Vapor Pressure Deficit: The Master Control for Transpiration

Vapor pressure deficit is the difference between the amount of water vapor the air currently holds and the maximum it could hold at the same temperature. When that gap is large, the air is thirsty and pulls water out of the leaf through open stomata; when it is small, transpiration slows to a trickle.^[1]Supports: VPD as the transpiration-relevant target rather than relative humidity[1]◆ Tier 1 · PrimaryUniversity ExtensionMichigan State UniversityView source ↗✓ Verified 2026-04-11

Relative humidity on its own is a misleading target because it is temperature-dependent: warm air holds more moisture than cool air, so as temperature rises the relative humidity falls even though the amount of water in the air has not changed. That is why two rooms at the same relative humidity can have very different VPD. VPD folds temperature and humidity into one number that tracks what the plant actually experiences.^[2]Supports: relative humidity is temperature-dependent and therefore misleading on its own[2]◆ Tier 1 · PrimaryUniversity ExtensionUniversity of MassachusettsView source ↗✓ Verified 2026-04-11

Leaf temperature matters too. Transpiring leaves run cooler than the surrounding air, and because the coldest surfaces reach the dew point first, moisture condenses on leaves before it condenses anywhere else in the room. Meridian therefore reports the leaf-based deficit: saturation at leaf temperature minus the actual vapor pressure of the air.^[3]Supports: leaf temperature is distinct from air temperature and is measured separately[3]◆ Tier 1 · PrimaryUniversity ExtensionMichigan State UniversityView source ↗

Target VPD climbs as a plant matures and its root system and canopy can support faster transpiration. The ranges below are a widely-used starting point; tune them to how your specific plants respond. The humidity column is computed from the VPD column, not measured separately, so it holds only at the stated air temperature.

You need three readings: air temperature, relative humidity, and (ideally) leaf temperature. Enter them below for a worked example, or open the full calculator to run your own numbers. The offset is editable: set it to zero and you get air-based VPD instead.

Controller documentation covering the VPD leaf-temperature offset setting and its default.

Holding a VPD band means controlling humidity and temperature together, and that starts with measuring both where the plants actually are. Put the sensor at canopy height rather than at the floor or on a wall: the canopy is where transpiration happens and where the temperature and humidity that matter are found. A sensor that logs history is worth more than one that only shows the current reading, because the overnight low is usually where problems start.

Overnight is the risk window. As air cools toward the dew point, moisture condenses on the coldest surfaces first, and leaves are among them. That film of water is what fungal spores need to germinate, which is why persistently low VPD in a dense canopy leads to rot rather than merely slow growth.^[2]Supports: condensation forms on leaves as air cools to dew point and promotes fungal spore germination[2]◆ Tier 1 · PrimaryUniversity ExtensionUniversity of MassachusettsView source ↗✓ Verified 2026-04-11