Plants do not respond to relative humidity. They respond to the difference between the water vapour pressure inside the leaf and the water vapour pressure of the air around it. That difference — vapour pressure deficit — is what drives water out of the leaf, and it is the variable that connects humidity to growth, nutrient transport and disease.
Relative humidity is a proxy for it. Often a poor one.
Why Relative Humidity Is the Wrong Variable
The air spaces inside a leaf are effectively saturated with water vapour at the leaf's temperature. Water moves out through the stomata whenever the vapour pressure outside is lower than the vapour pressure inside. The size of that gap sets the transpiration rate.
Relative humidity tells you how close air is to saturation at the air's own temperature. It says nothing about the leaf. Two greenhouses at 75 % RH can present entirely different conditions to their crops if their temperatures differ, and the same greenhouse at constant RH presents a changing condition to the crop as temperature moves through the day.
This is why growers who control on relative humidity alone find that the same setpoint produces different results in different seasons.
Calculating VPD Properly
The standard calculation, using air temperature only:
es(T) = 0.61094 × exp( 17.625 × T / (243.04 + T) ) [kPa]
VPD_air = es(T_air) − es(T_air) × RH/100
= es(T_air) × (1 − RH/100)The physiologically correct calculation uses leaf temperature for the leaf side:
VPD_leaf = es(T_leaf) − es(T_air) × RH/100The difference is not academic. Worked example, at conditions that occur routinely under lights:
| Input | Value |
|---|---|
| Air temperature | 24 °C |
| Relative humidity | 70 % |
| Leaf temperature | 22 °C (transpirational cooling) |
es(24 °C) = 2.978 kPa
e (air) = 2.978 × 0.70 = 2.085 kPa
es(22 °C) = 2.639 kPa
VPD_air = 2.978 − 2.085 = 0.89 kPa
VPD_leaf = 2.639 − 2.085 = 0.55 kPaAir VPD reports 0.89 kPa, comfortably inside the range usually quoted for vegetative growth. Leaf VPD reports 0.55 kPa, low enough that transpiration is being suppressed. Same greenhouse, same instant, two different conclusions — and the crop is experiencing the second one.
A 2 K difference between leaf and air temperature changes VPD by roughly 40 % at these conditions. Any VPD strategy that does not account for leaf temperature is working with a number the plant does not see.
Why Leaf Temperature Is Never Air Temperature
Leaves depart from air temperature in both directions, for different reasons at different times.
By day, leaves are usually cooler than air. Transpiration removes latent heat from the leaf surface. Under strong transpiration a leaf can sit 1–3 K below air temperature. The effect is self-limiting: if VPD rises and stomata close, transpirational cooling stops and leaf temperature rises — sometimes above air temperature under high radiation, because the leaf is absorbing light energy it can no longer shed by evaporation.
By night, leaves are usually cooler than air for a different reason. With no light input, the canopy exchanges long-wave radiation with the glazing and, through it, the sky. On a clear night the effective sky temperature is far below air temperature, and leaf surfaces settle below the air around them. This is the mechanism that produces leaf wetness before the air humidity alarm registers anything, described in greenhouse dehumidification and in the general treatment of surface condensation in condensation risk and dew point.
Under high-intensity supplemental lighting, leaves can be warmer than air. Radiant load from the fixtures is absorbed by the canopy. Facilities that moved from HPS to LED often find their leaf-to-air relationship changed, because the radiant and convective balance changed even where measured air temperature did not.
The practical consequence: leaf temperature has to be measured, not assumed. An infrared sensor aimed at the canopy is inexpensive relative to the decisions it informs.
What Goes Wrong When VPD Is Too Low
Persistently low VPD — humid air, or air close to leaf temperature — suppresses transpiration. Four consequences follow, in roughly the order growers encounter them.
Calcium transport fails first. Calcium moves through the plant in the xylem, carried by the transpiration stream, and it is effectively immobile in the phloem. Tissues that transpire little — young expanding leaves, enclosed heads, developing fruit — depend on the bulk transpiration flow to receive it. When transpiration is suppressed, those tissues are starved of calcium even though the root zone supply is adequate.
The visible results are familiar: tipburn in lettuce and other leafy crops, blossom end rot in tomato and pepper. Both are frequently treated as nutrition problems and addressed by increasing calcium in the feed, which does not work, because the limitation is transport rather than supply. Where these disorders track periods of low VPD — typically nights and dull humid days — the corrective action is environmental, not nutritional.
Guttation appears. When VPD approaches zero and root pressure continues to push water into the plant, water is exuded from hydathodes at leaf margins as visible droplets. Those droplets carry dissolved salts, leave residue as they evaporate, and constitute free water on the leaf surface.
Leaf wetness creates disease conditions. Free water on leaves — from guttation, from condensation, or from drip off cold glazing — is the precondition for Botrytis and other foliar pathogens. Duration matters more than occurrence: a surface that stays wet for hours presents a very different risk from one that wets briefly and dries.
Growth slows. With reduced transpiration comes reduced mass flow of water and dissolved nutrients from root to shoot. The plant is not water-stressed; it is under-supplied.
What Goes Wrong When VPD Is Too High
The opposite condition has a different failure path.
As VPD rises, transpiration rises with it — up to the point at which the plant begins closing stomata to conserve water. Stomatal closure protects the plant from desiccation, but stomata are the same pathway through which CO₂ enters. Closing them therefore reduces CO₂ uptake and photosynthesis at the same time.
This is the central trade-off of humidity management in controlled environments: the aperture that lets water out is the aperture that lets carbon in. A facility investing in CO₂ enrichment while running a VPD high enough to close stomata is paying for CO₂ the crop cannot take up. The coupling is developed in CO₂ uptake in controlled environments.
Sustained high VPD also produces the more familiar symptoms — wilting, leaf curl, reduced expansion, and in fruiting crops abortion of flowers or young fruit.
Target Ranges, and Why They Are Not Universal
Ranges commonly cited in controlled-environment practice fall around 0.8–1.0 kPa for vegetative growth and 1.2–1.5 kPa for generative or flowering stages, with some operations running late-flower higher to reduce disease pressure.
These figures should be treated as orientation rather than specification, for several reasons:
- Much of the widely circulated guidance derives from a narrow set of crops, and does not transfer directly to others.
- Optimal VPD varies with cultivar, growth stage, light intensity, CO₂ concentration and root-zone conditions. Under high light and enriched CO₂ a crop can support a higher VPD than the same crop under low light.
- Published ranges are usually stated without specifying whether they refer to air VPD or leaf VPD — and as shown above, the two can differ by 40 %. A target of 0.9 kPa means different things under the two definitions.
- The correct target for a facility is the one that its own crop records support, established over production cycles.
When adopting a published range, establish which definition it uses before applying it.
Night Is a Separate Control Problem
Day and night are physiologically different, and a single VPD target across both is rarely correct.
At night there is no light, no photosynthesis and no transpirational cooling, but there is radiative cooling of the canopy. Transpiration continues at a reduced rate — and this residual night transpiration is what continues to carry calcium to the tissues that need it. A night with essentially zero VPD suspends that transport for eight to twelve hours at a stretch.
At the same time, the canopy is at its coldest relative to the air, so leaf wetness risk is at its highest. The two requirements — maintain some transpiration, avoid free water — are both served by holding a modest VPD through the night rather than allowing humidity to rise unchecked.
Ventilation is least effective at exactly this time, because outdoor air is at its most humid in absolute terms in many climates and the heating penalty is highest. The conditions under which ventilation can and cannot deliver are set out in ventilation vs. dehumidification.
Measuring What the Crop Experiences
The gap between a facility's readings and the crop's conditions is usually a measurement placement problem.
Measure at canopy height. Sensors mounted at a convenient height in the aisle report a different microclimate from the one the crop occupies. Stratification of several kelvin between canopy and ridge is common.
Measure inside the canopy, not above it. Dense canopies have their own boundary layer, with higher humidity and lower air movement than the space above them. That microclimate is where transpiration and disease occur.
Measure leaf temperature. An infrared sensor aimed at the canopy converts VPD from an estimate into a measurement. Where budget allows only one addition to a climate monitoring set, this is usually the one with the highest return.
Measure in more than one place. A single sensor set in a large or compartmented house reports one location. Problem areas are usually the ones that are not instrumented.
Log leaf wetness duration if disease is the concern. Hours of free water is a better predictor than any humidity statistic.
From Crop Targets to Equipment Requirements
Once VPD targets are established by stage and period, they translate into an equipment specification through three steps.
The moisture load comes from the crop, and can be estimated directly from irrigation minus drain over a representative period. The method is set out in greenhouse dehumidification.
The operating condition is where the equipment has to deliver. For night-time VPD control that is a cool, humid condition — considerably less favourable than typical catalogue rating conditions, so capacity has to be read at the project condition rather than the rating condition. Why the two differ is explained in what a daily water removal rating means.
The control interface determines whether the equipment can serve a VPD strategy at all. Controlling to a VPD target requires the equipment to be sequenced by the climate computer against temperature and humidity together, not run on an independent humidistat.
Information to Prepare for a Project Discussion
- Crop, cultivar, production system, planting density and production calendar.
- VPD or humidity targets by growth stage and by day/night period, and whether targets refer to air VPD or leaf VPD.
- Whether leaf temperature is currently measured, and by what method.
- Logged climate data at canopy level covering a full production cycle.
- Irrigation and drain volumes over a representative period.
- Lighting type, installed intensity and daily schedule.
- CO₂ enrichment strategy and target concentration.
- Observed physiological disorders — tipburn, blossom end rot, guttation, foliar disease — with their timing and location in the facility.
- Existing climate equipment and the control interface available.
Discussing a Project
Yakeclimate designs and manufactures industrial dehumidification equipment for complex climate applications, with a focus on agriculture and energy projects.
We co-develop application-specific dehumidification equipment around the operating conditions, interfaces, and integration requirements of the wider project or system.
Crop strategy and VPD targets remain with the grower and the agronomy team. Our scope is the dehumidification equipment that allows those targets to be held.
Explore agriculture humidity control, review industrial ceiling dehumidifiers, or contact Yakeclimate to review the operating conditions for your facility.
FAQ
Frequently Asked Questions
What is VPD and how is it calculated?
Vapour pressure deficit is the difference between the vapour pressure inside the leaf, which is effectively saturated at leaf temperature, and the vapour pressure of the surrounding air. Using es(T) = 0.61094 × exp(17.625 × T / (243.04 + T)) in kPa, air VPD is es(T_air) × (1 − RH/100). The physiologically correct form uses leaf temperature on the leaf side: VPD_leaf = es(T_leaf) − es(T_air) × RH/100.
Does leaf temperature really change the answer?
Substantially. At 24 °C air, 70 % RH and a leaf at 22 °C, air VPD is 0.89 kPa but leaf VPD is 0.55 kPa — a difference of about 40 %. The first figure sits comfortably in the range usually quoted for vegetative growth; the second indicates suppressed transpiration. Leaves are typically 1–3 K below air temperature under active transpiration, and cooler still at night due to radiative exchange, so the difference is present most of the time.
Why does my lettuce get tipburn when the calcium feed is correct?
Tipburn is usually a calcium transport problem rather than a calcium supply problem. Calcium moves in the xylem with the transpiration stream and is effectively immobile in the phloem, so low-transpiring tissues such as young enclosed leaves depend on bulk transpiration to receive it. Prolonged low VPD suppresses transpiration and starves those tissues regardless of root-zone concentration. If the disorder tracks humid nights or dull humid days, the effective correction is environmental.
What VPD should I target?
Ranges commonly cited in controlled-environment practice are around 0.8–1.0 kPa for vegetative growth and 1.2–1.5 kPa for generative stages, but these should be treated as orientation only. Optimum varies with crop, cultivar, growth stage, light intensity and CO₂ concentration, and published figures often do not state whether they refer to air VPD or leaf VPD. Establish which definition a published range uses before applying it, and refine against your own crop records.
Can VPD be too high as well as too low?
Yes, and the failure mode is different. High VPD drives the plant to close stomata to conserve water. Because stomata are also the pathway for CO₂, closing them reduces photosynthesis at the same time — which is particularly costly in a CO₂-enriched facility. Sustained high VPD also produces wilting, leaf curl and, in fruiting crops, flower or fruit abortion.
Should day and night VPD targets be the same?
Usually not. At night there is no photosynthesis and no transpirational cooling, but residual transpiration continues to carry calcium to tissues that need it, and the canopy is at its coldest relative to the air, so leaf wetness risk is highest. Holding a modest VPD through the night serves both requirements, whereas allowing humidity to rise unchecked suspends calcium transport for the whole dark period and creates conditions for foliar disease.
