Vapour-Pressure Deficit

Vapour-pressure deficit, vapor pressure deficit, or VPD, is the difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated. Once air becomes saturated, water will condense out to form clouds, dew or films of water over leaves. It is this last instance that makes VPD important for greenhouse regulation. If a film of water forms on a plant leaf, it becomes far more susceptible to rot. On the other hand, as the VPD increases, the plant needs to draw more water from its roots. In the case of cuttings, the plant may dry out and die. For this reason the ideal range for VPD in a greenhouse is from 0.45 kPa to 1.25 kPa, ideally sitting at around 0.85 kPa. As a general rule, most plants grow well at VPDs of between 0.8 and 0.95 kPa.

In ecology, [Vapour-Pressure Deficit] it is the difference between the actual water vapour pressure and the saturation water vapour pressure at a particular temperature. Unlike relative humidity, vapour-pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.


Ecology is the branch of biology which studies the interactions among organisms and their environment.


Evapotranspiration is the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator.


This image has an empty alt attribute; its file name is Tomato_leaf_stomate_1-color.jpg
Source: Dartmouth – Images – Botanical Leaf

In botany, a stoma (plural “stomata”), also called a stomata (plural “stomates”) is a pore, found in the epidermis of leaves, stems, and other organs, that facilitates gas exchange.


Source: By M. W. Toews – Own work, CC BY 4.0,

Monitor – Control – Analyse
Environmental Controller – Hydro-X – TrolMaster

Trol Master – Hydro-X

The Hydro-X from TrolMaster can be used to control, and monitor, both a humidifier, and a dehumidifier. This allows growers to maintain an optimal humidity level as the lights turn on and off. The Hydro-X can also be used to control air conditioning devices, or fans, to cool the environment, giving the grower even more control as the weather outside changes. Using external monitors to check against and read for hot spots is not a bad idea when setting a climate controlled area. The proper use of circulated air can help insure that every bit of your air is being conditioned to the optimal levels.

How Do I Add Humidity?

Most people are familiar with a dehumidifier, but when it comes to adding humidity most people are less familiar with the option they have. In our search to find a solution to this question we have found two different style units which we have readily available.

Pump & Fan Humidifier
Ultrasonic Humidifier

We have found in our experience that either unit will suffice. However in situation where condensation is an issue one unit has preformed better than the other. Different situations call for different solutions. This information is not meant to confuse you, but to make you look into the idea of adding humidity and to be generally aware of the options that exist. For a larger room the Pump & Fan style humidifier seemed to work just fine, in small more confined spots the Ultrasonic humidifier did i great job of adding humidity without generating any condensation on the plants or surfaces in the room. However when we tried to use PVC to spread out the humidity from the Ultrasonic humidifier we had an issue with condensation. The same thing happened using the Pump & Fan humidifier in too small of a room. Without enough room for the water vapor to mix with the air our customers noticed a build up of condensation on both the plants and the surface of their grow room. However, all that said, anything is possible and every environment is different.

*We have created a few different custom options for several different customers. Feel free to call and ask and we will come up with a customer solution for you too.

-Aggressive Garden

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