What VPD actually measures
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 definition and its role in transpiration[1] Tier 1 · PrimaryUniversity ExtensionVapor Pressure Deficit and Greenhouse Climate ManagementUniversity Extension (placeholder)View source ↗ Verified 2026-05-12
Because saturation rises sharply with temperature, two rooms at the same relative humidity can have very different VPD. That is why humidity alone is a poor target — VPD folds temperature and humidity into one number that tracks what the plant actually experiences. Leaf temperature matters too: leaves typically sit a couple of degrees below air temperature, which shifts the true leaf-to-air deficit.[3]Supports: leaf temperature offset from air[3] Tier 2 · SecondaryUniversity ExtensionLeaf temperature, transpiration, and canopy energy balanceState Agricultural Extension (placeholder)View source ↗ Verified 2026-03-18
Reading the VPD chart
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.
| Stage | VPD range (kPa) | RH @ 24°C | Risk if too low |
|---|---|---|---|
| Seedling / cutting[1][1] Tier 1 · PrimaryUniversity ExtensionVapor Pressure Deficit and Greenhouse Climate ManagementUniversity Extension (placeholder)View source ↗ Verified 2026-05-12 | 0.4 – 0.8 | 70 – 85% | Damping-off, weak roots |
| Vegetative | 0.8 – 1.2 | 55 – 70% | Soft growth, fungal pressure |
| Early flower / fruit set[2][2] Tier 1 · PrimaryPeer-reviewed journalStomatal responses to vapour pressure deficit across speciesJournal of Experimental Botany (placeholder)View source ↗DOI: 10.0000/jxb.placeholder.2024 Verified 2026-04-30 | 1.2 – 1.4 | 50 – 60% | Poor calcium movement |
| Late flower / ripening | 1.4 – 1.6 | 45 – 55% | Botrytis, dense-canopy rot |
| Category | Min VPD (kPa) | Max VPD (kPa) |
|---|---|---|
| Seedling | 0.4 | 0.8 |
| Vegetative | 0.8 | 1.2 |
| Early flower | 1.2 | 1.4 |
| Late flower | 1.4 | 1.6 |
Chase the deficit, not the humidity number — VPD is the variable the leaf can feel.
Calculate VPD for your space
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.
VPD Calculator
Worked example shown. Open the calculator to run your own numbers.
Equipment that holds VPD steady
Holding a VPD band means controlling humidity and temperature together. A monitor that logs both — at canopy height, not at the floor — is the first investment.[3]Supports: measure at canopy height[3] Tier 2 · SecondaryUniversity ExtensionLeaf temperature, transpiration, and canopy energy balanceState Agricultural Extension (placeholder)View source ↗ Verified 2026-03-18
Generic Canopy Climate Monitor
A logging temperature/humidity sensor placed at canopy height gives you the inputs VPD depends on. Look for one that records history so you can see overnight swings, not just the current reading.
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The stomatal-closure response to rising VPD is the mechanism behind the upper end of each target band.
Common questions
Is VPD better than relative humidity as a target?
Do I need leaf temperature to use VPD?
What happens if VPD is too low for too long?
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