Getting Into the Weeds of Vapor Pressure Deficit (VPD)

What is VPD?

VPD measures the current moisture in the air compared to its saturation point. But to get to the root of how VPD shows up in a grower’s day-to-day, we checked in with Cultivation Consultant and self-proclaimed lover of plants, Tyler Simmons.

“Vapor pressure deficit is like the suction force that's felt on the leaves in terms of moisture being removed from the plant,” Tyler explained. “You can assume that the water vapor in the leaf is actually liquid, so it's 100% humidity. And then what's in the air is gas so there's a lower pressure there, and that forms a gradient that sucks water out of the pores of the leaf.” 

VPD is a driving force behind the way water moves through the plant and later evaporates from its leaves and other external parts through the process of transpiration. At its core, VPD is a function of two key variables: temperature and humidity. That means growers who use crop steering techniques to manage their cultivation must constantly work toward a balance that aligns with their objectives.

Finding the VPD Sweet Spot

“Plants are going to perform best within a certain target VPD range, and that range is going to change depending on where you're at in your growth cycle and what your specific goals are,” Tyler said. 

Growers must figure out the optimal VPD target range that will not only facilitate plant growth but also minimize the potential for botrytis and other molds while trying to produce the outcomes that ultimately lead to profitability. “If the balance between how quickly water is coming in and how quickly it's being removed is wrong, then you have a plant that's going to experience a vegetative growth stimulus,” Tyler explained, “which will delay ripening and potentially harm the structural characteristics of the flower, the morphology.” 

Cultivators are working to create the best possible environmental conditions for their plants. And according to Tyler, part of managing that overall coordinated effort involves getting a handle on VPD, which if left unchecked could negatively impact the quality of your product in the long run. “One of the reasons it's important to track and target different VPD levels is because you want to keep transpiration rates within a certain range: not too low and not too high,” he explained. “Bad things will happen if you go in either direction there.” 

At the end of the day, growers must stay mindful of their goals and align their practices accordingly. At the beginning when plants are small, for example, the optimal VPD is at the lower end of the range in order to reduce transpiration demands and facilitate vigorous plant growth. Exposing small plants to too high of a VPD can lead to a scenario Tyler describes as a “VPD death spiral”.

“If you have small plants in a very dry climate, the high VPD can cause moisture to be removed from the leaves faster than the root system can replenish”, he said. In an effort to prevent water loss, the plants close their stomata, slowing down photosynthesis significantly as CO2 absorption is also reduced. Because the plants are now releasing less moisture into the air, the humidity levels decline even further.

“As a byproduct of cooling the lights, air conditioners remove humidity from the air and relative humidity levels drop. To protect itself, the plant closes the stomata and the humidity drops even further, so the air gets drier,” feeding into a cycle in which, “plants end up very, very unhappy,” Tyler said.

How to set yourself up for VPD success 

Crop steering enables growers to control their plants by responding to different cues. But as Tyler makes clear, the ability to make informed decisions comes from seeing the data behind the plants and continuing to make adjustments throughout each growth phase in order to achieve the desired results.

Another VPD scenario cultivators must account for is near the end of the flowering cycle. As the crop nears harvest, the VPD must be increased to the high end of or above the plants' preferred range, to reduce the risk of mold or botrytis. Although this may cause some plant stress, doing so is a net positive as it creates unfavorable conditions for mold growth.

"Earlier on, you're going to favor a slightly lower VPD level that's in the perfect zone for really vigorous plant growth, reducing transpiration demands and plant stress. Then later on you might target a slightly higher VPD, where water is being removed more quickly from the plant. This serves two purposes," he explained. "It reduces botrytis risk, and it also gives a generative signal towards the end of the growing season that it's time to ripen."

This is where a cannabis production platform like AROYA can really help. Sensors bring greater transparency into what’s really happening inside the substrate, while software delivers the data that helps growers see the full picture in ways that occasional spot checks and even the most robust Excel sheet can’t. And using an all-in-one solution to track and monitor crops and review the complete grow cycle after harvest can go a long way toward helping growers identify the VPD ranges they need to be targeting along the way.

Regarding the question of using an infrared Leaf Surface Temperature (LST) or ambient air temperature reading for calculating VPD, “When you have plants spread across a room with leaves at a variety of levels and locations on the edge, top, bottom, or interior of the canopy, the leaf temperatures are so variable that it's very difficult to try to take the temperature from a single leaf and then reliably calculate VPD for the entire crop from that,” Tyler said. “While using LSTs has its advantages, in my experience using ambient air temps leads to more reliable results.” 

Then from there, Tyler applies what he learned to develop a broader overall strategy. “I have my day and night temperature and VPD targets planned for each phase. I'll use a calculator to reverse engineer the RH% required to hit that VPD,” he explained. “I'll input the temperature and VPD targets and it outputs the required RH% to achieve the VPD target. For each given VPD and temperature target, we input the humidity setpoint into the crop recipe. Once you have your targets planned out, it’s simply monitoring the climate data and adjusting the RH as necessary to meet those targets.”

Cannabis has experienced its share of challenges in recent years. But expanding legalization makes one thing clear: this industry isn’t going anywhere. And for commercial growers looking to cultivate success, data holds the key.