Vapor Pressure Deficit (VPD) is a measurement that quantifies the relationship between temperature and humidity within a growing environment. It determines the “drying power” of the air, which directly affects how a plant regulates its internal temperature and absorbs nutrients. The VPD chart serves as a concise visual guide, allowing controlled environment agriculture (CEA) growers to manage their climate for optimized plant health and growth.
The Science of Vapor Pressure Deficit
Vapor Pressure Deficit is the difference between the saturated vapor pressure inside a plant’s leaf and the actual vapor pressure of the surrounding air. Saturated vapor pressure is the maximum amount of moisture the air can hold at a specific temperature before it condenses. Inside the leaf’s stomatal cavity, the air is nearly 100% saturated with water vapor.
The difference between the internal and external vapor pressure creates a gradient that drives water outward, a process known as transpiration. This continuous movement of water carries dissolved nutrients throughout the plant. Transpiration also provides cooling, preventing the plant from overheating under intense light.
The amount of water vapor the air can hold increases exponentially as the temperature rises. Therefore, a change in temperature significantly alters the VPD, even if the relative humidity (RH) remains constant. If the VPD is too high, the air is very dry, causing the plant to transpire excessively and potentially close its stomata. Conversely, if the VPD is too low, the air is nearly saturated, which slows transpiration, reduces nutrient uptake, and increases the risk of mold growth.
Interpreting the VPD Chart Structure
The VPD chart is a graphical representation that maps the intersection of air temperature and relative humidity to produce a single VPD value, typically measured in kilopascals (kPa). Temperature is displayed on one axis, while relative humidity is on the other. The chart visually simplifies a complex calculation, allowing growers to quickly assess their environmental conditions.
By locating the current air temperature and RH on the chart, a grower finds the corresponding VPD value at their intersection point. Most charts utilize color-coded zones to indicate the health status of the environment. A central “green zone” represents the optimal VPD range for general plant function, encouraging efficient transpiration and nutrient delivery.
Areas surrounding the optimal range shift to a “yellow zone,” indicating acceptable but not ideal conditions. Conditions in the “red zone” signify stress, either from a VPD that is too high (rapid water loss) or too low (risk of fungal disease). This visual system provides immediate feedback on whether the temperature and humidity combination is promoting or hindering growth.
Applying VPD for Controlled Environment Agriculture
The practical application of the VPD chart involves selecting a target VPD range that aligns with the plant’s specific life cycle stage. Different stages of development require different transpiration rates to maximize growth and yield.
Young clones and seedlings benefit from a lower VPD, typically in the range of 0.4–0.8 kPa. This lower deficit minimizes water stress, which is important for plants establishing roots or those with undeveloped root systems.
As the plant transitions into the vegetative stage, the target VPD increases to a moderate range, often between 0.8–1.2 kPa, to promote rapid growth and nutrient uptake. This moderate deficit supports a strong transpiration stream, necessary for moving high volumes of water and nutrients to fuel leaf and stem development. The ideal VPD for the flowering or fruiting stage is typically higher, moving to a range of 1.0–1.4 kPa. The higher VPD during flowering helps to reduce the risk of mold and mildew, which thrive in the high humidity associated with low VPD.
Growers use the chart to guide adjustments to their environmental controls, such as altering the temperature setpoint or adjusting humidifiers and dehumidifiers. For example, if the current VPD is too low, increasing the air temperature or decreasing the relative humidity will raise the VPD toward the target.
A more advanced consideration involves monitoring the leaf temperature, which is often 3–5°F cooler than the surrounding air due to evaporative cooling from transpiration. Since the VPD is technically the difference between the saturated vapor pressure inside the leaf and the air, using the leaf temperature provides a more precise calculation. Integrating leaf temperature measurements with the VPD chart allows for the most precise environmental optimization, ensuring the plant is never forced to close its stomata due to excessive water loss.