VPD in a Container Farm: Good Temperature Is Not Enough

Why temperature alone was misleading

Air temperature averaged 22.36 ± 2.24°C, which can appear acceptable for leafy-green operation. Relative humidity, however, averaged 78.18 ± 11.44%, pulling VPD downward for long periods.

VPD combines temperature and humidity into a measure of the atmospheric drying demand experienced by leaves. A temperature-only controller cannot see this interaction.

Observation window

The dataset covered 22–24 June 2026, totaling approximately 48.1 hours. Air VPD ranged from 0.166 to 1.411 kPa, with a mean of 0.593 kPa.

The 0.6–1.2 kPa band was used as an operational evaluation range, not as a universal physiological optimum for every cultivar and growth stage.

Nighttime humidity was the dominant limitation

During the dark period, VPD compliance was 0%. Cooling without coordinated dehumidification can reduce temperature while leaving relative humidity too high.

This matters because a controller may satisfy a thermal setpoint and still produce a weak vapor-pressure gradient for transpiration.

Control implications

Temperature and humidity should be controlled jointly. Lighting state must be included because it changes heat load and the appropriate interpretation of VPD. Evaluation should also distinguish day and night rather than reporting only one aggregate percentage.

Energy must remain visible

Cooling energy during the observation was approximately 9.73 kWh. Any improvement in VPD compliance should therefore be evaluated alongside energy use, actuator cycling, and spatial uniformity.

Conclusion

A thermally acceptable container is not automatically a suitable crop environment. In this observation, humidity—especially at night—was the limiting variable. The next control design must coordinate temperature, humidity, lighting state, and energy.