What is VPD?

Vapor Pressure Deficit (VPD) measures the difference between the vapor pressure inside a plant and the surrounding air. It plays a crucial role in plant transpiration, impacting overall plant health. Transpiration facilitates the movement of nutrients from the roots to the upper parts of the plant. This water flow occurs as plants release water vapor into the air—similar to sweating—and is measured in pounds per square inch (psi) or kilopascals (kPa). When VPD exceeds 1.0 kPa, the air still has a significant potential to hold more water.

The Importance of VPD

Monitoring VPD helps growers detect plant stress and establish the ideal balance of temperature and humidity for optimal growth. It also provides valuable insights into CO2 absorption and nutrient efficiency, especially when monitored with high-quality sensors.

If VPD is too low, plants struggle to release or evaporate water from their leaves, preventing essential nutrients from moving from roots to growing cells. On the other hand, if VPD is too high, excessive evaporation can cause stomata to close, halting photosynthesis and potentially damaging the plant.

Factors That Influence VPD

Several factors influence VPD, including relative humidity (RH), temperature, and lighting. While many academic studies overlook lighting as a factor—often conducted in greenhouse environments—indoor grow rooms must account for the heat generated by artificial lighting when setting environmental controls.

Relative Humidity

Relative humidity refers to the current amount of water vapor in the air compared to the air’s total water-holding capacity. This capacity increases with temperature—hotter air holds more water than cooler air. For instance, the air’s ability to hold water nearly doubles for every 20-degree temperature increase. This means air at 80°F can hold twice as much water as air at 60°F.

Temperature

Plants, much like cold-blooded animals, rely on ambient temperature to regulate their metabolism and photosynthesis rates. At lower temperatures, photosynthesis slows down, regardless of available light. A 37°F increase in temperature can boost VPD by as much as 45%.

Striking the Ideal Balance

Achieving a balance between relative humidity and air temperature creates an ideal environment—often referred to as the “Goldilocks zone.” It’s not too hot, not too cold, and not too humid. While every plant species has unique requirements, most indoor cannabis growers use a proven formula as a starting point and then fine-tune the conditions to create the perfect environment for their plants.

 

Optimal VPD Levels for Indoor Cannabis Cultivation

During the Vegetative Stage:

 

• Temperature: 70-85°F
• Relative Humidity: 55-70%
• VPD: 0.84 - 1.06

Flowering Stage:

• Temperature: 65-80°F
• Relative Humidity: 40-50%
• VPD: 1.00 - 1.39