VPD stands for Vapour Pressure Deficit. It is a way of calculating the exact combination of temperature and relative humidity to achieve absolute peak performance from a plant. If you secure the right environmental conditions and are able to maintain them, the results are nothing short of astonishing.
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.
DO I REALLY NEED TO USE VPD?
Not at all. Depends on how much you appreciate a significant yield increase come harvest.
The principle behind plant growth is: light, food, and water. As you may know, plants consume CO₂ (carbon dioxide) and excrete O₂ (oxygen). They take in water and nutrients mainly through the roots, and transport it to the leaves where the light source then hits to energise the metabolic processes in the developing plant. With VPD, you can control (and hopefully increase) the rate at which these metabolic processes occur—though technically indirectly.
Underneath the leaves, there are tiny structures called stomata. These are openings that mediate the entry of water molecules and carbon dioxide, as well as serving as an exit port for oxygen and water molecules.
The stomata are sensitive to the environment. They can “read” how much humidity is in the air as well as the air column pressure. If needed, they can bring in some extra water through the leaves. Good news for avid foliar sprayers.
On the contrary, if the temperature is too high, the plant will try to transpire as much water as possible as means to cool-off. Much like we as well.
The problem is, when both temperature and relative humidity are either too high or too low, for instance, if the room temperature is too high and the relative humidity is also too high, the leaves are incapable of transpiring. When this happens, your cannabis production rate decreases.
The higher the ambient temperature, the higher quantity of water molecules it can absorb. This increases the surrounding air pressure. It is this air pressure that constricts the stomata from being able to open. They will do their best, but will not be as efficient, and far from working at peak performance.
You may think you have dialed in your grow room perfectly; but in all practicality, there is likely a great margin for improvement. It only makes sense to use VPD to dial in perfect conditions if you are able to fine tune to precise values of both temperature and humidity. In practical terms, most of the time you will simply be adjusting temperature.
BUT I ALWAYS RUN MY ROOM AT OPTIMAL LEVELS!
The more advanced you get, the finer the details get, but also the higher the rewards. Grow rooms are becoming ever more intelligent with the lowering of prices regarding technology. Just a few years back, a full kit of sensors would be prohibitively expensive. In this day and age, you can get continuous readings from your grow room straight to your smartphone, without breaking the bank.
Sure, everyone knows that during flowering, you will want to keep the temperatures at around 25°C and below 55–65% RH to avoid bud-rot. Under 35% RH may be too stressful for the plant. If you run CO₂ supplementation, you can easily run both higher temperatures and humidity levels safely.
But what if you could significantly increase your final yield, simply by knowing with absolute certainty the exact combination of temperature and RH to turbo-charge the stomata’s efficiency? This is the power of VPD.
HOW DOES IT WORK IN PRACTICE?
Start by getting an infrared thermometer gun. These can be found quite cheaply with lower precision tolerances, but are good enough to get the job done and let you see first hand how effective VPD actually is. Or, out of curiosity, to see how far off you are from optimal performance.
You start by pointing and shooting the gun’s laser over your top canopy while you take leaf surface temperature readings. These will vary across your canopy, so take a good few measurements at your most productive regions. Average these values to get a final temperature reading, and then grab your current RH% level.
Simply cross-reference these numbers in a chart like this one, and you will know where you sit regarding VPD. The best of it, no calculations are needed.
If (not surprisingly) you are too far away from optimal levels, just look at the closest, most viable option (temperature or RH%) and tweak your grow room accordingly.
That is essentially it. Do this from time to time to keep a good VPD in its place, and get prepared to be amazed. If you thought you knew your strain well, you might be in for a pleasant surprise.
To really take this to an entirely new level, consider purchasing a continuous measuring unit and see how temperature and RH% varies during a full day-night cycle, permitting you to adjust for ultimate peak performance.
