Managing the greenhouse environment in CEA

The best decisions are made when they are informed decisions. We are constantly faced with questions in hydroponic and controlled environment food crop production.By incorporating data into our decision making processes, a more objective approach is taken which. In turn, this can allow producers to feel more confident with their choices. Last month, I discussed monitoring water, nutrient solutions, and plant tissue. This month, the focus will be on monitoring environmental factors such as light, temperature, carbon dioxide and humidity.

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 (i.e. per container).

The amount of light plants receive in an completely indoor production facility relying completely on electrical lighting does not vary over the course of a season or even a day, unless lighting programs are changed. Alternatively, the light inside a greenhouse is variable, changing from morning to night and across seasons. 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, shade 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 food crops in controlled environments. The average daily temperature (ADT), day and night temperatures, and 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. Monitoring the ADT not only allows you to maintain target temperatures, it can also help when determining how much to adjust the ADT up or down to speed up or slow down development, respectively. Nearly all food crops benefit from a positive diurnal variation in air temperature, or warmer days and cooler nights, in controlled environments. While temperature setpoints are used to maintain day and night setpoints, monitoring them independently will help identify when temperatures are drifting too much. Finally, extreme temperatures can cause damage to plants. Injury from the cold can cause chilling or freezing damage, depending on the temperature. 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 seenor felt in the greenhouse. But for food crop production in controlled environments, CO₂ is directly related to plant growth and, ultimately, yields. When greenhouses are filled with actively growing plants and ventilation is limited — like the winter — CO₂ decreases in the greenhouse and can limit plant growth. You may be surprised to see how low CO₂ can get, which is precisely the reason monitoring it will help better understand how to manage it. If CO₂ concentrations in the growing environment are constantly dropping well below ambient outdoor concentration, investing in CO₂ burners or injection can have a very positive impact on crop yield by not only maintaining near-outdoor CO₂ concentrations but enriching them up to 800 to 1000 ppm.

The relative humidity or, more technically, the vapor pressure deficit (VPD) affects not only plants and their growth, but also pathogens as well. 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. In addition to plants, the development of pathogens such as powdery mildew is enhanced under higher relative humidity or low VPDs. By monitoring humidity or VPD in the growing environment, you can manage ventilation to avoid any abiotic or biotic problems with crop production.

Monitoring light, temperature, carbon dioxide and humidity in the growing environment 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 is the best approach to avoiding problems and maximizing productivity and quality.

Christopher (ccurrey@iastate.edu) is an associate professor of horticulture in the Department of Horticulture at Iowa State University.