What is Vapor Pressure Deficit and why does it matter?
To understand VPD, it is important to understand how humidity works. When we talk about humidity, we’re talking about the amount of water in the air in the form of vapor. There are two ways to measure humidity- absolute humidity and relative humidity. Absolute humidity tells us exactly how much water vapor is in the air while relative humidity (RH) tells us how much water is in the air as a percentage of how much water the air can hold at that temperature. This is where things can begin to get confusing, because temperature and humidity have a complicated dynamic. As temperatures increase, the air is able to hold more water vapor than at lower temperatures. This means that a room will see a drop in RH when the absolute humidity has not changed, but the temperature has increased.
So why is this important? All cultivators know that managing humidity is important. For example, high humidity levels can lead to mold growth, damaging crops and affecting yields. However, RH levels also dictate how much a plant will transpire, and thus, how much they will grow. This is because the water we give to plants merely acts as a vehicle for nutrients. Once the nutrients have been metabolized, plants transpire water back into the air and are able to bring in more water and nutrients again. But when RH levels are high, plants have a harder time doing this.
The reason for this is that water vapor in the air creates a certain amount of pressure, pushing back on plants as they try to transpire water. As you can imagine, more water vapor in the air (i.e. higher RH levels) means more pressure, causing plants to have a harder time transpiring. This brings us to what VPD actually is. As the name indicates, this is a measure of a deficit, or difference between the pressure that could be exerted by the water vapor at 100% RH at a given temperature and the actual pressure exerted by water vapor at your current RH at the same given temperature. Essentially, this is the way the plant would feel, measuring the difference between the pressure inside the leaf and the pressure of the air outside, giving us an idea of how easy or difficult it would be for the plant to transpire. VPD is measured in units of pressure and is essentially RH and temperature in a single value.
Much research has been down to find the ideal VPD for cannabis transpiration, ensuring plants drink slowly enough to be able to metabolize nutrients but fast enough to ensure they are taking up enough. Depending on the stage of growth, flowering plants should have a VPD of between 10kPa1 and 15.5kPa.
Source: Surna