Cannabis VPD Chart: Target Ranges by Growth Stage
A VPD chart for cannabis assigns a target vapor pressure deficit range to each growth stage. Common figures run from 0.5 to 1.0 kPa for clones and seedlings, 0.7 to 1.2 kPa during vegetative growth, and 1.0 to 1.5 kPa during flowering, as a 2025 study in the journal Frontiers in Plant Science summarizes the figures cited in the literature. These are practical values drawn from growing experience rather than results from controlled trials. The same study lists humidity among the least studied climate factors in cannabis cultivation.
The table further down shows the VPD that results from any combination of air temperature and relative humidity. For other combinations, the VPD calculator works it out directly. By default it shows leaf VPD for a leaf 2 °C cooler than the air; with the leaf offset set to zero, its values match the table below.
What vapor pressure deficit measures
Vapor pressure deficit (VPD) is the difference between the saturation vapor pressure and the actual vapor pressure of the air, given in kilopascals (kPa). The UN Food and Agriculture Organization (FAO) describes it, in its guidelines on calculating evapotranspiration, as an accurate indicator of the air's evaporative capacity.
Relative humidity on its own does not describe that capacity. It is only the ratio of the two vapor pressures, and saturation vapor pressure does not rise linearly with temperature. At 60% relative humidity, VPD is 0.94 kPa at 20°C but 1.70 kPa at 30°C.
Plants respond to drier air by partially closing their stomata. This is described in a modeling study in the Journal of Advances in Modeling Earth Systems (2019) and in a series of experiments on sunflowers published in AoB Plants (2020). For cannabis specifically, an industry article in Cannabis Science and Technology (2019) lists slowed growth and problems with mold and root rot as consequences of VPD that is too low. If VPD is too high, the article states, plants close their stomata, leaf tips burn, and leaves curl.
How to calculate VPD
The starting point is the saturation vapor pressure, the vapor pressure of fully saturated air at a given temperature. The Magnus formula describes it as an exponential function of temperature, with constants that vary slightly between sources. Germany's national weather service, the Deutscher Wetterdienst, uses a simplified Magnus variant with the constants 17.5043 and 241.2 in its MOSMIX forecasting system to derive relative humidity from dew point and temperature. The FAO's Irrigation and Drainage Paper No. 56 on crop evapotranspiration uses an equation of the same type that returns the result directly in kPa. It is the basis for the chart in this article:
Saturation vapor pressure at air temperature T in °C: es = 0.6108 · exp(17.27 · T / (T + 237.3)), result in kPa (FAO equation 11).
Actual vapor pressure from relative humidity RH in percent: ea = es · RH / 100 (rearranged from FAO equation 10).
Vapor pressure deficit: VPD = es − ea, or VPD = es · (1 − RH / 100).
At 24°C, saturation vapor pressure is 2.98 kPa. At 60% relative humidity, the air's actual vapor pressure is 1.79 kPa, giving a VPD of 1.19 kPa. Because the equation is nonlinear, the FAO averages values calculated at the daily maximum and minimum temperature rather than at the mean; calculating from the mean temperature alone would give a lower average saturation vapor pressure.
VPD chart by temperature and humidity
The columns show relative humidity, the values show VPD in kPa. They apply to air VPD, which assumes the leaf is the same temperature as the air. Figures are calculated with the FAO equation and rounded to two decimal places.
Temperature | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% |
|---|---|---|---|---|---|---|---|---|
18°C | 1.14 | 1.03 | 0.93 | 0.83 | 0.72 | 0.62 | 0.52 | 0.41 |
20°C | 1.29 | 1.17 | 1.05 | 0.94 | 0.82 | 0.70 | 0.58 | 0.47 |
22°C | 1.45 | 1.32 | 1.19 | 1.06 | 0.93 | 0.79 | 0.66 | 0.53 |
24°C | 1.64 | 1.49 | 1.34 | 1.19 | 1.04 | 0.90 | 0.75 | 0.60 |
26°C | 1.85 | 1.68 | 1.51 | 1.34 | 1.18 | 1.01 | 0.84 | 0.67 |
28°C | 2.08 | 1.89 | 1.70 | 1.51 | 1.32 | 1.13 | 0.94 | 0.76 |
30°C | 2.33 | 2.12 | 1.91 | 1.70 | 1.49 | 1.27 | 1.06 | 0.85 |
VPD targets for clones, vegetative growth and flowering
The stage-specific figures for cannabis are practical values from cultivation experience rather than results from controlled trials. The Frontiers in Plant Science study reports them as ranges cited in the literature, drawing in part on the 2019 article in Cannabis Science and Technology. Its authors, three engineers at a cultivation-facility design firm, describe 0.8 to 1.1 kPa in vegetative growth and 1.0 to 1.5 kPa in flowering as commonly regarded as ideal. They do not give a separate range for clones.
Stage | Frontiers in Plant Science (2025) | Cannabis Science and Technology (2019) |
|---|---|---|
Clone and seedling | 0.5 to 1.0 kPa | not given |
Vegetative growth | 0.7 to 1.2 kPa | 0.8 to 1.1 kPa |
Flowering | 1.0 to 1.5 kPa | 1.0 to 1.5 kPa |
Mapped onto the chart above, 24°C at 75% relative humidity, giving 0.75 kPa, falls within the clone range; 24°C at 65%, giving 1.04 kPa, falls within the vegetative range; and 26°C at 60%, giving 1.34 kPa, falls within the flowering range. The ranges overlap, and the same VPD can result from more than one combination: 1.34 kPa also occurs at 24°C and 55%.
Air VPD and leaf VPD
Charts based on air temperature assume the leaf is as warm as the air. A modeling study on leaf temperature in PLOS ONE (2015) found that leaves of well-watered plants under sunlight, high-pressure sodium lamps and LED lighting typically stayed within 2°C of air temperature. Under LED lighting, leaves were around 1.3°C cooler than under sodium lamps at the same light intensity. Under water stress and low air movement, the model showed leaves could warm up to 8°C under LED or 12°C under sodium lighting above air temperature.
Leaf VPD uses saturation vapor pressure at leaf temperature rather than air temperature. The Frontiers in Plant Science study assumed a leaf-to-air temperature difference of 1 to 3°C for this calculation. The effect this has can be shown using the FAO equation: at 26°C and 60% relative humidity, air VPD is 1.34 kPa. If the leaf is 2°C cooler than the air, leaf VPD drops to 0.97 kPa.
What a humidity study on cannabis found
In November 2025, Frontiers in Plant Science published a study on humidity at canopy level, which its authors describe as the first to compare two humidity ranges in CBD-rich cannabis. In growth chambers, 20 clones of a CBD-dominant cultivar were grown, 10 at 37 to 58% and 10 at 78 to 98% relative humidity. The results were as follows:
Stage | Relative humidity | Temperature | Air VPD | Leaf VPD |
|---|---|---|---|---|
Vegetative | 37 to 58% | 20.2°C | 1.29 kPa | 1.02 kPa |
Vegetative | 78 to 98% | 20°C | 0.05 kPa | 0.13 kPa |
Flowering | 37 to 58% | 17°C | 0.92 kPa | 0.60 kPa |
Flowering | 78 to 98% | 21°C | 0.25 kPa | −0.04 kPa |
Under high humidity, flowering began in week 13 instead of week 10, and harvest took place in week 15 instead of week 14. Flower biomass fell from 33.80 g to 9.90 g, a drop of 71%. Total CBD content was 0.61% instead of 2.80%, and other cannabinoids declined as well: CBC-A fell from 3.90 mg/g to 0.30 mg/g.
Where the evidence is thin
The authors of the Frontiers study note that the effect of high humidity on growth and cannabinoid content is largely unexplored across individual genotypes. Their own study covers a single genotype with 20 plants. They consider follow-up studies with other CBD-rich cultivars necessary.
The study does not directly test one of the commonly cited target ranges. Even the drier condition produced a flowering VPD of 0.92 kPa, below the 1.0 to 1.5 kPa flowering range the authors themselves cite. Two humidity levels were compared; whether values within the usual ranges perform better than values just outside them is not something the study addresses.
The stage-specific figures trace in part to the 2019 industry article, which describes them as commonly known ideal values. There is also the question of temperature: the chart above is based on air temperature, while leaf temperature can differ depending on lighting and water availability, as the 2015 modeling study shows.
Frequently asked questions
What VPD is used for cannabis during flowering?
VPD for flowering is usually given as 1.0 to 1.5 kPa, a figure cited both in a 2025 study in Frontiers in Plant Science and in a 2019 industry article in Cannabis Science and Technology. These are practical values from cultivation experience rather than figures confirmed by controlled trials.
How do you calculate VPD?
VPD is calculated using a Magnus-type equation for the saturation vapor pressure at air temperature, for example the FAO equation es = 0.6108 · exp(17.27 · T / (T + 237.3)) in kPa. Multiplying that figure by (1 − RH / 100) gives VPD; at 24°C and 60% relative humidity, the result is 1.19 kPa.
Why isn't relative humidity enough on its own?
Saturation vapor pressure does not rise linearly with temperature, so the same relative humidity produces a different VPD depending on temperature. At 60% humidity, VPD is 0.94 kPa at 20°C and 1.70 kPa at 30°C.
What is the difference between air VPD and leaf VPD?
Air VPD is calculated using air temperature; leaf VPD uses the temperature of the leaf itself. If a leaf is 2°C cooler than the air at 26°C and 60% relative humidity, the value drops from 1.34 kPa to 0.97 kPa.
Is there research on VPD in cannabis?
Only a few studies exist. A 2025 study in Frontiers in Plant Science compared two humidity ranges across 20 plants of a CBD-rich cultivar; under high humidity, flowering began in week 13 instead of week 10, and total CBD content fell from 2.80% to 0.61%.
Sources
As of: 22 October 2026
This article is for informational purposes only and is not a substitute for medical or therapeutic advice. If you have any health-related questions, please consult a doctor or another qualified professional.