VPD – The Vanity Metric of Your Grow Facility?

A 1-Month Snapshot of VPD Values from a Commercial Cannabis Facility

Figure 1: Typical VPD chart that assumes leaves are 2°C cooler than air temperature. VPD is measured in kPa. We have arbitrarily selected the green shaded area (about 0.8 to 1.2 kPa) as being ideal for an imaginary crop. The yellow areas show an acceptable but marginal VPD range and the red areas are either too high or too low. The chart is overlaid with 1-month worth of VPD data from a commercial cannabis facility (black dots). The VPD values from the facility spanned all color zones, however most of the time was spent in the red zone.

Above is a typical VPD chart (Figure 1). Humidity (%) is on the x-axis and temperature (°C) is on the y-axis. VPD is measured in kPa. Leaf temperature is assumed to be 2°C cooler than air temperature (a widely used assumption that actually doesn’t often hold true). Many VPD charts have been arbitrarily shaded to represent optimal, sub-optimal, and non-optimal VPD values and we have done this with our VPD chart. We took this VPD chart and overlaid a month’s worth of VPD data from a commercial cannabis facility. This chart, offered as part of the Braingrid analytics service, provides a unique view of VPD values. It allows growers to see where their facility is spending the most time. In the case of this facility, most of the time (52.4 %) was spent in the red (non-optimal) zone, followed by the yellow (sub-optimal; 32.4 %) zone. The least amount of time was spent in the optimal zone (15.2 %). These results were surprising and so we investigated further.

We wonder if VPD charts might be oversimplifying the story? VPD is a good indicator of plant transpiration rate, and maybe that’s all it should be used for – interpreting just how much water may be evaporating from plant leaves. However, the important thing to note here is, as VPD increases, evapotranspiration also increases (Massmann et al., 2018). Many growers worry that if pushed too far, a plant will transpire too much, but this is simply not the case! Research shows that at very high VPDs, plants respond accordingly and reduce their evapotranspiration to conserve water (Massmann et al., 2018). This is true for virtually all plants – from peanuts to pine trees (Addington et al., 2004; Devi et al., 2010) so labels like “optimal”, “sub-optimal”, and “non-optimal” – aren’t exactly wrong but seem inaccurate when interpreting VPD. Labels like “lower transpiration”, “moderate transpiration”, and “higher transpiration” may be more fitting for the purposes of a master grower.

Why was VPD in the “Non-Optimal” Range so Often?

Naming convention aside, we want to dive deeper into the data to solve this mystery of just why this facility (and plenty others) spent so much time in the “non-optimal” and “sub-optimal” zones. What we discover is that most of the VPD values fit with the expected thermodynamic properties of gas-vapor mixtures (represented by the red line in Figure 2). Air temperature affects how much water vapor the air can hold, so as the air temperature increases, it holds more water. If we assume that the amount of water in the air stays the same (in Figure 2 it is 11.5 g of water per 1 m³ of air), then as temperature rises, so will humidity in a non-linear way.

VPD values falling outside of this thermodynamic relationship can be linked to the HVAC and dehumidification systems, which change the volume of water in the air (blue circles in Figure 2). Abnormal VPD values may also occur when activities happen that disturb the temperature or water vapor content of the room. This includes cleaning, moving plants, and leaving the doors open for a long period. The times where these activities may have happened is highlighted with a green circle in Figure 2.

Figure 2: Relationship between temperature and relative humidity. If we assume that the amount of water in the air stays the same (in this case, 11.5 g of water per 1 m3 of air), then as temperature rises, so will humidity in a non-linear way. The red line represents this relationship and the black dots represent 1 month worth of VPD values from a commercial cannabis grow.

Will Yields be Lower if VPD Stays in the “Non-Optimal” Range?

For many crops, VPD actually has little to no effect on yield! Research shows that VPD has no effect on flower number or plant yield in some species (Erickson & Markhart, 2001). Even at very high VPD values, when the temperature is high and the humidity is low, plants are more negatively affected by the high temperatures (33 – 35°C) rather than by the high VPD (Erickson & Markhart, 2001; Ruxton et al., 2014). For crops that are affected by VPD, the VPD must be very high before yield is impacted. In fact, VPD must be above 3 kPa (i.e., at the very extreme end of the “non-optimal” zone) before any yield is lost (Ruxton et al., 2014). Above 3 kPa, every 0.1 kPa increase in VPD can result in 1.5 – 2.8% decreases in yield (Ruxton et al., 2014).

What about other factors, like powdery mildew, that negatively impact yield? Research also shows that most measures of air moisture, like relative humidity, absolute humidity, and VPD, are equally accurate at predicting powdery mildew spore germination (Carroll & Wilcox, 2003). Growers should feel free to use any of these air moisture measures that they are most familiar with. Most research on powdery mildew suggests that temperature is actually a stronger predictor of disease progression than air moisture (Gubler et al., 2016).

VPD is a great tool for understanding your grow facility but so is temperature, humidity, CO2, etc. VPD provides growers with another avenue for understanding complex plant processes. Deviations in VPD can also tell growers about how their HVAC and dehumidification system is functioning, or if activities like cleaning and moving plants are significant enough to overwhelm the regulation systems. Knowledgeable growers also see the limitations of VPD and realize that it is not necessarily the best weapon in their arsenal!