VPD in cannabis cultivation – why temperature and humidity only make real sense when considered together

VPD stands for Vapour Pressure Deficit and, in simple terms, describes the "drying power" of the air in relation to the plant. Technically, it refers to the difference between the water vapor currently present in the air and the amount of water vapor the air could hold at saturation. For plants, VPD is often more informative than relative humidity (RH) alone because VPD is directly related to transpiration and water loss through the leaves. Michigan State University explicitly describes VPD as the better metric for predicting plant water release and transpiration, whereas RH alone without temperature can easily be misleading.

What exactly VPD means

In practical terms, VPD shows how strongly the air can pull water from the plant. If the VPD is very low, the air is already highly saturated with moisture, and the plant transpires poorly. If the VPD is very high, the air is "thirsty," the plant loses water more quickly, and it is more prone to stress. The basic definition comes from plant physiology: VPD is the difference between the saturation vapor pressure and the actual vapor pressure of the air.

Why VPD is more important than just RH

Relative humidity sounds simple, but it is temperature-dependent. The same RH can lead to completely different evaporation pressures at different temperatures. Michigan State illustrates this very clearly: At 70% RH, the VPD at 15 °C is about 0.55 kPa, at 24 °C it is already at 0.90 kPa, and at 32 °C it is at 1.45 kPa. This is precisely why RH alone is not a complete control instrument for a grow room.

Why VPD is so relevant for cannabis

Cannabis plants react very clearly to humidity and temperature conditions because these directly determine how strongly the plant transpires, how open the stomata remain, and how stable the leaf climate is. A recent 2025 cannabis study showed that excessively high humidity and thus low VPD values can delay flower development, reduce biomass, and negatively affect cannabinoid concentration.

VPD and transpiration

The most important physiological connection is transpiration. When the VPD rises, the driving force for water loss initially increases. In many plant species, transpiration initially rises at higher VPD until the plant reacts at a critical point and closes its stomata more tightly to limit water loss. Often, photosynthetic performance and growth then decrease as well. A major review on plant reactions to rising VPD describes this exact connection very clearly.

VPD and nutrient uptake

Because the water flow through the plant is closely coupled with transpiration, VPD also influences nutrient uptake. This does not mean that "more VPD always means more nutrients," but rather that a balanced evaporation pressure supports the movement of water and dissolved ions through the plant. If the VPD becomes too low, this flow stalls; if it becomes too high, stress and stomatal closure occur. This is exactly why VPD in controlled cultivation is a climate metric, not just a humidity one.

What happens when the VPD is too low

A low VPD means: the air is too humid or too cool in relation to the humidity. In this state, the plant struggles to release water, the leaves dry off more slowly, and the climate becomes more favorable for fungi. For "finishing" in dense crops, Michigan State explicitly recommends a VPD of over 0.5 kPa because plants transpire better, cool themselves better, and the environment is less disease-promoting. The 2025 cannabis study additionally shows that persistently low VPD in cannabis can be accompanied by less biomass, delayed flowering, and lower cannabinoid levels.

What happens when the VPD is too high

A high VPD means: the air is too dry or too warm in relation to the existing humidity. This causes the water loss pressure to rise sharply. Plants often react with stomatal closure to avoid dehydration. The result can be reduced gas exchange, decreasing photosynthesis, and growth stress. The major review on rising VPD values describes precisely this cascade of higher atmospheric water demand, stomatal reaction, and subsequent drop in carbon uptake.

Leaf temperature makes VPD more accurate

A common growing mistake is to calculate VPD only using room temperature and RH. However, for the plant, the leaf-to-air VPD is actually the deciding factor—that is, the difference between the moisture status within the leaf and the air outside. e-GRO shows that VPDair and VPDleaf can differ significantly: In the example with 20 °C air temperature, 18 °C leaf temperature, and 50% RH, the VPD based on air temperature is 0.94 kPa, but with leaf temperature it is only 0.66 kPa. That is a significant difference.

Why leaf temperature is not just a minor detail in growing

The importance of leaf temperature is well-supported by science. It directly influences the micro-environment of the leaf and therefore stomata, photosynthesis, and VPD. An MDPI paper on leaf temperature and VPD emphasizes that leaf temperature and VPD are central environmental factors for photosynthesis and that leaves can assume very different temperatures even under the same air temperature. This is important for precise VPD control.

Which VPD ranges growers often target in practice

There is no universal ideal value that applies to every strain, light intensity, and growth phase. In practice, however, many indoor and greenhouse growers work with lower VPD values for propagation and early phases and somewhat higher values in vegetation and flowering. A greenhouse source from Michigan State cites about 0.3 kPa as a low target value for rooting cuttings and over 0.5 kPa for dense, mature crops. A VPD regulation paper from the greenhouse sector mentions about 0.5 to 1.2 kPa as a general plant guideline, while also noting that recommendations vary by growth phase and many practical guides use about 0.8 kPa for clones, approx. 1.0 kPa in vegetation, and approx. 1.2–1.5 kPa in flowering. These values are therefore guidelines, not laws of nature.

For cannabis, the direction is more important than a fixed number

For Cannaseuse, the cleanest formulation is therefore: in early phases, a slightly lower VPD is usually sensible so that young plants do not dry out. In vegetation and especially in flowering, a moderate to higher VPD can help maintain transpiration and reduce disease susceptibility in dense crops. This logic is reflected both in greenhouse sources and in the current cannabis humidity/VPD study.

How to practically control VPD during the grow

VPD is controlled via temperature and humidity. If the temperature rises and the RH remains the same, the VPD also rises. If the RH rises and the temperature remains the same, the VPD falls. This is exactly what the UGA summary on the relationship between temperature, RH, and evapotranspiration describes: Higher temperature at the same RH increases evaporation pressure; lower RH at the same temperature does as well. In practice, this means:
more VPD = warmer or drier
less VPD = cooler or more humid

What you need for meaningful VPD control

The minimum requirements are a reliable hygrometer and a thermometer at plant height. Even better is an additional tool for leaf temperature, because VPDleaf is more accurate than VPDair. In scientific studies and environmental guidelines, temperature and RH are measured continuously; in high-quality setups, VPD is now even used as the primary climate control variable.

Why VPD is not an end in itself

VPD is a very useful tool, but no substitute for an overall understanding. Light, air movement, root health, irrigation, and strain differences are always involved. A greenhouse source from 2025 hits the nail on the head: VPD is a helpful guideline for evapotranspiration, but not the only factor that determines success. This is exactly how VPD should be understood in cannabis growing—as a powerful control instrument, not as an isolated miracle number.

FAQ – Frequently asked questions about VPD in cannabis growing

What is VPD explained simply?

VPD describes how strongly the air can absorb water from the plant. The higher the VPD, the stronger the "drying pressure" of the air on the leaf.

Why is VPD more important than just humidity?

Because VPD views temperature and humidity together. The same RH can lead to completely different conditions for the plant at different temperatures.

What happens when the VPD is too low?

The plant transpires poorly, the leaf dries off more slowly, and the risk of humid, disease-promoting conditions increases. In cannabis, persistently low VPD can impair flowering, biomass, and cannabinoid content.

What happens when the VPD is too high?

The water loss pressure increases sharply, plants are more likely to close their stomata, and they can lose photosynthetic efficiency and growth as a result.

Do I have to measure leaf temperature for VPD?

For rough practical values, air temperature plus RH is often sufficient, but it becomes more accurate with leaf temperature. VPDleaf can differ significantly from the VPD calculated with air temperature.

Which VPD values are good?

There is no rigid universal solution. As a rough practical guide, lower values for cuttings and early phases, around 0.3–0.8 kPa, are often cited, then about 0.8–1.2 kPa for vegetation, and approx. 1.2–1.5 kPa for later flowering. These ranges are guidelines and must suit the strain, light, and setup.

Conclusion

VPD is one of the most sensible metrics for a modern grow climate because it shows how temperature and humidity act together on the plant. A good VPD supports transpiration, nutrient flow, leaf health, and stable development. A VPD that is too low often makes the climate too humid and sluggish; a VPD that is too high puts the plant under water stress. For Cannaseuse.de, therefore, the following applies:
Whoever understands VPD understands climate not just in percentage humidity, but in true plant physiology.