Chapter 10

Temperature Requirements for Greenhouse Crops

Water Vapor-Pressure Deficit (VPD)

Greenhouse growers manage many environmental factors in their greenhouses, sometimes including humidity. Relative humidity refers to the amount of water vapor in the air versus what it can hold. The amount of water that air can hold varies with temperature; warmer air has a greater water-holding capacity than cooler air. The water-holding capacity of air approximately doubles with every 20-degrees F (11°C) increase in temperature. Therefore, air that is at 80 degrees F (27°C) can hold twice the amount of water compared with at 60 degrees F (16°C). Instead of relative humidity, the more accurate way to express the driving force of water loss from a leaf is vapor pressure deficit (VPD). Its value is independent of temperature. VPD is the difference between the amount of moisture in the air and how much moisture the air could potentially hold when it is saturated. It is often measured in pounds per square inch (psi) or kilopascal (kPa).

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