An Introduction to Vapor Pressure Deficit

By Haley Bridgnell
VPD Bioengineering Chart

An Introduction to Vapor Pressure Deficit

An Introduction to Vapor Pressure Deficit

How well do you understand temperature and relative humidity?

By Harry Connor

Vapor Pressure Deficit (VPD) is used by growers to assess how dry the air is at a given moment. If the air is too dry, photosynthesis will stop in order to protect the plant from exhaustion. 

Photosynthesis is the process of converting sunlight, water and carbon dioxide (CO2) into oxygen and, most importantly, sugars, which are used for plant growth. 

Growers often allow the air in their greenhouse to become too dry during the day causing their crop to shut down during critical daylight hours for growth. Measuring for VPD allows growers to maximize their peak daylight hours for photosynthesis, ultimately leading to larger and healthier crop growth.

 

How well do you understand temperature and relative humidity?

Many growers use temperature and humidity readings to monitor environmental conditions in their greenhouses. When in fact, maintaining a stable grow environment is more complicated than this. What growers should understand is that increasing air temperature allows that parcel of air to hold more water. It also follows that decreasing air temperature results in the air potentially holding less water. With this understanding, problems arise when we compare relative humidity values at different temperatures during the course of the day expecting the environmental story to be consistent.

Unlike Ceres greenhouses, indoor growing facilities use energy-intensive cooling and heating systems to keep temperatures stable. Our energy-efficient greenhouses on the other hand, utilize their passive solar design to maximize incoming solar radiation. This means internal greenhouse temperatures rise and fall with the power of the sun making relative humidity a tricky metric to use as temperatures change throughout the day.

The table below shows what happens when we have a constant relative humidity value for five separate scenarios and how this affects plant health as temperature changes. The takeaway is that using relative humidity at different temperatures can be a real headache in a greenhouse application. So instead of tracking environmental conditions using temperature and relative humidity, we recommend tracking VPD.

VPD chart

The Importance of Vapor Pressure Deficit

The reason we recommend using VPD is that it is independent of temperature and relative humidity. VPD measures the pressure exerted by the water vapor in the air and compares it to the theoretical saturation point of the air. The “deficit” is the difference between the actual vapor pressure and the saturated vapor pressure value. The smaller the difference, the more moisture laden the air will be. The larger the difference, the drier the air will be.

In the table above, a VPD value of 2 indicates high plant stress (large difference), where a VPD of 0.62 indicates a more humid environment. Plant stress means the metabolic activity of the plant is too high. If high metabolic activity persists, the plant’s stomata will close in order to prevent damage while simultaneously stopping photosynthesis. As growers, we want to encourage photosynthesis and avoid periods of stress.

There are greenhouse automation systems out there that allow for VPD control. One of them is the Ceres SunSense™ controller that allows growers to monitor and optimize their greenhouse conditions in one easy interface. 

Temperature and humidity are not useless metrics by any means however, new methods of analysis are always being discovered. If you want to be an innovative grower using the latest in data analytics, or you have any questions about VPD in general, please do not hesitate to contact us.

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