Chapter 8

Effects of Temperature on Greenhouse Crops

Temperature Requirements for Greenhouse Crops

Temperature is an important environmental factor that influences the growth of greenhouse crops. The response and growth of crops occur over a wide range of temperatures referred to as minimum, optimum and maximum temperatures. Using the minimum temperature (or base temperature) we can subjectively place crops into different temperature response categories: cold-tolerant plants, cold-temperate plants, and cold-sensitive plants. Crops do not respond in the same manner to temperatures at all stages of growth. For example, the optimum temperature for vegetative growth may not be suitable for flower development. Given adequate humidity and air movement, most crops generally grow better if the day temperature is warmer than the night. Night temperatures that are too warm or too cold are more often than not, much more damaging to crops than day temperatures. Temperature is a valuable tool to modify and regulate crop timing by temperature changes during the day and night (DIF) and controlling the average daily temperature (ADT).

Cardinal Temperatures for Greenhouse Crops

Every crop has specific cardinal temperatures at which growth and development are best. There are three cardinal temperatures for every crop. The minimum cardinal temperature is the lowest temperature at which crop growth can occur. The optimum cardinal temperature is the temperature when crop growth is at its greatest. Finally, the maximum cardinal temperature is the highest temperature at which crop growth can occur.

Base Temperature

The base temperature is a cool temperature at which a plant stops growing. The base temperature can vary among species and even cultivars, and is estimated to range from roughly 30 to 54 degrees F (−1 to 12° C) for the most floriculture crops.

Optimum Temperature

Growers should also know what the optimum temperature is for a crop. The optimum temperature is the temperature at which plant development is most rapid. The optimum temperature can be around 70 degrees F (21° C) for cool-season crops, or as high as 90 degrees F (32° C) for warm-season crops. Cool-season plants also have a lower optimum temperature than do warm-season crops.

Maximum Temperature

Additionally, if temperatures exceed the optimum temperature, developmental rates again begin to decrease. Once the maximum temperature is reached, development stops due to plant stress. When possible, plants should not be exposed to temperatures above their optimum.

Cold-tolerant and Cold-sensitive Crops

Greenhouse crops can be broken down into three basic categories: 1) cold-tolerant crops, those with a base temperature of 39 degrees F (4 C) or lower and generally should be grown at an average daily temperature (ADT) of 60 F to 65 degrees F (16 to 18 C); 2) cold temperate crops, those with a base temperature between 40 to 45 degrees F (4 to 7 C), and generally should be grown at an ADT of 65 to 70 degrees F (18 to 21 C), and 3) cold-sensitive crops, those with a base temperature of 46 degrees F (8 C) or higher and should generally be grown at an ADT of 70 to 75 degrees F (21 to 24C).

Suboptimal Production Temperatures for Cold-tolerant Crops

Cold-tolerant crops lend themselves well to low production temperatures. Although the optimum temperature for cold-tolerant crops is approximately 70 degrees F (21° C) or higher, they will grow and flower well at 40 to 60 degrees F (4 to 16° C).

Temperature Requirements for Crop Development Stages

When contemplating greenhouse temperature management it is important to take into consideration the influence of temperature on plant growth and development for the various stages of growth—seed germination; vegetative and growth; and reproductive, flowering, and fruit. A plant species optimum temperature changes as a plant grows from a seedling to a mature plant. Generally, seed germination and early seedling growth occurs most rapidly at warmer temperatures. Warmer temperatures are generally favored by younger plants.

Day/night Temperature Differential for Greenhouse Crops

The day/night temperatures at which crops are grown in a greenhouse are those that have been found to maximize growth, yield, and quality. As a general rule, greenhouse crops are grown at day temperature 5 to 10 degrees F (2.8 to 5.6 to °C) higher than night temperatures on cloudy days and 15 degrees F (8.3 °C) higher on clear days. This assumes that the temperature at which the crop is grown can be controlled.

Day-night Temperature Relationships (DIF)

The growth habit and flowering time of many ornamental and greenhouse crop plants can be shaped quite dramatically by alternating day and night temperatures. Plant response to DIF varies depending on the species and cultivar. This relationship between day and night temperatures can be expressed as the difference in the day and night temperature, abbreviated DIF:

Height Control by DIF

Plant height can be decreased by decreasing the day temperature and also by increasing the night temperature. Conversely, plant height is increased by increases in day temperature as well as by decreases in night temperature. The effect is upon the length of stem internodes rather than the number of leaves. The magnitude of the response to DIF is not the same across all DIF values. The increase in internode length as DIF increases (more positive) is greater than the decrease in internode length as DIF decrease (more negative).

Implementation of DIF in Warm Seasons

Although there is little problem in lowering the day temperature in northern locations during the winter, low DIF effects can still be achieved in the spring and even in the winter in warm climates. This can be accomplished by lowering the day temperature for the first two hours of the day immediately after sunrise. This is based on the fact that the greatest rate of internode elongation occurs during the night, with a maximum peak at sunrise.

Computer Control of DIF

DIF control can be managed using thermostats and manual settings but this can be difficult to control if managing a number of crop zones requiring different DIF values. Under this scenario the best option is to rely on greenhouse computer software that includes DIF control that can calculate the average daily temperature needed for controlling the date of crop maturity.

Average Daily Temperature for Greenhouse Crops

Average daily temperature (ADT) is an important concept in greenhouse temperature management. It is an average temperature that considers the length of time each day that a greenhouse is at a specific temperature rather than simply what the high and low points were for that day. The ADT is important in that plant development generally responds to this value rather than the high and low temperatures of a greenhouse. Plants develop increasingly slower as the average daily temperature decreases, and increasingly faster as the average daily temperature increases.

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