Vapor pressure deficit (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 doesn’t tell us much about the forces driving evaporation 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 Vapor Pressure Deficit Affects Evaporation
We consider the water content of a living leaf to be 100% RH, meaning completely saturated. While the air is normally undersaturated. This enables water to flow from the plant to the air.
If VPD is high, meaning the air is relatively dry, transpiration rates will increase. As the vapor pressure deficit lowers, transpiration will decrease or even cease completely.
There is a direct connection between plant metabolism and 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 VPD is problematic as well. 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 stops the intake of CO2, which also impedes growth.
How to Measure VPD
Vapor pressure deficit 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 air conditions of 22oC and 80% RH, vapor pressure is 2.12 kPa. If RH were 100%, meaning complete saturation, at the same temperature, the vapor pressure would be 2.64 kPa.
The difference between saturation and the actual water content, in terms of pressure, is the VPD.
VPD = 2.64 kPa (saturation) – 2.12 kPa (actual) = 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.
The optimal vapor pressure deficit range for most plants is considered to be between 0.4 kPa and 1.6 kPa. This, of course, differs among plants. Each plant has its own optimal conditions for different stages of development.
As a rule of thumb, VPD should be on the higher side of the optimal range during later stages.
Ideal VPD for cannabis
In cannabis, the ideal VPD is narrower:
- Vegetative stage: 0.8-1.0 kPa
- Flowering stage: 1.2-1.5 kPa
How to Affect Vapor Pressure Deficit
There are 2 ways to influence vapor pressure deficit:
- To raise VPD: increase temperature
- To lower VPD: decrease temperature
- To raise VPD: decrease humidity (turn dehumidifier on)
- To lower VPD: increase humidity (turn dehumidifier off)
It’s important to note that there are many things in a greenhouse that affect these 2 factors. For example, lights emit heat and will increase the temperature, resulting in higher vapor pressure deficit levels.
Greenhouse VPD Management
In a greenhouse environment, water constantly evaporates. This drives VPD downward, as the air gets closer to the dew point.
Dehumidification is the obvious solution. Physically removing water from the air decreases humidity and 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. Plants only experience the air with which they come into immediate contact. So, when the boundary layer remains hovering around the plant, it will experience a lower VPD than is measured in the rest of the space. This inevitably results in inconsistent measurements that may not reflect the slow metabolism rate actually experienced.
Circulating air efficiently helps 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. This makes monitoring easier and more effective, while providing uniform optimal conditions, including VPD, throughout the growing space.