Chapter 10

Temperature Requirements for Greenhouse Crops

Cardinal Temperatures

Temperature is an important environmental factor that influences the growth of greenhouse crops. There are three important temperature points in plant growth: minmum temperature, optimum temperature and maximum temperature. They are called cardinal 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. The actual temperature of a plant affects how slow or fast a crop develops. Temperature is a valuable tool to modify and regulate crop timing by temperature changes during the day and night (DIF) or by 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.

Minimum Temperature

The minimum temperature (or base temperature) is a species-specific temperature at which a plant stops growing. The base temperature (Tb) can vary among species and even cultivars and is estimated to range from roughly 30°F to 54°F (−1°C to 12°C) for the floriculture crops that have been investigated so far. For example, the base temperature for seed petunia is about 39 degrees F (4°C), which means that at or below this temperature, petunias essentially stop growing.

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. Therefore, they tend to undergo heat stress symptoms at lower temperatures.

Maximum Temperature

Maximum temperature stress can occur in crops when the day temperature, night temperature, or both are above some crop-specific temperature. Maximum temperatures can also reduce plant photosynthesis.

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