Grow guide

Vapor Pressure Deficit

Updated October 2, 2026

What is VPD?

Vapor pressure deficit (VPD), also written vapour pressure deficit, is a metric that combines air temperature, leaf temperature, and relative humidity (RH) into a single unit of pressure.

How to calculate VPD?

The chart shows two VPDs. Leaf VPD compares the leaf with the air around it: it is what drives transpiration, and what the chart's zones are for. Air VPD only looks at the air, as if the leaf were at air temperature.

Saturation vapor pressure (Tetens)

SVP(T) = 0.61078 × e^{17.2694 T / (T + 237.3)}

Leaf VPD

VPD_leaf = SVP(T_leaf) − SVP(T_air) × RH / 100

Air VPD

VPD_air = SVP(T_air) × (1 − RH / 100)

T_leaf: Leaf (canopy) temperature, in °C
T_air: Air temperature, in °C
RH: Air relative humidity, in %
SVP: Saturation vapor pressure, in kPa
VPD: Vapor pressure deficit, in kPa

To measure it, you need a thermometer / hygrometer for the grow room temperature and RH, and ideally an infrared thermometer for the canopy temperature. Without one, position your thermometer's remote probe at canopy level and take the reading after a few minutes, or assume that the leaf is 2°C cooler than the air.

Or let the VPD chart do the math

Why is VPD important?

VPD expresses how temperature and relative humidity actually affect the plants - growth, stress and diseases. VPD is a great indicator of plant transpiration rate. Transpiration is necessary for photosynthesis, nutrient uptake, and plant cooling. Plant stress is brought about by either excessive transpiration (high VPD values) or the inability to transpire adequately (low VPD values). Therefore, VPD can help growers make decisions about irrigation, heating / cooling and dehumidification in their grow room / greenhouse.

What is the ideal VPD?

The ideal VPD changes with the crop and with the growth stage. Young plants have small roots, so they need a low VPD. As the plant grows, the ideal VPD increases.

The ideal leaf VPD for each crop and growth stage, as on the chart
Growth stage	Leaf VPD
Cannabis
Early veg / propagationAir: 65–80% RH, 21–30 °C (70–86 °F)	0.4–0.8 kPa
Late veg / early flowerAir: 40–70% RH, 21–30 °C (70–86 °F)	0.8–1.2 kPa
Mid / late flowerAir: 40–50% RH, 18–26 °C (64–79 °F)	1.2–1.6 kPa
Tomatoes
Early growth	0.2–0.5 kPa
Vegetative	0.5–0.8 kPa
Flowering to mature fruiting	0.8–1.2 kPa
Leafy greens
Vegetative growthAir: 18–25 °C (64–77 °F)	0.8–0.95 kPa
Cucumber
All growth stages	0.4–1.4 kPa

These values are leaf VPD, as on the chart. To show the zones of a crop or of one growth stage on the VPD chart, select it in “Chart settings”, under “Crop”.

What ifVPD is too low?
Humidity is high and plants are unable to evapotranspirate enough water to enable the transport of nutrients to cells, even though the stomata may be fully open.
Some plants may even exude water through special leaf tip or edge structures (hydathodes or water glands), forming drops, in a process called guttation.
In cases where theVPD is extremely low, water may condense onto leaves and other plant parts. This can provide a medium for fungal/mold growth and disease.
Plants are unable to evaporate water and turgor pressure within the cells can cause splitting and cracking of fruits.

What ifVPD is too high?
Humidity is low and the rate of evaporation from the leaves can exceed the supply of water into the roots. This will cause the stomata to close and photosynthesis to slow or stop. The leaves might be at risk of high temperature injury since evaporative cooling is reduced due to the lack of water to evaporate. To avoid injury and death from wilting, many plant species will either curl their leaves or orient them downward in an attempt to expose less surface area to the sun. This can significantly reduce the growth rate and quality of crops.

How to control VPD?

To increase the VPD, moisture must be removed from the air, or the moisture holding capacity of the air must be increased by raising the temperature. Moisture removal can be accomplished by using dehumidifiers (expensive), or by replacing moist air with drier air (typically through ventilation). This is the standard practice for avoiding direct condensation onto crop or greenhouse surfaces.

To decrease the VPD, operate fog or misting devices to add water vapor to the air, or cool the air. Both would reduce VPD level and the evapotranspiration stress in the crop.

References

Shamshiri, R.R., Jones, J.W., Thorp, K.R., Desa, A., Che, M.H. & Sima, T. 2018. Review of optimum temperature, humidity, and vapour pressure deficit for microclimate evaluation and control in greenhouse cultivation of tomato: a review, https://doi.org/10.1515/intag-2017-0005

Jin D., Jin S., Chen J. 2019. Cannabis Indoor Growing Conditions, Management Practices, and Post-Harvest Treatment: A Review, https://doi.org/10.4236/ajps.2019.106067

Amitrano,C.;Rouphael,Y.; Pannico, A.; De Pascale, S.; De Micco, V. Reducing the Evaporative Demand Improves Photosynthesis and Water Use Efficiency of Indoor Cultivated Lettuce. Agronomy 2021, 11, 1396. https://doi.org/10.3390/agronomy11071396

Amitrano,C.;Rouphael,Y.; De Pascale, S.; De Micco, V. Modulating Vapor Pressure Deficit in the Plant Micro-Environment May Enhance the Bioactive Value of Lettuce. Horticulturae 2021, 7, 32. https://doi.org/10.3390/horticulturae7020032

Bakker, J. C. (1991). Analysis of humidity effects on growth and production of glasshouse fruit vegetables. Ph.D. dissertation, Agricultural University, Wageningen, Netherlands, p. 155.

Konopacki, P. J.; Treder, W.; Klamkowski, K. (2018). Comparison of vapour pressure deficit patterns during cucumber cultivation in a traditional high PE tunnel greenhouse and a tunnel greenhouse equipped with a heat accumulator. Spanish Journal of Agricultural Research, Volume 16, Issue 1, e0201. https://doi.org/10.5424/sjar/2018161-11484