In controlled environment agriculture, managing the atmosphere is fundamental to maximizing plant growth and health. While growers monitor temperature and humidity separately, these metrics do not fully capture the air’s drying power. Vapor Pressure Deficit (VPD) is a more precise measurement that integrates the effects of both temperature and humidity into one value. VPD offers a sophisticated gauge of the atmospheric demand on a plant, moving beyond simple Relative Humidity (RH) readings to optimize the growing environment.
Understanding Vapor Pressure Deficit
Vapor Pressure Deficit is a measure of the difference between the amount of moisture currently in the air and the maximum amount of moisture the air could hold at its current temperature. This value essentially quantifies how “thirsty” the air is, or its capacity to draw moisture away from a surface. VPD is expressed in units of pressure, typically kilopascals (kPa).
The concept relies on two values: saturated vapor pressure (the maximum moisture the air can hold) and actual vapor pressure (the moisture truly present). Warmer air holds significantly more moisture than cooler air, meaning the saturated vapor pressure changes significantly with temperature. The deficit is simply the difference between these two values.
VPD is more reliable for growers than Relative Humidity (RH). RH is a percentage expressing current moisture relative to maximum capacity, which constantly changes with temperature. VPD provides an absolute measure that directly reflects the atmospheric force driving water loss from the plant, making it a superior indicator of plant activity.
VPD’s Role in Plant Physiology
The primary reason VPD is important is its direct influence on the plant’s rate of transpiration, which is the movement of water through the plant and its evaporation from the leaves. Transpiration is the engine that drives nutrient delivery from the roots to the rest of the plant. When VPD is in the optimal range, it creates a stable pressure differential that encourages the plant to transpire efficiently.
A higher VPD means drier air, creating a strong “pull” that accelerates transpiration. This increased water flow facilitates the mass flow of nutrients, such as calcium, from the roots to the growing tips. If VPD becomes too high, the air’s drying power stresses the plant, causing the stomata—microscopic pores on the leaf surface—to close to conserve moisture.
When stomata close, they shut down carbon dioxide intake, drastically slowing photosynthesis and growth. Conversely, if VPD is too low, the air is saturated, and the pressure differential needed to pull water is weak. This low transpiration rate hinders nutrient movement, leading to deficiencies and slower growth. Low VPD also increases humidity, raising the risk of fungal pathogens and molds.
Calculating and Monitoring VPD
VPD must be calculated using two primary inputs: air temperature and relative humidity. Growers typically use a combination of a thermometer and a hygrometer to collect these two readings within the plant canopy. More advanced, integrated environmental monitoring systems can automatically compute the VPD value in real-time, simplifying the process.
The most common way for growers to determine their current VPD without complex mathematics is by using a VPD chart. These charts plot temperature on one axis and relative humidity on the other, with colored zones indicating the corresponding VPD value in kilopascals. By cross-referencing the two measured inputs, a grower can quickly determine if the environment is in the ideal, caution, or stress zone.
Using Leaf Surface Temperature (LST)
A more precise calculation incorporates the leaf surface temperature (LST) instead of the ambient air temperature. Plants cool themselves through transpiration, meaning the leaf surface is often 3 to 5°F cooler than the surrounding air. Using LST provides a more accurate picture of the evaporative demand the plant is truly experiencing. LST is measured using a non-contact infrared thermometer.
Adjusting the Environment for Optimal VPD
Maintaining the correct VPD requires manipulating temperature and humidity. Growers adjust the target VPD based on the plant’s stage of development, a technique called environmental zoning.
VPD Targets by Stage
- Young plants (seedlings and clones) require a lower VPD (0.4 to 0.8 kPa) because they have underdeveloped root systems and must conserve water. This is achieved by maintaining higher humidity.
- During the vegetative stage, the plant tolerates a higher rate of transpiration, making the optimal range 0.8 to 1.2 kPa. This higher deficit encourages robust nutrient uptake and rapid growth.
- In the flowering stage, VPD is often increased further (up to 1.4 kPa) to promote faster water movement while lowering humidity to discourage mold or rot in dense flower structures.
The primary levers for controlling VPD are temperature manipulation (heating and cooling) and humidity manipulation (humidifiers, dehumidifiers, and ventilation). Raising the temperature with fixed humidity increases the air’s capacity to hold moisture, which increases the VPD. Conversely, introducing moisture or reducing temperature decreases the VPD. Growers must continuously monitor and adjust both factors to stay within the target range.