Find your just-right temperature

One of the biggest benefits to growing hydroponic crops in controlled environments is the ability to control environmental conditions and cultural practices. From managing light to mineral nutrients, we can use the environment to influence crop growth and development. The control that is afforded to us when growing crops hydroponically in greenhouses or other environments is one of the biggest advantages to this production system.

Every factor involved in growing hydroponic food can affect plant quality, including those factors we have already discussed in this series such as production systems, water, mineral nutrition and light. One of the most important factors that control the development of food crops in hydroponic systems is temperature. In this fifth installment of our Hydroponic Production Primer, we will be discussing the effect of temperature on the growth and development of food produced hydroponically.

Average daily temperature

When we designate our air temperatures, we commonly set both a day temperature (DT) and a night temperature (NT). As the name implies, DT is when light is present and the NT is during darkness. While both of temperatures influence growth and development, the primary factor controlling the speed or rate of growth is the average daily temperature (ADT). The ADT is the mathematical average of air temperatures over a 24-hour period. Plants integrate DT and NT over a 24-hour period and growth and development are strongly influenced by this integrated, average temperature.

The ADT controls the rate of plant growth, whether we are talking about leaf development, fruit growth, etc. A generic temperature response curve is seen in Fig. 1. On the “cool” end of the temperature response curve, we have the base temperature (Tbase). The Tbase is the temperature below which plant growth stops; this does not necessarily mean that plants will die, but growth will cease. As the temperature increases from the Tbase, the rate of growth increases up to a maximum rate. The air temperature at which crop growth is maximized is the optimal temperature (Topt). As temperatures increase above Topt, growth declines until it reaches the maximum temperature (Tmax), after which plant growth stops.

The time to harvestable products is largely a function of the average daily temperature (ADT), and this can be used to regulate the rate of development and subsequent harvests.

Why is it useful to think about the effect of air temperature like this? Ultimately, understanding the fundamental relationship between ADT and crop growth can help make managing air temperature simpler. Generally speaking, we want to avoid temperatures that are above Topt so we do not inhibit development; this will be covered more later in this article.

You can get an approximation of your ADT using the following equation: ADT = [(DT × day hours) + (NT × night hours)]/24. However, the best way to determine your ADT is to use your greenhouse environmental computer or environmental datalogger and calculate the actual ADT based on air temperature measurements in the greenhouse.

Extreme temperatures

In addition to the DT, NT and ADT, we are concerned about daily minimum and maximum temperatures. Both extremely high and low temperatures can have negative effects on plants and both should be avoided throughout production.

Extremely cool or cold temperatures are generally only a potential during later fall, winter and early spring when outside temperatures are colder than our desired greenhouse temperatures. Usually, extreme cold temperatures become a problem when heating fails and temperatures drop. The damage from cold temperatures will depend on the sensitivity of the plant to cold. Some plants are sensitive to cold temperatures, such as basil (Fig. 2), whereas some other crops, such as kale, have greater tolerance to low temperatures. In order to avoid dangerously low temperatures, make sure that your greenhouse alarms are working properly and that you have a back-up plan in place.

Excessively high temperatures also have negative impacts on plant growth and development. Extremely high temperatures are more common when outdoor temperatures are warm and light intensity is high, starting in later spring, throughout summer and into early fall. During these times, the radiant energy entering the greenhouse from high light intensities and the warm ambient air temperatures can make cooling a challenge, and temperatures in the greenhouse can raise more than is desirable. Heat stress can cause several different problems. First, though less immediately apparent, is reduced growth from supraoptimal temperatures. Another more immediate response to warm temperatures is flower abortion (Fig. 3). While inhibiting flower development is not a problem for most leafy greens and herbs where foliage is the harvested product, flowering is essential for crops where fruits are harvested, such as tomatoes, peppers and strawberries.

DIF

The difference between the day and night air temperatures, or DIF, is determined by subtracting the night temperature (NT) from the day temperature (DT). This is represented mathematically as DIF = DT – NT. Stem elongation is promoted by a positive DIF and suppressed as DIF approaches zero or becomes negative. As a result, many containerized ornamental plant producers track DIF, since it affects growth control. For hydroponic food crops, stem elongation is not a factor that producers generally are concerned with.

Hydroponic food crops are generally grown with a positive DIF. It is generally beneficial to have a cooler NT relative to the DT. One reason is that warmer night temperatures enhance respiration, depleting carbohydrates and diminishing produce quality. There is also an economic advantage to having a positive DIF, since heating is most expensive during the night.

Take-home message

The yield of hydroponically produced food is strongly affected by temperature. The time to harvestable products is largely a function of the ADT, and this can be used to regulate the rate of development and subsequent harvests. Extreme temperatures — both from cold and heat — can also impact crop quality, including appearance or marketability and yields. Finally, a diurnal temperature difference with a positive DIF is not only beneficial for crop quality for hydroponically grown food, it is also an economical heating strategy. Armed with this knowledge, it is the time to start thinking about how you can improve temperature management for food crop production in controlled environments.

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