The vapor pressure deficit (VPD) is the difference between the moisture currently in the air and the maximum moisture it can hold when saturated. As we discussed in our previous blog post, VPD is an important parameter to measure and control because it directly influences plant transpiration.
In this blog post, we’ll explain how to determine the VPD of the grow room using temperature and humidity, the ideal VPD levels for cannabis and the equipment needed to monitor and adjust VPD.
Measuring VPD
VPD is calculated based on the grow room’s temperature and humidity. Temperature indicates the maximum moisture the grow-room air can hold—the warmer the air, the more it can hold—while relative humidity indicates how much moisture is already in the air.
With these two measurements, growers can determine the current VPD using a VPD chart (Figure 1). To read the chart, locate the temperature vertically (°C on the left, °F on the right), and find where it intersects with the grow room’s humidity across the top. The intersection shows the VPD in kilopascals (kPa).[1]
Figure 1. VPD table by inter-agriculture.org.
In this example, when humidity is low (30%) and temperature is high (95°F/35°C), the VPD reaches its peak at 3.90 kPa. In contrast, when the air is saturated (100% humidity), the VPD drops to 0 kPa, regardless of temperature.
VPD charts visually demonstrate how changes in temperature and humidity influence VPD, helping growers decide on adjustments to reach the ideal VPD for each growth stage.
Optimal VPD
The optimal VPD for cannabis varies by growth stage. For juvenile plants, a VPD of 0.8 kPa is ideal, while 1.3–1.4 kPa is optimal for vegetative growth and flowering.[2]
Cannabis thrives at 25–30°C. For juvenile plants, achieving a VPD of 0.8 kPa requires relative humidity around 75% at 25°C or 80% at 30°C. For later-stage plants, relative humidity should be between 55–60% at 25°C or 65–70% at 30°C to maintain a VPD of 1.3–1.4 kPa.
Aim to keep VPD between 0.4 kPa and 1.6 kPa. Levels below 0.4 kPa may hinder transpiration, while levels above 1.6 kPa can increase transpiration excessively.[3]
VPD management strategies
As discussed previously, VPD is affected by humidity, temperature and light intensity. For optimal plant health, these three factors must be managed according to the specific growth stage and environmental conditions. Balancing heat, light and moisture at the right “setpoint” is crucial and may require adjusting these factors in the grow room, as needed.
Raising the temperature, light and airflow while lowering humidity increases VPD. Conversely, reducing temperature, light and airflow while raising humidity decreases VPD.
Proper equipment is key to managing these factors. Use HVAC systems and fans to control temperature. Divide the grow room into temperature-controlled zones for precise management, especially when plants are at different growth stages.
In dry regions, ventilation windows can lower humidity by allowing drier outdoor air to replace humid indoor air. In humid regions, HVAC or dehumidification systems are typically necessary to lower humidity. Exhaust fans can also help expel warm, humid air. Effective air circulation distributes temperature and humidity evenly throughout the growing area, preventing localized pockets of high VPD near plants, particularly under intense lighting.
To raise humidity and lower VPD, use humidifiers or misting systems. Be careful when misting, as excessive humidity can encourage fungal outbreaks.
Good irrigation practices can also help manage VPD. For instance, when VPD is high, short and frequent watering can ensure a steady water supply that prevents plants from closing their stomata without overwhelming the roots.
Lastly, VPD sensors paired with automated systems enable HVAC, irrigation, misting and lighting to adjust automatically based on real-time VPD readings, ensuring optimal growing conditions around the clock.
Emerald Harvest Team
[1] VPD can also be measured in pounds per square inch (psi).
[2] Jin, Dan, Shengxi Jin, and Jie Chen. 2019. “Cannabis Indoor Growing Conditions, Management Practices, and Post-Harvest Treatment: A Review.” American Journal of Plant Sciences 10: 925-946. https://doi.org/10.4236/ajps.2019.106067.
[3] Bernard-Perron, David. 2018. “Humidity for a Healthy Grow.” Cannabis Business Times, May 2. https://www.cannabisbusinesstimes.com/home/article/15696276/humidity-for-a-healthy-grow.
