Optimizing controlled environment food crop production: Managing light, temperature & CO2

The best decisions are made when they are informed ones. We are constantly faced with questions in hydroponic and controlled environment food crop production. Incorporating data into our decision-making processes allows us to feel more confident.

When it comes to controlled environment food crop production, one of the most important environmental factors is photosynthetic light. You may have heard the saying, “A 1% increase in light is a 1% increase in yield.” Since food crops are sold by weight, this relationship between light and yields can be more important compared to other crops like containerized flowering plants, which are sold by unit.

We can measure light both instantaneously (µmol·m–2·s–1) or cumulatively (mol·m–2·d–1). Both of these measurements are useful for different reasons. Instantaneous light intensity is useful for making lighting decisions over the course of the day.

For instance, once the instantaneous light intensity increases above a certain set point, shades may be drawn over a crop. Or, as it drops below a setpoint, supplemental lighting may turn on. Alternatively, monitoring the cumulative daily light integral (DLI) is good for making seasonal decisions, such as when to apply permanent shading on greenhouse glazing materials or when to stop or start supplemental lighting programs.

Temperature is another important factor for CEA food crops. The average daily temperature (ADT), day and night temperatures, as well as extreme minimums and maximums in the growing environment all warrant monitoring.

Plants integrate temperature over a 24-hour period, and the rate of development, like leaf unfolding or flower or fruit growth, is controlled by the ADT. Nearly all food crops benefit from a positive diurnal variation in air temperature, or warmer days and cooler nights, in controlled environments.

Monitoring day and night setpoints independently will help identify when temperatures are drifting too much. Injury from the cold can cause chilling or freezing damage. More common, however, is heat stress during the summertime, which can cause flower and fruit abortion for fruiting vine crops.

Carbon dioxide (CO₂) is an environmental factor that can be easier to forget, since it can’t be seen or felt in the greenhouse. But for CEA food production, CO₂ is directly related to plant growth and, ultimately, yields. When greenhouses are filled with actively growing plants and ventilation is limited — like during the winter — CO₂ decreases in the greenhouse and can limit plant growth.

You may be surprised how low CO₂ can get, which is precisely the reason monitoring it will help you better understand how to manage it. If CO₂ concentrations are constantly dropping well below ambient outdoor concentrations, investing in CO₂ burners or injection can have a positive impact on crop yield by not only maintaining near-outdoor CO₂ concentrations but enriching them up to 800 to 1,000 ppm.

The relative humidity — or more technically the vapor pressure deficit (VPD) — affects the growth of plants and pathogens. As relative humidity increases or VPD decreases in the growing environment, plants reduce their transpiration rate. This can make it more challenging to get nutrients like calcium and magnesium (that are taken up by bulk flow when water is taken up) into plants, while also making it more challenging for plants to cool down.

Alternatively, a very low relative humidity or more negative VPD can cause excessive transpiration and water use. The development of pathogens such as powdery mildew is enhanced under higher relative humidity or low VPDs. By monitoring humidity or VPD, you can manage ventilation to avoid any abiotic or biotic problems.

Monitoring light, temperature, carbon dioxide and humidity will position you to better manage crops by being a more informed grower. While we do work in highly controlled environments, using environmental data to inform decision-making will maximize productivity and quality.

Christopher J. Currey is an associate professor of horticulture in the Department of Horticulture at Iowa State University. Contact him at ccurrey@iastate.edu.