This article explores how vapor pressure differentials (VPD) affect transpiration rates in cannabis plants. Transpiration is a process in which water and essential nutrients move through the plant from cell to cell. Understanding of VPD at different temperatures and relative humidities, and its effect on this process, is important to gain maximum plant growth. We also cover the impact of set points on growth environments as well as energy costs, along with an analysis on how data surrounding watering rates and transpiration can affect equipment selection.
We all learned about the water cycle in junior high—rain, evaporation, clouds, rain, and so on. This cycle takes on increased importance in the cannabis industry because maintaining the right space conditions for an indoor grow is essential to the success of a cultivation operation. Temperature and humidity play a large role in how cannabis plants will function, directly impacting both plant yield and overall quality. We must look beyond the simplified precipitation cycle and understand that space conditions directly affect a plant’s ability to sweat, or transpire.
A common misconception is that the transpiration of cannabis plants will affect the relative humidity within a grow room. In reality, that’s backwards—using the paradigm that the room conditions affect the plants’ ability to transpire rather than the plants’ transpiration affecting the room condition is a good perspective to have when reading this article. In an ideal setting, the room setpoint conditions (temperature and humidity) would be held perfectly stable and would never deviate regardless of what happens in the room. Keeping the room conditions perfectly stable is the job of the mechanical equipment used to control the environmental conditions of the room. If we can maintain any given set of room conditions, the question is: under what conditions does cannabis thrive?
As we will discuss, plant transpiration drives plant growth and vapor pressure differential (VPD) drives plant transpiration. Temperature and humidity both impact the vapor pressure differential, a factor that must be considered when making decisions about facility systems—especially when it comes to the tricky deliberations surrounding heating, ventilation, and air conditioning (HVAC) equipment selection. In order for plants to thrive in an indoor grow space, the VPD needs to be at a particular level, which can be different for every situation and every stage of growth. Since the temperature and humidity affect the VPD, the temperature and humidity both need to be at the correct levels—at the right “setpoint”—which means heat and moisture are going to need to be added or taken away from the grow room at different times.
Understanding the drivers behind this process is key to selecting the appropriate HVAC system for your operation. In our experience, a setpoint difference of just 10% can have a significant impact on HVAC system sizing, upfront cost, and ongoing energy costs. It’s worth exploring if a small difference in design setpoint will make a large impact on your HVAC system costs, without having much impact on the product yield of your operation.
Understanding VPD and Transpiration Rates
Temperature and humidity are defining factors for VPD, sometimes called vapor pressure deficit, which is what truly affects the health of a cannabis plant. Vapor pressure is the pressure at which liquid becomes a vapor. Here is a real-world example of vapor pressure in action: When you boil water on the stove, you heat the water, increasing the pressure to a point at which it reaches the vapor pressure of the atmosphere around it and becomes steam. In cannabis cultivation, VPD refers to the difference between vapor pressure within a plant and the vapor pressure of the air surrounding the plant. VPD is responsible for driving a process in the plant known as transpiration, which directly impacts plant health.
Transpiration is a process in which water and other essential nutrients move through a plant from cell to cell. It is also how plants regulate their own temperature and obtain the carbon dioxide they need out of the air. VPD drives transpiration and the nutrient uptake from the roots of a plant to the upper area of a plant. Water movement occurs as a result of plants releasing water vapor into the air through openings called stomata—almost as if they are sweating.
If VPD is too small, peak growth rates are not achieved, and problems like mold or root rot can become an issue. If VPD is too large, the plant stomata will close in an attempt to limit transpiration, which can result in issues like tip burn and leaf curl. VPD can be directly calculated from the temperature and relative humidity (RH) of both the plant and the grow room. Both of these concepts are explained in detail below. The surface temperature of the plant and the dry bulb temperature of the room are approximately the same, but since the plant has water forming, the plant surface will be at 100% RH when it is transpiring. For a given design temperature, we can modulate the VPD by changing the RH of the grow room.
A VPD range of 0.8–1.1 (kPa) is commonly known as ideal in the vegetative stage, while a VPD range of 1.0–1.5 (kPa) is commonly known as ideal in the flowering stage. Tables I and II show that the same ideal VPD range can be obtained at different temperatures and relative humidities.
To put it simply: Consistent temperature and relative humidity in the space --> consistent vapor pressure deficit --> plant transpiration --> plant growth.
This concept is further illustrated in Figure 1 on the psychrometric chart, which is a commonly used tool that graphically illustrates the relationship between air temperature and relative humidity as well as other properties.
What Are the Metrics?
As explained above, maintaining proper setpoints of indoor grow rooms is essential to the success of an operation, but what do we measure and how do we measure it? There are a few things to understand about measuring temperature and humidity and determining VPD:
- Wet and dry bulb temperature readings: Dry bulb temperature is the temperature reading most of us are familiar with; the temperature that is shown on the thermostat in a home. Wet bulb temperature is the temperature that a thermometer reads when its bulb is wrapped in a moist cloth. The wet bulb temperature indicates how much moisture is present in the air. When relative humidity is at 100%, the wet and dry bulb temperatures are equal. If the difference between the dry and wet bulb temperatures is small, there is a large amount of moisture in the air. There is so much moisture in the air that it is similar to having a wet rag around the thermometer bulb. If there is a large difference between the dry and wet bulb temperature readings, the air is dry.
- Relative humidity: Relative humidity (RH) is a measurement of the amount of moisture in the air expressed as a percentage of the maximum possible moisture in the air at a given dry bulb temperature. As humidity increases, the air of an indoor space will eventually reach a state of saturation. When air has reached its capacity for maximum possible moisture, water will leave the air in the form of clouds, dew, or condensation. In warmer temperatures, air is able to hold more moisture. If the amount of moisture in a space were to remain constant and the temperature increased, the RH would decrease. This is because the total amount of moisture present is the same, but it is possible for the air to take on more moisture, therefore the air is further from the maximum possible moisture, resulting in a lower percentage. Conversely, if the moisture content were to remain constant while the temperature decreased, the RH would increase because the moisture in the air is closer to the maximum possible moisture, resulting in a higher percentage.