Written by: Bartholomew Alen

Temperature and humidity are among the first climate parameters growers keep an eye on. Nevertheless, two rooms with the same relative humidity can have completely different effects on plants. The reason is simple: 60% humidity at 22°C produces a different evaporation power than 60% at 28°C. This is exactly where it will be VPD for cannabis interesting.

VPD stands for Vapor Pressure Deficit , in German vapor pressure deficit. In simple terms, the value describes how big the difference is between the maximum possible amount of water vapor and the actual moisture in the air. For the plant, this difference is an essential part of the driving force behind transpiration: the flow of water from the roots through the plant to the leaves.

A sensibly set VPD can help to control the growing climate in a more stable and understandable way. However, it is not a magical yield regulator. Genetics, root health, light, irrigation, nutrient supply, air movement and plant age remain just as important. This guide therefore not only shows target areas, but also how to correctly classify measured values, avoid typical mistakes and... VPD table for cannabis practically used.

What exactly does VPD mean in cannabis?

Warm air can hold more water vapor than cold air. The relative humidity shows to what extent the air is saturated with water vapor at the measured temperature. However, it alone does not say how much the atmosphere can absorb water from a leaf. The VPD closes this gap.

In plant physiology, VPD refers to the difference between the saturation vapor pressure and the actual water vapor pressure. The greater this difference, the greater the atmospheric demand for water. Plants react to this through their stomata, the so-called stomata. Under favorable conditions, open stomata allow both the absorption of CO₂ and the release of water vapor. If the air becomes too dry, the stomata can partially close, which changes water loss, CO₂ uptake and photosynthesis. ^[1]

Why temperature and humidity alone are not enough

A blanket recommendation like “always keep humidity at 55%” ignores temperature. At 22°C and 55% relative humidity, the air-based VPD is approximately 1.19 kPa. At 28 °C and the same humidity it is already around 1.70 kPa. The air may feel similar to us, but presents a significantly higher evaporation requirement for the plant.

This explains why a grow can show dry leaf edges, high water consumption or slowed development despite seemingly “correct” humidity. Conversely, an acceptable temperature combined with very high humidity can lead to sluggish transpiration and a humid microclimate in the crown.

VPD helps to read both values together. However, it does not replace direct plant observation. A healthy target value is of little use if the substrate is permanently waterlogged, the roots do not receive enough oxygen or strong lamps locally overheat the leaf surface. The Grow Island article also explains how climate, light and plant stress interact Cannabis Stress Guide .

Cannabis VPD table: practical target areas by phase

The following areas are not universal constants of nature. They are practical guidelines that must be adapted to genetics, rooting, light intensity, irrigation and cultivation system. Recent cannabis literature identifies similar corridors while emphasizing that direct research on humidity in cannabis is still limited. ^[2]

In the seedling and cutting phase, a lower VPD is preferred because the roots are still poorly developed. With unrooted cuttings, water absorption via the root system is initially severely limited. As root mass increases, the demand for evaporation is allowed to increase gradually.

During the vegetation phase, a medium VPD can support active water and mineral flow. During flowering, the relative humidity is usually reduced in a controlled manner in order to keep the microclimate in increasingly dense inflorescences less humid. This is no guarantee against fungal infestation, but together with air movement, hygiene and a crown that is not too dense, it reduces an important risk factor. You can find out more about this in the guide Recognize, avoid and properly treat mold in cannabis .

Cannabis VPD table by temperature and humidity

This table shows the air-based VPD in kPa. To simplify, it assumes that leaf and air temperatures are the same. In practice, the blade surface may be warmer or cooler, so the values ​​are intended as a quick guide rather than a high-precision blade VPD calculation.

Example: At 26°C and 60% RH, the air-based VPD is around 1.34 kPa. This can be good for the flowers, but would usually be too dry for a freshly cut cutting. At 24°C and 70% RH the value is approximately 0.90 kPa, which is more suitable for an established young plant or early vegetation period.

A VPD calculator uses the saturation vapor pressure at a specific temperature and subtracts the actual vapor pressure of the air. For a simplified calculation with the same leaf and air temperatures:

A more accurate leaf VPD calculator uses measured leaf temperature for saturation vapor pressure, while actual vapor pressure is determined from air temperature and relative humidity. From this it follows:

This sounds theoretical, but it is simple in everyday life: Measure the air temperature and RH at the height of the plant crown, determine the leaf temperature with a suitable infrared thermometer and enter all values ​​into a leaf VPD calculator. It is important not to measure on shiny reflections or directly on wet leaf surfaces.

Leaf temperature, LED and the difference between room VPD and leaf VPD

Many simple tables equate leaf and air temperatures. In fact, leaf temperature depends on radiation, transpiration, air movement, plant water status and lamp technology. Under some LED setups, leaves remain cooler than the surrounding air, while high light intensity and low air movement can also warm individual leaf zones.

For example, if the leaf is 2°C cooler than the air, its saturation vapor pressure will be lower. The real leaf VPD can therefore deviate significantly from the table value. That's why a measured leaf difference is more helpful than the general assumption that "LED always means minus two degrees".

Light intensity should not be viewed in isolation either. In a controlled investigation Cannabis sativa favorable photosynthesis conditions were 25–30 °C and high PPFD; at the same time, transpiration increased with temperature and light, while very high temperature and light intensity could affect photosynthesis and water use efficiency. ^[3] You can find information on suitable lighting control in the article How far should the LED grow light be from cannabis?

What happens if the VPD is too low?

A low VPD means that the air can only hold a small amount of additional water. The demand for perspiration decreases. This is helpful in the short term for seedlings and unrooted cuttings, but in the long term a very low value can become problematic for established plants.

Possible signs and consequences

• slow drying of the substrate and low water absorption;
• sluggish perspiration and altered mineral transport;
• soft or excessively lush growth;
• condensation on cool surfaces;
• humid microclimate in a dense plant crown;
• Increased risk of certain fungal diseases, especially if there is poor air movement.

A recent controlled study on a CBD-dominant cannabis chemotype compared high and low humidity. Extremely high RH with very low VPD was associated with delayed flower development and lower cannabinoid concentrations in this specific experimental setup. ^[2] One should not derive a universal percentage rule for every genetic from this; However, the study shows that long-term extreme humidity is physiologically relevant.

What happens if the VPD is too high?

When VPD is high, the air demands a lot of water from the plant. As long as roots and conducting tissue can supply enough, transpiration often increases. If demand exceeds supply capacity, the plant becomes water-stressed. She can partially close her stomata to conserve water. This means that less CO₂ can be absorbed at the same time.

Possible signs and consequences

• unusually rapid water consumption;
• hanging leaves despite sufficiently moist substrate;
• leaf edges rolled up or dry leaf tips;
• strong salt concentration in the root area due to rapid water removal;
• apparent calcium or magnesium problems even though fertilizer is present;
• slowed growth with prolonged drought stress.

General plant research shows that increasing VPD affects stomatal conductance, transpiration and carbon uptake. However, the reaction is species-specific and depends, among other things, on soil water, acclimatization and duration of exposure. ^{[1] [4]}

Correctly control day, night and light changes

When the lights are turned off, the temperature often drops faster than the moisture is removed from the room. Because cold air can hold less water vapor, the relative humidity increases. In the worst case scenario, the dew point is reached and water condenses on leaves, tent walls or flowers.

Stable night control therefore begins before the light changes. Dehumidification and exhaust air should not only react when RH has already risen significantly. At the same time, an extremely dry night time value is not automatically better: the goal is a controlled climate without condensation and without unnecessarily large jumps.

1. Log temperature and RH for at least one full day-night cycle.
2. Check particularly carefully the first 30-60 minutes after the light change.
3. Reduce large temperature drops if the setup allows.
4. Let the circulating air continue to run moderately at night, without constantly blowing heavily on plants.
5. Control dense flowers and the interior of the crown, not just the indoor air.

This is how you optimize the growing climate step by step

1. Check measuring devices

Two cheap hygrometers can differ from each other by several percentage points. Compare sensors side by side and calibrate them if the model allows this. A small measurement error can produce a noticeable difference in the calculated VPD in warm temperatures.

2. Choose a realistic target corridor

First choose a range to suit the development phase instead of an exact value. For example, an established plant in the vegetation can be targeted to 0.9–1.1 kPa. Then observe for several days before optimizing further.

3. Change only one factor at a time

If you change temperature, humidity, lamp power and watering at the same time, the plant reaction is difficult to determine. If possible, only adjust one variable and document the effect.

4. Think about irrigation

A higher VPD can increase water consumption. This does not mean that watering should be carried out more frequently according to a rigid calendar. Check pot weight, substrate moisture and drainage. The detailed Grow Island guide Water hemp plants properly explains the connections between the root zone and water balance.

5. Document plant reactions

Record daily minimum, maximum, light phase, dark phase, watering amount and visible changes. A weekly trend is more meaningful than a single momentary value. Good climate control is created through trends, not through constant readjustments every minute.

Advantages, limitations and risks of VPD control

Possible benefits

• Temperature and humidity are evaluated together rather than in isolation.
• Climate changes can be planned comprehensibly between development phases.
• Irrigation and transpiration trends become easier to interpret.
• Problematic day-night transitions and humid microclimates are identified more quickly.
• In controlled indoor systems, the control of humidification, dehumidification and exhaust air can be carried out more precisely.

Boundaries

• Many common VPD target values come from horticultural practice and general plant physiology; Cannabis-specific comparative studies are still limited.
• Different genetics, substrates and root systems do not respond identically.
• A spatial value does not fully represent the microclimate within dense flowers or under the canopy.
• A mathematically perfect value cannot compensate for problems with light, roots, salinity or disease.

Risks if used incorrectly

• Dehumidifying too aggressively can dry out young plants.
• Excessive humidification can increase condensation and microbial risks.
• A misplaced or uncalibrated sensor can misdirect the entire climate control system.
• If you only follow a table, you can miss clear stress signals from the plant.

Cannabis VPD FAQs

Which VPD is optimal during flowering?

About 1.1–1.5 kPa is used as a rough guide, with early bloom near the lower end and late bloom near the upper end. Leaf temperature, genetics, water supply and avoiding a humid microclimate are crucial.

Is a VPD of 1.5 kPa too high?

For cuttings and small young plants, usually yes. For a healthy, well-rooted plant in later flowering, 1.5 kPa may be appropriate at times. Permanently higher values ​​should only be achieved with good control of leaf reaction, root zone and water consumption.

Do I absolutely need a VPD calculator?

No. For a simple grow, reliable temperature and RH measurements as well as a suitable table are sufficient. A calculator becomes particularly useful if you want to include leaf temperature, use automation, or compare climate data more accurately.

Can a perfect VPD prevent mold?

No. It can help to avoid permanently too humid conditions, but is only part of the prevention. Air exchange, hygiene, plant spacing, canopy structure, genetics and regular monitoring remain crucial. If the crown is very dense, well thought-out thinning can improve air movement; However, unnecessary stress should be avoided. See this Cannabis defoliation .

Why does my sensor show good values, but the plant shows stress?

Possible causes include incorrect sensor position, different leaf temperature, local wind stress, root problems, incorrect watering, salt accumulation or too high light intensity. Check the system as a whole instead of automatically changing the VPD further.

Conclusion: VPD for cannabis as a compass, not a dogma

The VPD for cannabis makes temperature and humidity understandable together. Instead of tracking two isolated numbers, you get an indication of how much the indoor air is driving perspiration. In practice, around 0.4–0.8 kPa for seedlings and cuttings, 0.8–1.2 kPa in the vegetation phase and 1.1–1.5 kPa in flowering are sensible starting points.

However, the best VPD table is only as good as the measurement behind it. Measure at crown height, take leaf temperature into account if possible and pay particular attention to the change in light. Adjust the climate gradually and read the plant at the same time: leaf position, water consumption, growth, substrate drying time and root health provide the necessary context.

VPD is therefore not a replacement for experience, but a tool that makes observations more precise. If you use target corridors instead of supposedly perfect individual values, you can control the growing climate more stably and recognize more quickly whether a change actually helps.

About Grow Island

Grow Island combines practical experience with understandable knowledge about cannabis, plant care, genetics and controlled growing conditions. Our goal is to provide adult readers with comprehensible information, to separate scientific findings from empirical values ​​and to promote responsible use of the plant.

Scientific sources

1. Grossiord, C.; Buckley, T.N.; Cernusak, L.A.; Novick, K.A.; Poulter, B.; Siegwolf, R.T.W.; Sperry, J.S.; McDowell, N. G. (2020): Plant responses to rising vapor pressure deficit. New Phytologist , 226(6), 1550–1566. DOI: 10.1111/nph.16485 .
2. Corredor-Perilla, D. et al. (2025): Elevated relative humidity significantly decreases cannabinoid concentrations while delaying flowering development in Cannabis sativa L. Frontiers in Plant Science , 16, 1678142. DOI: 10.3389/fpls.2025.1678142 .
3. Chandra, S.; Lata, H.; Khan, I.A.; ElSohly, M. A. (2008): Photosynthetic reaction of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants , 14(4), 299–306. DOI: 10.1007/s12298-008-0027-x .
4. Yuan, W. et al. (2019): Increased atmospheric vapor pressure deficit reduces global vegetation growth. Science Advances , 5(8), eaax1396. DOI: 10.1126/sciadv.aax1396 .