VPD is one of several methods growers use to measure humidity in a greenhouse and gauge its impact on plant development.
Vapor pressure deficit measures the difference, in terms of pressure, between the air’s water content, and its saturation point – the maximum amount the air can carry at its current temperature. The point of complete saturation is also called the dew point.
VPD = Vapor Pressure (saturation) – Vapor Pressure (air)
This pressure difference is what drives transpiration from the plants’ stomata.
Many growers use RH (relative humidity) to measure the humidity level in a greenhouse. This measures the percentage of the air’s water content saturation at a certain temperature. While RH is great for managing humidity, it does not tell us much about the forces driving transpiration from the plants.
VPD, as opposed to RH, correlates with plant transpiration rates, making it a proactive tool for plant empowerment, rather than simply a climate parameter.
How VPD Affects the Rate of Evapotranspiration
The water content of a living leaf is considered to be 100% RH, meaning completely saturated. While the air is normally undersaturated. This enables the one-directional flow of water, from the plant to the air.
If VPD is high, meaning the air is relatively dry, transpiration rate will increase. As the vapor pressure deficit lowers, transpiration will decrease or even cease completely.
Plant metabolism is directly connected to transpiration. As water evaporates from the leaves, it “pulls” water through the roots. This water contains the nutrients required for plants to grow, develop and bear fruit.
Low VPD, meaning low transpiration, would cause plant metabolism to slow down, inhibiting development, while also increasing susceptibility to diseases.
But high VPD is not advised either. At high levels, meaning dryer air, the evaporation rate from the leaf’s stomata may be too fast, causing it to dry out. Very high vapor pressure deficit could lead to water stress, a situation in which the plant goes on the defense, in an effort to retain water. The stomata close in order to prevent evaporation, which in turn halts the intake of CO2, which also impedes growth.
The optimal VPD range is considered to be between 0.8 kPa and 1.2 kPa. This, of course, differs among plants, as each may have its own optimal conditions for different stages of development.
How VPD is Measured
VPD is measured by subtracting the pressure of vapor in the air from the vapor pressure which would be found in saturated air, at a certain temperature.
Example: At an air temperature of 22Co and 80% RH, vapor pressure equals 2.12 kPa. If RH were 100%, meaning complete saturation, at the same temperature, the vapor pressure would equal 2.64 kPa.
The difference between saturation and the actual water content, in terms of pressure, is the VPD.
VPD = 2.64 kPa – 2.12 kPa = 0.52 kPa
Vapor pressure is calculated using temperature and RH. There are many charts online which provide this information, but we recommend using a vapor pressure calculator such as the one provided by LetsGrow.
Due to the nature of a greenhouse, in which water is constantly evaporated, VPD tends to push downward, as the air gets closer to the dew point.
Dehumidification is the obvious solution. Physically removing water vapor from the air increases VPD, as the air moves further away from the dew point.
A less intuitive method is air circulation. As the leaves emit water, they surround themselves with a thin layer of cool humid air – a boundary layer. As plants only experience the air with which they come into immediate contact, when the boundary layer remains hovering around the plant, the VPD experienced will effectively be lower than that measured in the rest of the space. This inevitably results in inconsistent measurements which may not reflect the slow metabolism rate actually experienced.
Circulating air efficiently assists in dispersing the boundary layer. In addition to increasing the VPD experienced by the plants, good circulation also assists in minimizing fluctuations in the greenhouse, making monitoring easier and more effective, while providing uniform optimal conditions, including VPD, throughout the growing space.