If you are able to dial in the right temperature and humidity in your grow-room, this is a must-read.

VPD has been known for quite a while, but fell out of the agro-horticulture circuits because large-scale operations rely on natural climate, as opposed to indoor growing. Only more recently, with the advent of the greenhouse and an influx of hobby cannabis growers, has it become possible to actually control environmental conditions with evermore precision to make practical use of VPD.

What was once only capable by university scientists is now possible to achieve by anyone, anywhere!

All you need is to get a cheap infrared thermometer gun and a relative humidity monitor (which you should already own by now), and start aiming the gun at your canopy, read the leaf temperature, and adjust either the temperature or the relative humidity (RH%)—whichever is more convenient.

It is as simple as that in practical terms. But the complete theory can get very confusing, very quickly.

THE THEORY BEHIND VPD

We will try to keep it as simple as possible. The mathematics behind this concept are quite complex indeed, but we do not need to get into them. Thankfully, there are many charts out there that will make your calculations as simple as crossing two numbers together—temperature x relative humidity.

But… for the sake of accuracy, VPD is the difference (or deficit) between how much water air can hold, compared to how much it is actually holding at that given moment. This difference is expressed in kilopascals, a pressure metric. The higher the ambient temperature, the higher concentration of water molecules can be maintained in the air, until relative humidity reaches 100% and condensation occurs. So, as both temperature and relative humidity shift, the deficit can be greatly increased or decreased.

You can use the Pulse One, grow room monitor to easily track your vpd

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