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VPD - Vapour Pressure Deficit

As implied by the word "deficit" we're talking about the difference between the theoretical pressure exerted by water vapour held in saturated air (100% RH at a given temperature) and the pressure exerted by the water vapour that is actually held in the air being measured at the same given temperature.

In essence, it is the consequence of the imbalance between temperature and relative humidity.

⬇︎LOW Humidity + ⬆︎ HIGH Temperature = as air warms it expands and has the ability to hold more water

⬆︎HIGH Humidity + ⬇︎LOW Temperature = as air cools and contracts vapour concentrates and can fall or condense out of the air as water

Temperature control and humidity maintenance are essential for the prevention of VPD. There are several ways to monitor your grow room and ensure these optimal levels (see table) are maintained.

To learn more about humidity, check out our humidity blog -

There are several products you can get to manage your humidity levels.

The temperature of your grow space is equally as important. For more information on the importance of temperature when growing, read our temperature blog post -

The essential item for any grow room with ventilation equipment is a fan speed controller.  By controlling the speed your fan, you are ensuring your plants do not get too cold or warm.


To ensure the balance of temperature and humidity remains balanced, closely monitor these factors using a thermometer/hygrometer.

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