evaporative cooling. This could be wet pads and a fan system, high pressure fog or a low pressure misting system in combination with mechanical and natural ventilation. If that doesn’t work, then a shade curtain can be pulled. A shade curtain is usually only drawn for two to four hours during the day. It’s pulled during the peak solar heat gain period. A shade curtain can cut the temperature by 2ºF-4ºF.”

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The challenge of reducing humidity

Sabeh said between controlling the greenhouse temperature and humidity, humidity is the more challenging variable, especially if it is for dehumidification.

“If a grower is trying to remove moisture from the greenhouse, that presents a lot of challenges,” she said. “The standard method of removing moisture from the greenhouse is through ventilation. But that assumes that the moisture level or the humidity outside the greenhouse is lower than it is inside the greenhouse.

“If a grower is looking to increase the humidity or humidification, for a greenhouse located in the southwest U.S. where it is very dry, moisture can be added to the greenhouse using evaporative cooling. Another benefit of evaporative cooling is a reduction in the temperature that cools the greenhouse temperature. Evaporative cooling works very well in a dry climate to do both of those things.”

Sabeh said growers in the Midwest and Southeast can experience more challenging climates because they have a high heat solar gain like growers in the Southwest experience, but they also have high humidity levels requiring them to ventilate.

“The climates in the Midwest and Southeast make it very challenging to grow plants in a greenhouse because of the humidity,” Sabeh said. “The only line of defense for growing plants in that kind of climate is ventilation. Growers want to exchange as much air as possible with the outside to remove moisture and solar heat gain during the day. Typically that is inadequate. If the outside temperature is 90ºF and the relative humidity is 90 percent, growers certainly don’t want those conditions in their greenhouses.

“If the temperature and humidity are high, growers don’t have the opportunity to use evaporative cooling because they can’t reduce the temperature enough,” she said. “They can shade the greenhouses, but that only lowers the temperature by 2ºF-4ºF from outside conditions. If it is 90ºF and 90 percent humidity, pulling shade results in 86ºF and 90 percent humidity, and that is not going to provide the vapor pressure deficit a grower is trying to achieve.”

Sabeh said growers might consider closing up their greenhouses to avoid bringing in hot, moist air, but that creates additional challenges.

“Closing the greenhouse can cause the greenhouse to heat up from the sun plus the plants are releasing moisture resulting in the greenhouse just getting hotter,” she said. “So far I haven’t really seen anyone come up with a very cost effective method to mitigate that heat and moisture. Certainly a grower could use a refrigerant-based cooling system similar to an air conditioning system that would provide dehumidification. But the size and scale of those systems are cost prohibitive.”

Maintaining the proper vapor pressure deficit

Sabeh said temperature and humidity are very closely linked through the vapor pressure deficit (VPD).

“As long as a grower is able to control the greenhouse temperature, that usually means he is able to control the humidity level to the point where the vapor pressure deficit is where it should be,” she said. “Even if VPD is not the target that a grower is going for, that is actually the target that he is trying to reach with temperature control with or without humidity control.”

VPD is the difference between the amount of moisture in the air and how much moisture the air can hold when it is saturated.

“There is an optimum level for VPD,” Sabeh said. “For leafy greens and culinary herbs, which prefer a lower VPD, the accepted VPD range is 0.65 to 0.9 kilopascal (kPa) with 0.85 kPA being optimum. Tomatoes, cucumbers and peppers tend to like it drier. The VPD range for tomatoes is 0.9 to 1.2 kPa.

“For leafy greens there is more surface area for moisture to escape the plants. The plants like to be in a more humid space so they don’t release too much moisture too fast.”

Providing adequate airflow

Sabeh said airflow in the greenhouse is really important for breaking up the layer of moisture around the leaf surface of the plants.

“If the leaves are transpiring water, the leaf surface itself is considered saturated,” she said. “The leaf surface is exchanging moisture with the air around it. The more moisture in the air around the leaf surface, the less tendency to transfer moisture from the leaf surface to the air around it.

“This is basically what the vapor pressure deficit is. It is the difference between how much moisture there is at the leaf surface at a given temperature vs. how much moisture there is in the air at that same temperature. If it is within the right range, then the plants are happy because the leaves are freely exchanging moisture with the air. If the vapor pressure deficit is too low that means the air has a lot of moisture in it so there is going to be less transfer of moisture from the leaves to the air. The plants can’t transpire as quickly and nutrients can’t be delivered as quickly to the rest of the plant. If the vapor pressure deficit is too high, the air is really dry, and the plants shut down. As a protection strategy, the plants will close their stomata so that they don’t transpire moisture to the air because it would occur too fast. The loss of water through transpiration would occur faster than the plants could take up water.”

Sabeh said horizontal airflow fans are the traditional method for producing airflow and air currents in a greenhouse.

“Horizontal airflow fans are usually suspended from the trusses or the structure of the greenhouse and blow air in a circular pattern over the tops of the plants without actually blowing directly down on the plants,” she said. “Just the circulation and motion is enough to create turbulence to cause air mixing around the plants to encourage transpiration and convection.

“By breaking up the little saturation pocket of air around the leaves, it facilitates that moisture transfer from the leaves to the air. Under more humid conditions, as air is blown over the leaf surface, a grower can facilitate more transpiration from the plants than if no airflow was blowing over it. Airflow is one of those variables not addressed as often as temperature and humidity control. It is sorted of neglected.”

With the increasing interest in vertical farms, Sabeh said growers are using large grow racks to try to create temperature and humidity conditions in three dimensions.

“Under these conditions it is very easy for air to get trapped over the center of a rack,” she said. “Vertical farmers are really cognitive of airflow because they see these hot spots or these wet spots in the middle of the grow racks so they know they need airflow.

“It is the same situation as if plants are grown in a greenhouse. If more airflow is provided in a greenhouse, more moisture could be removed from the plant surface and help the plant with cooling by convection.”

Maintaining the proper carbon dioxide level

Sabeh said although carbon dioxide is not necessarily impacted by the outdoor climate, greenhouse growers are controlling it relative to the outdoors.

“In a greenhouse where growers are burning fuel to generate carbon dioxide and ventilating at the same time there is a challenge of how much carbon dioxide should be delivered and how is it going to be retained? Is there a way to mitigate the carbon dioxide’s immediate loss to the outside air through greenhouse ventilation?

“One strategy for not overusing carbon dioxide is to provide plants with a boost from carbon dioxide enrichment. Carbon dioxide can be provided first thing in the morning during first light before the greenhouse vents are open. Basically the plants take a deep breath when the sun starts to come out and the stomata open. The sunlight or the supplemental lights are turned on and the plants take up that carbon dioxide. When a grower starts to ventilate because the moisture has built up overnight or the temperature starts to increase because the sun is rising, enrichment with carbon dioxide can be stopped so that it is not being blown out of the greenhouse through the vents and exhausted by the fans. Some growers use carbon dioxide enrichment all day as long as there is enough light from the sun or from artificial light.”

Sabeh said growers can mitigate the loss of carbon dioxide by trying to deliver it as close to the leaves as possible.

“Some growers use under-floor or under-bench ducts to deliver carbon dioxide,” she said. “Some growers may use PVC tubing or fish tubing to distribute carbon dioxide through the crop and directly to the leaves. This is ideal if a grower can find a way to deliver the carbon dioxide in an effective manner without getting in the way of all of the other equipment and people working in the greenhouse.

“This is why some people are looking at the potential advantage of growing in vertical farms. There is an enclosed space and in most cases it is being done in buildings that are not leaky. There are some growers who have considered closed greenhouses. The cannabis industry is really interested in this, but the problem is there is an outrageous energy bill to try and close the greenhouse and not use any ventilation or mechanical cooling.”

Relationship between greenhouse climate variables

Sabeh said the optimum level of carbon dioxide varies for each crop. She said 700-1,500 parts per million carbon dioxide is the level that most growers are trying to use.

“Carbon dioxide is most useful to the plants when there is a lot of light and good temperature and humidity levels or a good VPD,” she said. “Carbon dioxide is transferred through the leaf stomata, the same as moisture through transpiration. At the right VPD the stomata are open to the maximum and are letting out moisture and gulping up carbon dioxide.

“The first thing is having the right VPD to maximize stomata opening. The second thing is photosynthesis, which is driven by light. If the air is being enriched with carbon dioxide, but the light level is very low, much of the carbon dioxide will be wasted. There has to be enough light to facilitate a high enough rate of photosynthesis or the plants can’t use the carbon dioxide. All three of these variables work together. A good VPD is needed for stomata opening. An adequate light level is needed for photosynthesis. And carbon dioxide is needed to maximize the photosynthesis cycle.”

For more: Nadia Sabeh, (916) 775-3724; nadia@doctorgreenhouse.com,
http://www.doctorgreenhouse.com.

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