The role of VPD in plant health

Nowadays, it is relatively common to hear the term VPD, especially among newer growers. But what is VPD after all? What does it measure?

First, let's clarify the terminology. There are two meanings for VPD, and I have seen a lot of confusion between them.

We have VPD as vapor pressure deficit and VPD as vapor pressure difference. They measure different quantities, although both use the same unit kPa (kilopascal). The first refers to the potential for water to evaporate into the atmosphere. The second measures the difference in pressure between the substomatal cavity and the surrounding air in plants. For horticulture, it is the second meaning -vapor pressure difference - that really matters.

Vapor pressure difference in plants
The interior of the leaf (in the intercellular spaces near the stomata) is almost at 100% relative humidity, meaning it is at saturated vapor pressure. If the external air is less humid (i.e, has a lower vapor pressure), a vapor pressure gradient is created between the inside of the leaf and the surrounding air. This gradient causes water vapor to diffuse outwards — from high pressure (inside the leaf) to low pressure (in the air). This is the process of evaporation through the stomata, known as transpiration.

The Vapor pressure deficit is calculated using a formula that determines the pressure differential inside and outside the stoma.

A practical example
Suppose the leaf temperature in a medicinal crop is 25ºC. We can assume that inside the stomatal cavity, the RH is close to 100% so the vapor pressure at 25º is 3.17 kPa. This gives us the vapor pressure inside the stoma. Now, let's imagine the air in the greenhouse at 30ºC and 60% relative humidity; under these conditions, the vapor pressure is 2.55 kPa.

The vapor pressure difference = saturated vapor inside the leaf – actual vapor pressure of the greenhouse air.

In the example: 3.17 kPa - 2.55 kPa = 0.62 kPa

This means that there is a positive pressure from the inside to the outside of the leaf. But does a higher value always mean the plant transpires more? Not necessarily.

Up to a certain value -which may vary depending on the crop's phenological stage or species—transpiration increases with VPD. Beyond that point, however, the opposite effect occurs, and the transpiration potential drops.

Vapor pressure deficit chart of the air according to temperature and RH

Why is it important to know the Plant's Transpiration Performance?
VPD provides information on the crop's transpiration rate, that is, the speed at which water moves from the roots, through the stem, and out through the stomata. This is not just a process of hydration or cooling; it also involves the transport of nutrients taken up from the substrate or irrigation water.

A key example is calcium transport. Plants only transport calcium and other nutrients efficiently while evaporating. Calcium is crucial for building the plants' systemic resistance and other growing processes.

In summary, the leaf transpiration rate is determined by the vapor pressure difference between the air and the stomata. This, in turn, determines nutrient transport and plant growth.

Temperature and humidity set the VPD because the amount of water air can hold depends on temperature: warmer air holds more water. Thus, the vapor pressure difference drives evaporation in plants when the stomata are open.

To summarize, inside the stoma, it is nearly saturated (100% RH). If the air is also at 100% RH, there is no pressure difference and thus no evaporation. If the air's RH is lower than 100%, there is enough pressure difference for evaporation. When the leaf releases water, it enters the air as vapor, and the plant replenishes it from the roots. This process can be measured and manipulated to optimize plant development by irrigating at the right time and/or modifying the greenhouse environment.

How VPD affects crops
Vapor pressure deficit (VPD) values directly influence several crop processes. In terms of transpiration, higher VPD increases the rate at which crops transpire, necessitating more frequent rehydration but potentially resulting in faster growth. Regarding stomatal opening, plants close their stomata to prevent excessive water loss and maintain hydration when VPD rises above a certain threshold. This closure impacts CO₂ absorption, as increased VPD leads to reduced CO₂ absorption and photosynthesis, particularly in C3 plants, due to the restricted gas exchange. Additionally, nutrient absorption is affected by high VPD, which initially facilitates faster nutrient uptake via increased transpiration. However, stress can result from extreme VPD levels; very high VPD causes excessive transpiration, while very low VPD can impede water and nutrient movement. Managing VPD properly is crucial as it helps allocate assimilated sugars to the appropriate plant parts at optimal times.

VPD levels by crop stage and solar radiation
During the early stages of crop development, the water and nutrient requirements are minimal, but these needs increase as the crops mature. It is important for the vapor pressure deficit (VPD) to progressively increase in tandem with crop development. Within each growth stage, VPD should be maintained within specific limits, which vary depending on solar radiation. For instance, during the germination stage, which lasts until the start of the vegetative phase, the VPD should be kept between 0.4 and 0.8 kPa. Similarly, cuttings and clones should also have a VPD of 0.4 to 0.8 kPa. As crops transition from the end of the vegetative phase to the beginning of the flowering phase, the VPD range should be increased to 0.8 to 1.2 kPa. Finally, during the mid- to the end of the flowering stage, VPD should be further adjusted to a range of 1.2 to 1.6 kPa.

Many tables online show recommended VPD values by growth phase. These values can vary by crop stage or species. Remember, VPD is a guideline for evapotranspiration, but many other factors affect crop success.

How to change the VPD value in plants
To manage vapor pressure deficit (VPD) effectively, adjustments can be made to temperature and humidity based on the plant's needs, growth stage, and prevailing radiation levels.

Photo right: Data logger

To increase VPD, one can raise the temperature by using methods such as heating or closing windows and screens, or decrease humidity with dehumidifiers, ventilation management, or HVAC systems. Conversely, to decrease VPD, it is advisable to lower the temperature or increase humidity by opening windows or using humidifiers. These adjustments help maintain optimal growing conditions and support healthy plant development.

Air movement also increases evaporation by removing vapor from the leaf surface, helping maintain the pressure differential and contributing convective energy. This is especially important at night to ensure ongoing nutrient transport.

To manage equipment and achieve VPD targets, it is crucial to measure crop temperature. Devices range from manual infrared thermometers to advanced thermographic cameras that monitor temperatures at multiple points and communicate with cloud platforms that can communicate with your climate controller. Infrared technology works by capturing the infrared radiation emitted by the crop, allowing you to measure temperature without direct contact. Some examples of devices are:

Manual infrared thermometers: Allow spot-checks but are inefficient for large-scale monitoring.

Data logger infrared thermometers: Record and store crop temperature data for later analysis or cloud upload.

Plant temperature sensors: Measure crop or fruit temperature using infrared principles. These sensors can detect crop stress and condensation risks early. The sensor provide notification when any deviations occur between the crop temperature and the greenhouse temperature

Plant temperature sensor

Thermographic cameras: Capture infrared images (thermograms) showing temperature differences across the crop. These cameras provide real-time monitoring and can integrate with climate control systems for automated adjustments. By measuring both crop and greenhouse temperatures at multiple locations, growers gain insight into vertical and spatial temperature differences. If the climate computer is connected to the platform, data from thermographic cameras can be combined for automated environmental management, such as receiving alerts if fruit or flowers drop below dew point.

Thermographic cameras

In conclusion, understanding and managing vapor pressure difference (VPD) represents a transformative advance in protected‐crop cultivation. By precisely balancing temperature and humidity to maintain optimal VPD levels, growers can unlock more efficient transpiration, enhanced nutrient uptake—especially calcium transport—and stronger, more resilient plants. With accessible tools ranging from infrared thermometers to integrated climate‐control systems, this approach empowers even small‐scale producers to fine‐tune their greenhouse environment, driving higher yields and superior crop quality. Embracing VPD management is not just a technical upgrade; it's a strategic leap forward for modern horticulture.

This article was written by João Constantino, Canna.biz's Head of Cultivation, and Juan Francisco Moreno, J. Huete Greenhouse's Technical Director.