Many growers know about vapor pressure deficit (VPD) and why it is important, but I am constantly seeing one variable left out, which throws off the whole basis of VPD and optimal plant transpiration as a result.
Now, I’m not going to explain in detail what VPD is, or recommend that you should follow it. In fact, many growers don’t follow it at all, and still produce top-quality cannabis. VPD is not needed, and therefor, this missing variable I will soon talk about is also not needed. We all grow different ways, and learn from each other, which strengthens the collective growing power. In other words, grow how you want to, and improve yourself with each grow, no matter the methods and tools you use or do not use.
However, when talking about cannabis cultivation, we are talking about the optimization of plant growth and improving as many factors as we can that results in more preferred harvested crops. VPD is one such parameter we can utilize to optimize our cultivation, but there is a slight misconception about it that I will try to fix here, after I have seen countless occurrences of plant stress when taking VPD at its face value.
Without going into too much detail in the science or math, which I am willing to do later if there are any questions, VPD is a function of dry bulb (air) temperature and relative humidity. We use it to estimate the transpiration rate of plants, which directly correlates to growth rate.
The main problem here is, that is partly a lie. We shouldn’t be using dry bulb temperature in the equation, but instead, factor in the offset of the average leaf temperature. Because of moisture present in your cannabis leaves (I hope), evaporative cooling and transpiration mechanisms alter the temperature of the leaves, as compared to the air temperature, even when measuring the air properties at canopy level.
Just a single degree (F) will have a fairly large effect on the resulting VPD value, and with a cooler leaf surface temperature, and taking the air temperature at the canopy level, we are not doing our plants any good – plants do not care about the air temperature and humidity directly, only the effect the ambient environment has on its leaves and other photoreceptive parts.
This is why some automation controllers, such as AC Infinity’s, have a leaf offset parameter for you to plug in. VPD with a leaf temperature offset of zero, is not a good indicator of VPD under most conditions.
So how do we fix this? There are several ways, but a simple method is to take a laser or IR thermometer, and take samples of the canopy - several different leaves at the canopy level, and take the average of this. These values will change over time, especially after the plant receives water, or during night when it is doing less transpiration and more respiration, so it is a good idea to periodically check up on them.
Now, if you have an automation controller with a leaf temperature offset parameter to specify, it is not a good idea to enter in a value derived from the probe’s air temperature and your thermometer leaf readings. This basically boils down to because all thermometers are different. Instead, it is a better idea to also take samples of different objects in your grow space with your handheld thermometer, after measuring the leaf surfaces, around the leaf surfaces, and averaging these as well.
With the average canopy leaf temperature, and the average surrounding air temperature, you can derive a LTO value by subtracting the former from the latter. That is, if your surrounding air temperature sample average is 82F, and your leaf surface sample average is 79F, your LTO is -3F. This LTO value is what should be used in the calculation of VPD, adding it to the air temperature, rather than the direct air temperature at any given point.
Now, this is a little tedious, and these values will change over time, especially with diurnal changes. But the truth is, we are trying to optimize transpiration rate, which can only be done by taking fairly accurate samples of the moist leaves themselves, in addition to the surrounding air.
There is more I would have liked to write here, but just this should help optimize your grow just a little more if you were not familiar with any of this, and you happened to be following the concept of VPD.
I’ll happily answer any questions in the comments below if there is any confusion, or you’d like me to go more in-depth on something.
I’ll conclude this with mentioning that in my first couple of years growing indoor, I didn’t do this extra step, and saw many dozens of plants become diseased with (mostly) powdery mildew, as they were stressed, making them more susceptible to serious issues.
I hope this helps at least someone else. Happy growing!
