If you’ve never heard of VPD before it may seem like a highly technical or cumbersome tool to use but it’s an invaluable metric to help gauge environmental conditions and plant stress. In a nutshell, it measures how leaf temperature, air temperature, and relative humidity affect the pressure difference between the leaf and the environment. Each of these variables is already very important in growing, but when we combine them into a single measurement it helps see how they interact with one another.
The flow of water through plants is responsible for nutrient transport, cooling of leaf tissues, photosynthesis, turgidity, the list goes on… Almost all aspects of plant growth depend on the movement of water. VPD helps us get a grasp on how a plant might feel under a given set of environmental conditions.
Around 95% of the water that a plant uptakes is lost through transpiration. We can think of a plant as a sort of wick that pulls water up from the rootzone and constantly loses it to the atmosphere where the lost water is mostly utilized for turgidity, or plant structure. The difference in water saturation between the plant tissues and the environment determines how quickly plants transpire. The thing is, temperature directly affects the saturation capacity of air; as air increases in temperature, its ability to hold water vapour also increases. If you have a set amount of vapour in the air and the temperature rises the RH will drop. Conversely, as the temperature drops the RH rises. This is why we get dew when the temperature falls at night; the air already has a certain amount of vapour in it during the day, then it gets cooler and water condenses out as the holding capacity decreases.
VPD is such a vital measurement to take into consideration because the factors that determine the rate of transpiration are outside of the plant’s control. It’s kind of like how cold-blooded animals aren’t able to fully regulate their body temperature and need to bask in the sun. The obvious difference is that a turtle can move in and out of the sun when needed, whereas all a plant can do is regulate and limit transpiration and wait for more favourable weather when it detects extreme conditions. When a plant is forced to limit transpiration like this it also loses its ability to regulate internal temperature and move nutrients through the vascular system so the situation becomes dire quickly. This is the worst case scenario, think of a wilted plant in the blazing sun with dry soil.
On the other extreme, with a very high RH, when the difference in saturation levels are low, plants are unable to transpire because this gradient is required to ‘pull’ water out of the leaves. Without this gradient transpiration can’t happen. Think about how sweat is used to cool the body; when sweat evaporates it cools the skin, but if the air is already fully saturated sweat won’t evaporate and no cooling will occur. For plants these types of conditions not only hinder them from growing properly, but can also promote things like fungal infection and pest infestations.
Since plants rely on this passive mechanism to control the internal flow of water they are unable to thrive if the environment is not balanced. This is why tracking VPD is so helpful; it takes all these different factors into account and gives a metric that’s much more informative than any of them individually.
Now that we have some idea of how VPD affects plant growth and why it’s an important tool, especially for indoor growers who have the ability to tinker with their environmental controls, let’s break down how to calculate VPD yourself…
Since VPD is a measure of the difference in saturation levels between plant and environment we’ll need to find both the saturated vapour pressure (known as VPsat) of the plant and subtract from it the vapour pressure of the air (VPair).
Plants are fully saturated with water meaning that the only variable we need from the crop is the leaf temperature. An infrared thermometer is the best tool for measuring this. If we plug our leaf temperature (in degrees celsius) into the following equation we get the saturated vapour pressure (VPsat) within the plant measured in Kilopascals (kPa).
All that remains now is to subtract the vapour pressure of the air from the saturated vapour pressure we just derived. We use the same equation for VPsat and then multiply it by the RH of our environment to find VPair.
Since there are a number of online VPD calculators available to use for free there is no need to do the calculations yourself; we’re just breaking it down to help understand how it works. Once you’ve got your variables, just plug them into a calculator and see how balanced your growing environment is (just remember that different crops have different optimal VPD ranges so it’s important to make sure you’re referencing an appropriate chart).
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