Vapor Pressure Deficit (VPD) is a precise measure of the difference between the amount of moisture the air currently holds and the maximum amount it can hold when fully saturated at that temperature. This metric, typically expressed in kilopascals (kPa), provides a more accurate picture of the atmosphere’s drying power than simple relative humidity readings. When VPD levels are too low, the air is nearing saturation, which stalls a plant’s ability to regulate its internal systems. For growers utilizing controlled environments, understanding how to raise a low VPD is necessary for maintaining optimal plant health and maximizing growth potential. This article outlines strategies for increasing VPD using temperature and humidity controls.
The Role of VPD in Plant Physiology
A low VPD indicates that the air surrounding the plant leaves is nearly saturated with moisture, which fundamentally slows down the process of transpiration. Transpiration is the plant equivalent of sweating, where water is released as vapor through small leaf pores called stomata. This movement of water acts as a hydraulic pump that drives the uptake and transport of nutrients from the root zone to the developing foliage.
When the atmospheric demand for water vapor is low, the plant’s internal water movement decreases significantly. This reduction in flow can hinder the uptake of immobile nutrients, particularly Calcium and Boron, which rely heavily on the transpiration stream for delivery. Maintaining a higher VPD ensures the plant sustains a healthy transpiration rate, facilitating the continuous movement of water and dissolved minerals. Active transpiration also cools the leaf surface, preventing heat stress and maintaining optimal photosynthetic efficiency.
Core Strategies: Manipulating Temperature and Humidity
Raising the Vapor Pressure Deficit relies on manipulating the two primary variables that determine it: air temperature and relative humidity. The underlying principle is increasing the air’s capacity to hold moisture without increasing the actual amount of moisture present, thereby increasing the “deficit.” Increasing the ambient air temperature is a highly effective way to achieve this.
Warmer air has a greater capacity to hold water vapor than cooler air. For a constant amount of moisture, raising the temperature dramatically increases the saturation point. This widens the difference between the saturated vapor pressure and the actual vapor pressure, resulting in a higher VPD. This method is often the most energy-efficient for slight VPD adjustments.
The second core strategy involves directly reducing the relative humidity (RH), which is the most direct way to increase the vapor pressure deficit. Relative humidity is the actual amount of water vapor in the air expressed as a percentage of the maximum amount the air can hold at that temperature. By removing water vapor from the environment, the actual vapor pressure drops, widening the gap to achieve a higher VPD. This reduction is typically achieved through dehumidification.
Practical Implementation of Environmental Controls
Implementing the core strategies of temperature and humidity manipulation requires specific environmental control equipment and careful scheduling. To actively lower the relative humidity and raise the VPD, growers primarily rely on commercial-grade dehumidifiers. These units condense water vapor out of the air, directly reducing the absolute moisture content and providing immediate control over the humidity variable.
An alternative method for humidity reduction is exhausting humid air and introducing drier, conditioned air. This approach is effective but must be balanced against the loss of enriched air, such as supplemental carbon dioxide. For temperature manipulation, supplemental electric or gas heating elements can be used to raise the ambient temperature, particularly during the dark cycle. Even a minor temperature adjustment can significantly elevate VPD because temperature increases the air’s water-holding capacity.
Beyond managing the room’s bulk climate, increasing air movement helps raise the local VPD immediately surrounding the plant leaves. Transpiration creates a thin layer of highly saturated air, known as the boundary layer, directly on the leaf surface. Utilizing oscillating fans breaks up this humid boundary layer, exposing the stomata to drier ambient air and encouraging a higher rate of transpiration. Proper air circulation also prevents localized high-humidity microclimates that can foster disease.
Adjusting VPD Targets Across Growth Cycles
The ideal Vapor Pressure Deficit is not static and must be progressively adjusted to match the plant’s changing physiological demands throughout its life cycle. Plants in the propagation phase, such as cuttings or seedlings, require a very low VPD, typically in the range of 0.4 to 0.8 kPa. This high-humidity environment minimizes water loss from young, rootless tissue, reducing stress and allowing the plant to focus on establishing a root system.
As the plant transitions into the vegetative growth phase, the VPD target should be gradually raised to a moderate range, generally between 0.8 and 1.2 kPa. This higher deficit encourages active transpiration and nutrient uptake, supporting the rapid development of foliage and biomass. In the final stage of flowering or fruiting, the VPD can be pushed higher, often to a range of 1.0 to 1.4 kPa. The higher evaporative demand supports the nutrient and water requirements of development and helps reduce the risk of fungal pathogens.