What is VPD?
Vapor pressure deficit (VPD) is one of the main drivers of plant evapotranspiration. VPD is the difference between the maximum amount of water the air can hold and the amount of water it contains. It’s also one of the several methods growers use to measure humidity in a greenhouse or growing environment.
How to Measure VPD?
VPD = Vapor Pressure (at saturation) – Vapor Pressure (air)
You can refer to this form:
There are many online calculators and apps that can also help you calculate VPD：
What Are the Effects of Vapor Pressure Deficit on Plants?
VPD plays a huge role in plant transpiration. A high VPD (above 1.0 kPa) means the air is dry. In fact, air can still hold a lot of water. In this case, the plant will be over-drying because of transpires too quickly. There is a large difference in vapor pressure between the leaves and the air. And it will affect stomatal opening (smaller as VPD increases) and CO2 absorption (decrease).
A low VPD (below 0.4 kPa) means the air is close to saturation. A VPD of zero means the air is 100% saturated, so plants cannot transpiration efficiently. In fact, the water vapor does not leave the plant, but remains on the leaves. This will promotes some diseases, especially fungal disease.
In addition, VPD also has some other effects on plants, such as: carbon dioxide turnover, nutrient uptake by roots, plant stress, stomata opening.
How to improve VPD deficiency?
In a greenhouse, water will continuously evaporate due to the indoor temperature. This pushes the VPD decrease as the air approaches the dew point. To maintain optimal range, there are two basic ways to reduce humidity and increase VPD: dehumidification and air circulation.
Dehumidification is the obvious solution. Physical removal of moisture from the air reduces humidity and increases VPD as the air moves away from the dew point.
Another method is air circulation. Circulating the air efficiently helps to disperse the boundary layer. In addition to increasing the vapor pressure deficits experienced by plants, good circulation also helps minimize fluctuations in the greenhouse. This makes monitoring easier and more efficient, while providing uniform optimal conditions throughout the growing space, including VPD.
It is clear to see that understanding the concept of VPD and how to control it is very important for modern growing, if you have any other growing or product questions, please feel free to contact us!