Vapor Pressure Deficit Made Simple: 3-Minute Crash Course

Front Row Ag Technical Specialist Tyler Simmons explains Vapor Pressure Deficit (VPD) in simple terms and why it's one of the most important environmental metrics in controlled environment agriculture. In this video, he covers:

• What Vapor Pressure Deficit (VPD) is and why it matters
• How VPD relates to relative humidity and temperature
• The "Goldilocks zone" for water and nutrient uptake
• How air moisture creates the suction that moves water through plants
• How stomata respond to environmental stress
• Why balancing humidity, temperature, and airflow improves consistency
• The role of VPD in optimizing plant performance

Transcript:

VPD stands for Vapor Pressure Deficit. The word deficit simply means the difference between the amount of water inside a leaf and the amount of water vapor in the surrounding air.

Inside a leaf, water exists as a liquid.

Outside the leaf, water exists as vapor in the air.

As more water is added to the air, the air becomes more humid. The difference between the water inside the leaf and the water in the air gets smaller. At 100% humidity, the air is completely saturated—it's essentially raining because the air can't hold any more water.

On the other hand, when the air is very dry, there's a much larger difference between the water inside the leaf and the water vapor outside. You can think of it like placing something wet into a very dry environment.

That large difference creates a pulling force, almost like suction, that draws water out of the leaf.

That's really all VPD is:

The difference in vapor pressure between the inside of the leaf and the surrounding air.

When VPD is high, the difference is large. The air is relatively hot and dry compared to the inside of the leaf, so it pulls water out very aggressively.

When VPD is low, the difference is small. The air is cooler and more humid, so it pulls water from the leaf much more gently.

That pulling force doesn't just remove water from the leaf—it creates a chain reaction throughout the plant.

As water leaves the leaf, it pulls more water up the stem, which in turn pulls water and dissolved nutrients up from the roots.

This movement is essential because it's how plants transport both water and nutrients.

However, if the air becomes too dry and VPD gets too high, the plant begins losing water faster than it's comfortable with.

To protect itself, it starts closing the tiny pores on the leaf surface called stomata.

The problem is that those same stomata are also where the plant absorbs carbon dioxide (CO₂) for photosynthesis.

Once the stomata begin closing, the plant takes in less CO₂, photosynthesis slows, and growth begins to decline.

The opposite extreme isn't ideal either.

If the air becomes too humid, there's very little pulling force to move water through the plant.

Without enough transpiration, water and nutrients move much more slowly from the roots to the leaves.

The goal is to stay somewhere in the middle—a "sweet spot" where water and nutrients are moving efficiently without stressing the plant.

CO₂ also plays a role in this process.

As CO₂ concentrations increase, the stomata don't need to open as widely because there's plenty of carbon dioxide available. The plant can absorb everything it needs through smaller openings.

If CO₂ levels are low, the stomata open wider to capture as much CO₂ as possible. A side effect of those wider openings is that the plant also releases much more water vapor into the air.

That's why adding CO₂ generally reduces transpiration slightly.

However, there's a limit.

Beyond a certain concentration, adding more CO₂ provides very little additional benefit. Under hot, dry conditions, excessive CO₂ can actually increase plant stress because it encourages the stomata to stay even more closed when the plant already needs to cool itself and exchange gases.

Ultimately, VPD is about balance.

Too much drying force stresses the plant and causes it to shut down.

Too little drying force slows the movement of water and nutrients.

The healthiest plants grow when that balance stays in the optimal range, allowing them to transpire, absorb CO₂, and move water and nutrients efficiently.