VPD Explained: The Complete Guide to Temperature, Humidity & Plant Health

Creating the right indoor growing environment is about more than simply keeping temperature and humidity within acceptable ranges. Plants experience these conditions together, and the relationship between them can have a major influence on how efficiently a plant loses water, absorbs nutrients and carries out photosynthesis.

This relationship is known as Vapour Pressure Deficit (VPD).

VPD has become an increasingly important measurement in indoor growing, hydroponics and controlled-environment horticulture because it gives growers a more useful picture of the drying demand surrounding a plant than temperature or relative humidity alone.

Understanding VPD can help you make better decisions about temperature, humidity, ventilation, irrigation and overall climate control.

In this complete Holland Horticulture guide, we explain what VPD is, how VPD affects plants, how temperature and humidity influence it, why leaf temperature matters, how to read a VPD chart and how to control VPD in an indoor growing environment.

What Is VPD?

VPD stands for Vapour Pressure Deficit. Put simply, VPD describes the difference between the amount of moisture the air could hold when saturated and the amount of moisture that is actually present.

VPD is normally expressed in kilopascals (kPa).

Rather than looking at temperature or relative humidity independently, VPD helps describe how strongly the surrounding atmosphere is encouraging moisture to move from the plant into the air.

This matters because plants continually lose water vapour through tiny openings on their leaves called stomata. This process is known as transpiration.

As a simple rule:

• Low VPD means the air has a relatively low drying demand.
• High VPD means the air has a stronger drying demand.

The aim is not simply to achieve the highest or lowest possible VPD. The goal is to create a suitable and stable environment for the plant, its species and its stage of development.

Why Is VPD Important For Indoor Plants?

VPD matters because it is closely connected to plant transpiration.

Water absorbed through the root system moves through the plant before some of it eventually leaves through the stomata as water vapour. This transpiration stream is involved in several important plant processes.

A suitable growing environment can support:

• Healthy transpiration
• Water movement through the plant
• Transport of mineral nutrients
• Gas exchange
• Leaf cooling
• Photosynthesis
• Consistent plant development

VPD therefore provides growers with useful information about the relationship between the plant and the atmosphere surrounding it.

VPD, Transpiration And Photosynthesis Explained

To understand why growers monitor VPD, it helps to understand what happens at the leaf.

Plants contain microscopic pores called stomata. These pores regulate gas exchange between the plant and the surrounding atmosphere.

When stomata are open, carbon dioxide can enter the leaf for photosynthesis while water vapour can leave through transpiration.

The surrounding environment influences this process.

If atmospheric demand becomes excessively high, plants may respond by partially closing their stomata to reduce water loss. This can conserve water, but it can also reduce carbon dioxide uptake and therefore restrict photosynthesis.

At the opposite extreme, very low atmospheric demand can suppress transpiration and reduce the movement of some nutrients through the plant.

This is why VPD should be viewed as a balance rather than a number to maximise.

What Is The Difference Between VPD And Relative Humidity?

Relative humidity and VPD are closely related, but they are not the same measurement.

Relative humidity (RH) describes how much moisture is currently in the air relative to the maximum amount that the air could hold at that temperature.

VPD describes the difference between saturation vapour pressure and actual vapour pressure.

The important difference is that the moisture-holding capacity of air changes with temperature.

This means that the same relative humidity can represent different VPD conditions at different temperatures.

For example, a grow room at 20°C and 60% RH does not create the same atmospheric demand as a grow room at 28°C and 60% RH.

This is one of the main reasons VPD can provide a more useful environmental measurement than looking at humidity alone.

How Temperature Affects VPD

Temperature has a major effect on VPD because warmer air has a higher saturation vapour pressure.

If temperature rises while the amount of moisture in the air remains similar, VPD will generally increase.

In practical terms:

Higher temperature + unchanged humidity = generally higher VPD.

This can increase the atmospheric demand placed on the plant.

Temperature management should therefore never be considered completely separately from humidity management.

How Humidity Affects VPD

Relative humidity also has a direct relationship with VPD.

At a given temperature, increasing relative humidity generally lowers VPD because the surrounding air is closer to saturation.

Reducing relative humidity generally increases VPD because the air is further from saturation.

In simple terms:

Higher humidity = generally lower VPD.

Lower humidity = generally higher VPD.

This relationship explains why simply chasing a single humidity target without considering temperature can be misleading.

Why Leaf Temperature Matters When Calculating VPD

One of the most important aspects of VPD is also one of the most frequently overlooked: leaf temperature.

The temperature displayed on a grow room thermometer measures the surrounding air. However, the temperature of the plant's leaves can be different.

Lighting intensity, air movement, transpiration, room temperature and other environmental factors can all influence leaf temperature.

Because transpiration occurs at the leaf surface, using leaf temperature can provide a more accurate representation of the vapour pressure difference the plant is actually experiencing.

This is sometimes referred to as leaf VPD.

How To Measure Leaf Temperature

A practical way to measure leaf temperature is with a non-contact infrared thermometer or a suitable environmental sensor.

Take readings from several healthy leaves around the main canopy rather than relying on a single leaf.

Comparing average leaf temperature with air temperature gives you a better understanding of the environment your plants are experiencing.

How Is VPD Calculated?

VPD calculations use temperature and humidity to determine the difference between saturation vapour pressure and actual vapour pressure.

At its simplest:

VPD = Saturation Vapour Pressure − Actual Vapour Pressure

For more accurate plant-level calculations, leaf temperature can also be taken into account.

In everyday indoor growing, however, growers do not normally need to calculate every reading manually. VPD charts, environmental controllers and digital sensors can calculate or display VPD automatically.

 

How To Read A VPD Chart

A VPD chart allows growers to compare temperature and relative humidity to estimate the resulting vapour pressure deficit.

Typically, one axis displays temperature while the other displays relative humidity. Where the two values meet, the chart provides an estimated VPD value in kPa.

For example, if your growing environment is:

• 25°C air temperature
• 65% relative humidity

You can locate 25°C on the chart and follow it across to 65% RH to find the corresponding VPD.

However, remember that a basic air-temperature VPD chart does not necessarily account for differences between air temperature and actual leaf temperature.

What Is A Good VPD For Plants?

There is no single perfect VPD value for every plant.

Suitable VPD depends on several factors, including:

• Plant species
• Stage of development
• Light intensity
• Leaf temperature
• Root-zone conditions
• Irrigation strategy
• Growing medium
• Air movement

This is important because VPD charts found online are often presented as universal rules. In reality, they are better used as guides rather than absolute targets.

General VPD Ranges Through The Plant Lifecycle

The following ranges can be used as broad starting points for many commonly grown indoor plants. Individual species may prefer different conditions.

Growth Stage	General VPD Range	Environmental Approach
Seedlings & Cuttings	Approx. 0.4–0.8 kPa	Gentle atmospheric demand
Established Vegetative Growth	Approx. 0.8–1.2 kPa	Moderate transpiration
Flowering & Fruiting	Approx. 1.0–1.5 kPa	Stronger atmospheric demand

Important: These figures are general horticultural reference ranges rather than universal targets. Always consider the requirements of the plant species you are growing and observe how the plant responds.

VPD For Seedlings And Cuttings

Young seedlings and newly rooted cuttings have relatively small and developing root systems.

A very high VPD can create excessive atmospheric demand before the root system is capable of replacing water efficiently.

This is why propagation environments are commonly maintained at relatively high humidity with stable temperatures and gentle airflow.

As plants establish stronger roots and produce more foliage, environmental conditions can gradually be adjusted.

For more information about creating a suitable early growing environment, read our complete plant propagation guide.

VPD During Vegetative Growth

Once plants are established and growing actively, they can generally tolerate and benefit from greater transpiration than young seedlings.

During vegetative growth, maintaining balanced temperature, humidity and airflow helps support:

• Active photosynthesis
• Healthy leaf development
• Water and nutrient movement
• Strong structural growth

This is also when the relationship between irrigation and atmospheric demand becomes increasingly important.

Learn more in our vegetative growth guide.

VPD During Flowering And Fruiting

As plants mature, environmental management often changes again.

Larger plants release significant amounts of moisture into the growing environment through transpiration, while dense foliage can create pockets of higher humidity.

Maintaining good airflow and controlling humidity becomes particularly important during flowering and fruit production.

Growers should avoid simply reducing humidity as far as possible. Temperature, leaf temperature and plant response should still be considered together.

What Happens When VPD Is Too Low?

A low VPD means the surrounding air has relatively little capacity to accept additional water vapour from the plant.

If VPD becomes excessively low, transpiration may slow considerably.

Potential consequences can include:

• Reduced transpiration
• Slower movement of some nutrients
• Persistent moisture around foliage
• Condensation when surfaces reach dew point
• Conditions that may favour some fungal diseases

Low VPD should not automatically be interpreted as high humidity alone. Temperature must also be considered.

What Happens When VPD Is Too High?

A high VPD means the atmosphere has a strong drying demand.

Under excessively high VPD conditions, plants may lose water faster than their roots can replace it.

Plants can respond by partially closing their stomata to conserve water.

Potential signs or consequences of excessive atmospheric demand include:

• Rapid water loss
• Wilting
• Leaf stress
• Increased irrigation demand
• Reduced stomatal opening
• Reduced carbon dioxide uptake
• Slower photosynthesis and growth under prolonged stress

The solution is not necessarily simply adding more water to the root zone. The growing environment itself may need adjusting.

How To Lower VPD In A Grow Room

If VPD is consistently too high for the plants being grown, growers can usually reduce it by adjusting temperature, humidity or both.

Methods may include:

• Increasing relative humidity with a humidifier
• Reducing excessive room temperature
• Adjusting extraction rates where appropriate
• Reducing excessive heat from lighting
• Improving environmental control
• Checking whether airflow is excessively strong directly across plants

Make changes gradually and monitor the response rather than making large environmental adjustments at once.

How To Increase VPD In A Grow Room

If VPD is consistently too low, it can normally be increased by lowering humidity, increasing temperature within a suitable range or improving moisture removal.

Methods may include:

• Increasing extraction
• Using a dehumidifier
• Improving internal air circulation
• Managing plant density
• Reducing unnecessary sources of moisture
• Adjusting temperature where appropriate

Our grow room ventilation guide explains how extraction, intake and air circulation work together to control the indoor environment.

How Ventilation Affects VPD

Ventilation and VPD are closely connected because ventilation changes both temperature and humidity.

Extracting warm, humid air and replacing it with fresh air can significantly alter the growing environment.

However, the effect depends on the conditions of the incoming air.

For example, bringing cool, dry winter air into a grow room may have a very different effect from bringing warm, humid summer air into the same space.

This is why modern environmental management increasingly relies on sensors and variable-speed fans rather than simply running extraction equipment at full power continuously.

Why Air Circulation Matters For VPD

The conditions measured by a sensor elsewhere in the room may not perfectly represent the microclimate surrounding the leaves.

Plants can create a thin layer of relatively still, humid air around the leaf surface known as the boundary layer.

Appropriate air movement helps disturb this boundary layer and creates more consistent conditions across the canopy.

This is one reason circulation fans are important even when the room already has an extraction system.

Air movement should be distributed throughout the canopy rather than blasting individual plants continuously with a strong fan.

VPD And Grow Room Humidity

Humidity control becomes increasingly challenging as plants grow larger.

A room containing a large canopy can release considerable moisture through transpiration.

Depending on the environment, humidity can be managed using:

• Extraction fans
• Fresh air intake
• Circulation fans
• Dehumidifiers
• Humidifiers
• Heating and cooling equipment
• Environmental controllers

The goal should be to maintain a stable environment rather than allowing large swings between excessively dry and excessively humid conditions.

VPD And Irrigation: Why The Two Are Connected

VPD does not only affect the leaves. Changes in transpiration influence how quickly plants use water from the root zone.

When atmospheric demand increases, water consumption may increase.

When atmospheric demand decreases, plants may use water more slowly.

This means irrigation schedules should not always remain identical when environmental conditions change.

Growers using automated irrigation systems should pay attention to both the root zone and the aerial environment rather than treating them as completely separate systems.

For more information about irrigation fundamentals, read our complete guide to watering indoor plants properly.

VPD And Nutrient Uptake

Water movement through a plant is connected to the transport of mineral nutrients.

Because VPD influences transpiration, extreme environmental conditions can indirectly influence nutrient movement and plant performance.

However, an apparent nutrient problem should never automatically be blamed on VPD.

Growers should also check:

• Root-zone pH
• Nutrient concentration
• Root health
• Irrigation frequency
• Growing-medium moisture
• Root-zone temperature

Environmental and root-zone conditions need to work together.

VPD During UK Summer Conditions

UK indoor growing environments can change significantly during warm summer weather.

Higher outside temperatures can increase grow room temperatures, while humidity conditions may vary considerably from day to day.

During hot periods, growers should monitor:

• Air temperature
• Relative humidity
• Leaf temperature
• VPD
• Water consumption
• Extraction performance

Simply increasing extraction may not always solve the problem if the incoming air is already hot.

For a more detailed summer strategy, read our UK heatwave growing guide.

VPD During Winter

Winter creates a different environmental challenge.

Cold outside air can contain relatively little moisture. Once that air enters an indoor growing environment and is heated, relative humidity can fall significantly.

This can result in unexpectedly high VPD even though room temperature appears perfectly acceptable.

Growers may therefore need to balance:

• Heating
• Extraction
• Humidity
• Fresh air exchange

This is another example of why temperature or humidity should not always be managed independently.

How To Measure VPD Accurately

Accurate environmental data is essential if you intend to use VPD for climate management.

At minimum, you need reliable measurements of:

• Air temperature
• Relative humidity

For a more plant-focused measurement, also monitor:

• Leaf temperature

Sensor positioning matters. Avoid placing environmental sensors:

• Directly underneath intense lighting
• Directly in front of a circulation fan
• Immediately beside a humidifier
• Against the wall of a grow tent
• Far above or below the plant canopy

Where possible, position the main temperature and humidity sensor around canopy level in a representative part of the growing area.

Common VPD Mistakes

Chasing A Perfect Number

A VPD chart is a guide, not a guarantee of plant performance.

Plant genetics, lighting, root health, irrigation and nutrition still matter.

Ignoring Leaf Temperature

Using air temperature alone can provide a useful estimate, but leaf temperature can improve the accuracy of plant-level VPD calculations.

Changing Conditions Too Quickly

Large changes to temperature or humidity can create more environmental stress than the original VPD reading.

Make adjustments gradually.

Ignoring Night-Time Conditions

Turning the lights off changes temperature and often relative humidity.

Growers should monitor the environment across the complete day/night cycle rather than checking VPD only while lighting is operating.

Relying On One Sensor Reading

Large grow rooms can contain different microclimates.

Temperature, humidity and airflow may vary across the canopy, particularly in densely planted environments.

Treating VPD As More Important Than Plant Health

VPD is an environmental management tool, not a replacement for observing your plants.

Healthy growth, root condition, irrigation behaviour and plant appearance should always be considered alongside sensor readings.

Smart VPD Control And Grow Room Automation

Modern environmental controllers can make VPD management considerably easier.

Instead of controlling individual pieces of equipment independently, smart systems can use temperature and humidity data to coordinate equipment such as:

• EC extraction fans
• Circulation fans
• Humidifiers
• Dehumidifiers
• Heaters
• Cooling equipment

Some modern controllers can also calculate VPD automatically and adjust connected equipment according to environmental targets.

This represents a major shift in indoor growing: moving from manually controlling individual devices towards managing the complete plant environment as one connected system.

Is VPD More Important Than Temperature Or Humidity?

VPD does not replace temperature or humidity measurements.

Instead, it combines their relationship into a measurement that can provide additional information about atmospheric demand and plant transpiration.

A good environmental strategy therefore monitors:

• Temperature
• Relative humidity
• VPD
• Leaf temperature where possible
• Air movement
• Root-zone conditions

Think of VPD as another layer of environmental information rather than a replacement for everything else.

Frequently Asked Questions About VPD

What does VPD stand for?

VPD stands for Vapour Pressure Deficit. It describes the difference between the moisture pressure at saturation and the actual moisture pressure of the surrounding air.

What does VPD mean for plants?

For plants, VPD provides an indication of the atmospheric demand driving water vapour from the leaf into the surrounding air. It is therefore closely connected to transpiration, stomatal behaviour and water use.

Is high VPD good or bad?

Neither high nor low VPD is automatically good or bad. Excessively high VPD can create too much atmospheric demand and encourage plants to conserve water by closing stomata, while excessively low VPD can suppress transpiration. The appropriate range depends on the plant and growing conditions.

Does increasing humidity lower VPD?

Yes. At the same temperature, increasing relative humidity generally lowers VPD because the air moves closer to saturation.

Does increasing temperature increase VPD?

Generally, yes, if humidity conditions remain otherwise comparable. As temperature rises, saturation vapour pressure increases, which can increase VPD.

Do I need a VPD controller?

No. Plants can be grown successfully without a dedicated VPD controller. However, controllers that monitor temperature, humidity and VPD can make maintaining a stable indoor environment easier, particularly in more advanced growing setups.

Where should a VPD sensor be positioned?

Temperature and humidity sensors should generally be positioned around plant canopy level in a representative location, away from direct airflow, intense radiant heat and humidifier outlets.

Is leaf temperature important for VPD?

Yes. Leaf temperature can differ from room air temperature, and because transpiration occurs at the leaf surface, accounting for leaf temperature can provide a more accurate plant-level VPD measurement.

Final Thoughts: Understanding VPD Is About Understanding Your Plants

VPD can initially appear complicated, but the principle behind it is straightforward.

Temperature determines how much water vapour the air can potentially hold. Humidity tells us how much moisture is already present. VPD describes the resulting atmospheric demand placed on the plant.

Understanding this relationship allows growers to move beyond simply chasing individual temperature and humidity numbers and start thinking about the growing environment as a complete system.

But VPD should never be treated as a magic number.

The best indoor growing environments combine appropriate temperature, humidity, airflow, lighting, irrigation, nutrition and root-zone management while responding to the needs of the plants themselves.

At Holland Horticulture, our aim is to help growers understand not just which equipment to use, but why environmental control matters and how each part of an indoor growing system works together.