Relative humidity tells us how much water vapour is in the air relative to saturation. It does not, by itself, tell us how strongly that air is pulling water from a plant. Vapour pressure deficit (VPD) combines temperature and humidity into a measure of atmospheric drying demand. That makes it a more useful lens for understanding transpiration, stomatal behaviour and crop water demand—but VPD should never be interpreted separately from the root zone, radiation, leaf temperature and the crop itself.
In a greenhouse, I do not want to know only what the climate computer says. I want to know whether the plant can actually meet the demand that climate is creating.
Why can relative humidity be misleading?
Relative humidity is temperature-dependent. The amount of water vapour needed to saturate air increases as temperature rises. This means that the same relative humidity at two different temperatures does not represent the same atmospheric demand on a leaf.
VPD describes the difference between the vapour pressure the air could hold at saturation and the vapour pressure it actually contains. In plant physiology, the relevant gradient is closely related to the difference between the moist interior of the leaf and the surrounding atmosphere.
That is why a greenhouse at 70% relative humidity can represent a very different environment for a crop at 20°C than at 30°C.
What happens to a plant when VPD rises?
As VPD rises, the vapour-pressure gradient between leaf and atmosphere becomes steeper. All else being equal, that increases the physical driving force for transpiration.
But plants are not passive evaporating surfaces. Stomata regulate gas exchange. Research across vascular plants shows that stomatal conductance commonly decreases as VPD rises, helping restrict water loss. The actual response varies among species, cultivars, environmental conditions and hydraulic traits.
This creates an important distinction: higher atmospheric demand does not automatically mean transpiration will continue increasing without limit. At sufficiently high demand, stomatal regulation and hydraulic limitations can restrict water loss—and at the same time restrict carbon dioxide entry and photosynthesis.
Does high VPD automatically mean drought stress?
No.
A well-watered crop with an effective root system may meet substantial atmospheric demand. Another crop under the same air conditions may struggle because its root-zone water supply, root distribution or hydraulic conductance cannot keep pace.
A useful way to think about crop water status is therefore as a balance between demand above ground and supply below ground. A 2023 review in the Journal of Experimental Botany describes this through the soil–plant hydraulic pathway: increasing VPD raises atmospheric demand, while the plant’s ability to maintain transpiration depends on hydraulic conductance from the root environment through the plant to the leaves.
That is why VPD is not a substitute for root-zone measurements.
Will more irrigation solve high VPD?
Not necessarily.
If the substrate is becoming too dry because water demand has increased, irrigation strategy may indeed need adjustment. But simply increasing irrigation volume without understanding the root zone can create a second problem.
Water has to reach active roots at the right frequency and distribution. Excess irrigation can increase drainage losses and, in poorly aerated systems, reduce oxygen availability. A wet substrate also does not guarantee that the plant can transport water rapidly enough to match atmospheric demand.
This is the same diagnostic principle explored in Why Drip Irrigation Can Still Waste Water: water leaving an emitter is not the same as water becoming usefully available to the crop.
Can VPD be too low?
Yes. Very low VPD means the air has little capacity to accept additional water vapour. Transpiration can become low, and the relationship between water movement, nutrient transport, leaf temperature and stomatal behaviour changes.
That does not mean low VPD is always harmful. It means that neither “high humidity” nor “low VPD” should automatically be treated as a target without considering crop, development stage, light, temperature and disease risk.
Is there one ideal VPD for every greenhouse crop?
No. A universal VPD set point would ignore crop physiology.
Species differ in stomatal sensitivity, leaf structure and hydraulic behaviour. Cultivars can differ as well. Young plants do not necessarily respond like a mature fruiting crop. Radiation, carbon dioxide concentration, air movement, root-zone moisture and temperature all alter the context in which a VPD value should be interpreted.
The better question is not “What is the perfect VPD?” but: “What atmospheric demand is this crop experiencing, and how is the plant responding to it under these conditions?”
What should I look at alongside VPD?
I would never diagnose a crop from VPD alone. I would look at air temperature and relative humidity, radiation, leaf temperature, irrigation timing, substrate or soil moisture, drainage, root-zone temperature, EC, crop stage and the behaviour of the canopy.
Then I would walk the crop.
Are leaves losing turgor at a particular time of day? Does stress begin in the brightest part of the greenhouse? Are younger and older leaves behaving differently? Is the root zone actually drying, or is the crop restricting transpiration despite adequate water? Does the pattern disappear when radiation falls?
Numbers become useful when they help explain the plant.
VPD is a conversation between air, leaf and root
VPD gives us a better description of atmospheric demand than relative humidity alone. But it is still only one part of the plant–environment system.
The atmosphere creates demand. Stomata regulate exchange. The hydraulic system moves water. Roots have to obtain that water from soil or substrate. Irrigation has to replenish what the crop can actually use.
When those parts remain in balance, a crop can function under substantial evaporative demand. When one becomes limiting, the symptoms may appear somewhere else entirely.
That is why I use VPD as a starting point for diagnosis—not as the diagnosis itself.
Key takeaways
- Relative humidity alone does not describe atmospheric water demand because it depends on temperature.
- VPD combines temperature and humidity into a more physiologically useful measure of atmospheric drying demand.
- Increasing VPD generally increases the physical driving force for transpiration, while plants can respond by reducing stomatal conductance.
- High VPD does not automatically mean drought stress; plant response depends on water supply and the soil–plant hydraulic pathway.
- More irrigation is not automatically the correct response to high VPD.
- There is no single ideal VPD for every crop, cultivar, growth stage and greenhouse condition.
- Interpret VPD together with radiation, leaf temperature, root-zone moisture, EC, root-zone temperature, irrigation and direct crop observation.
For the below-ground side of the same system, continue with The Heat You Cannot See: Why Root-Zone Temperature Matters. For the wider plant-physiology framework, see Plants: Understanding Growth as a Complete System.
Further reading
McAdam, S.A.M. & Brodribb, T.J. (2015). The Evolution of Mechanisms Driving the Stomatal Response to Vapor Pressure Deficit, Plant Physiology. This review examines stomatal responses to VPD across vascular plants.
Koehler, T. et al. (2023). Transpiration response to soil drying versus increasing vapor pressure deficit in crops: physical and physiological mechanisms and key plant traits, Journal of Experimental Botany. Particularly useful for understanding the interaction between atmospheric demand and soil–plant hydraulics.
Grossiord, C. et al. (2021). Systemic effects of rising atmospheric vapor pressure deficit on plant physiology and productivity. A quantitative review covering 112 species and multiple physiological responses to elevated VPD.