Vapor Pressure Deficit (VPD) measures the difference between the amount of moisture in the air and the maximum amount it can hold at its current temperature. It quantifies the air’s “drying power,” indicating how much more water vapor it can absorb before becoming saturated. This measurement is expressed in kilopascals (kPa) and reflects the atmosphere’s evaporative potential.
The Role of VPD in Plant Processes
VPD directly influences transpiration, the process by which plants release water vapor into the atmosphere. Plants use tiny pores on their leaves, called stomata, to regulate this exchange of water vapor and carbon dioxide. When VPD is moderate, plants efficiently transpire, drawing water and dissolved nutrients from their roots through their vascular system. This movement is driven by the vapor pressure gradient between the nearly saturated air inside the leaf and the drier air outside.
A suitable VPD encourages stomata to remain open, facilitating carbon dioxide uptake for photosynthesis and allowing for effective nutrient transport throughout the plant. If VPD becomes too high, the air’s increased drying power can cause stomata to partially close to conserve water, reducing carbon dioxide intake and potentially leading to nutrient deficiencies. Conversely, very low VPD indicates highly humid air, which slows transpiration and can impede water and nutrient movement, potentially stressing the plant.
Calculating and Measuring VPD
Vapor Pressure Deficit is calculated from air temperature and relative humidity (RH), rather than being measured directly. The calculation involves determining the saturation vapor pressure (SVP)—the maximum water vapor the air can hold at a given temperature—and the actual vapor pressure (AVP), which is the amount of water vapor currently in the air. VPD is the difference between these two values (SVP – AVP).
Growers often use VPD charts, which plot air temperature and relative humidity to indicate corresponding VPD values. To use a chart, one locates the measured air temperature and relative humidity to find their intersection and identify the VPD. Modern cultivation environments frequently employ integrated environmental sensors that automatically measure temperature and humidity, then calculate and display the VPD in real-time, simplifying monitoring.
Ideal VPD Levels for Plant Growth
Maintaining specific VPD ranges benefits plants as they progress through different life stages, supporting their unique physiological needs. For propagation, including seedlings and clones, a very low VPD, typically between 0.4 and 0.8 kPa, is generally preferred. This lower evaporative demand helps delicate young plants with underdeveloped root systems retain moisture and establish themselves without excessive water loss. High humidity during this stage promotes root development and prevents desiccation.
Vegetative Growth Stage
During the vegetative growth stage, plants benefit from a low-to-medium VPD, often in the range of 0.8 to 1.2 kPa, with an ideal target around 1.0 kPa. This range encourages robust transpiration, which in turn facilitates efficient water and nutrient uptake and transport throughout the plant. It supports the rapid development of leaves and stems, fostering vigorous growth.
Flowering Stage
As plants transition into the flowering stage, a medium-to-high VPD, typically between 1.0 and 1.5 kPa, or potentially up to 1.6 kPa, is often recommended. This elevated VPD encourages the plant to focus energy on producing fruits and flowers by promoting higher transpiration rates, which enhances nutrient delivery to developing reproductive structures. Additionally, a slightly drier environment reduces the risk of excess moisture accumulating on flowers, helping mitigate potential issues like mold or fungal diseases.
Managing Environmental Conditions to Control VPD
Controlling VPD involves balancing air temperature and relative humidity within the growing environment. Effective VPD management relies on continuous monitoring and strategic adjustments of these environmental factors to meet the plant’s specific needs at each growth stage.
To lower a high VPD, indicating overly dry air, growers can increase relative humidity using humidifiers or misting systems. Lowering the air temperature can also decrease VPD, as cooler air holds less moisture at saturation. Reducing the speed of ventilation or air circulation can help retain moisture in the immediate plant canopy.
Conversely, to raise a low VPD, signifying overly wet air, adjustments aim to reduce humidity or increase temperature. Dehumidifiers are effective tools for removing excess moisture, thereby increasing VPD. Increasing the air temperature also raises the air’s capacity to hold water, which can increase VPD if absolute moisture content remains constant. Enhancing air circulation with fans or increasing ventilation can help move humid air away from the plant canopy, promoting a higher evaporative deficit.