Quick answer: Find your air temperature in the left column and your relative humidity across the top; the number where they meet is the vapour pressure deficit (VPD) in kPa. Low VPD (below about 0.4 kPa) means humid air, slow transpiration and a higher risk of disease and condensation. High VPD (above about 1.5 kPa) means dry air and plant stress. Many greenhouse and indoor crops grow well around 0.5–1.2 kPa, lower for propagation and higher for mature, actively growing plants. To lower VPD, add humidity or lower the temperature; to raise it, dehumidify or warm the air.
Quick answer
- Below 0.4 kPa: too humid; risk of mold and leaf wetness.
- About 0.4–0.8 kPa: propagation and young plants.
- About 0.8–1.2 kPa: vegetative growth for many crops; up to about 1.5 for some mature crops.
- Above 1.5 kPa: dry; plants may close stomata and wilt.
What VPD means
VPD stands for vapour pressure deficit: the difference between how much water vapour the air could hold at its temperature and how much it actually holds, measured in kilopascals (kPa). It is the "drying power" of the air. A high VPD pulls water out of leaves quickly; a low VPD means the air is close to saturated and leaves transpire slowly. Growers prefer VPD to RH because the same RH means a different drying power at different temperatures: 70 % RH is a VPD of 0.62 kPa at 18 °C but 1.27 kPa at 30 °C.
Air VPD chart (kPa)
| Air temp | 40 % RH | 50 % RH | 60 % RH | 70 % RH | 80 % RH | 90 % RH |
|---|---|---|---|---|---|---|
| 14 °C (57 °F) | 0.96 | 0.80 | 0.64 | 0.48 | 0.32 | 0.16 |
| 16 °C (61 °F) | 1.09 | 0.91 | 0.73 | 0.55 | 0.36 | 0.18 |
| 18 °C (64 °F) | 1.24 | 1.03 | 0.83 | 0.62 | 0.41 | 0.21 |
| 20 °C (68 °F) | 1.40 | 1.17 | 0.94 | 0.70 | 0.47 | 0.23 |
| 22 °C (72 °F) | 1.59 | 1.32 | 1.06 | 0.79 | 0.53 | 0.26 |
| 24 °C (75 °F) | 1.79 | 1.49 | 1.19 | 0.90 | 0.60 | 0.30 |
| 26 °C (79 °F) | 2.02 | 1.68 | 1.34 | 1.01 | 0.67 | 0.34 |
| 28 °C (82 °F) | 2.27 | 1.89 | 1.51 | 1.13 | 0.76 | 0.38 |
| 30 °C (86 °F) | 2.55 | 2.12 | 1.70 | 1.27 | 0.85 | 0.42 |
| 32 °C (90 °F) | 2.85 | 2.38 | 1.90 | 1.43 | 0.95 | 0.48 |
Calculated with the saturation vapour pressure equation in FAO Irrigation and Drainage Paper 56 (FAO, Chapter 3). Example: at 24 °C and 70 % RH, air VPD is 0.90 kPa.
How to calculate VPD
- Saturation vapour pressure at the air temperature T (°C):
e_s = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa
- Actual vapour pressure: e_a = RH ÷ 100 × e_s
- Air VPD = e_s − e_a
At 24 °C: e_s = 2.98 kPa; at 70 % RH, e_a = 2.09 kPa; VPD = 0.90 kPa.
Leaf VPD
Plants respond to the difference between the vapour pressure inside the leaf (saturated at leaf temperature) and the air:
Leaf VPD ≈ e_s(leaf temperature) − e_a
If the leaf is 2 °C cooler than the air (22 °C leaf in 24 °C / 70 % RH air): e_s(22) = 2.64 kPa, so leaf VPD = 2.64 − 2.09 = 0.56 kPa, much lower than the 0.90 kPa air VPD. Leaves under strong lights can be warmer than air; leaves at night and under a clear sky are often cooler. Measure leaf temperature with an infrared thermometer if you control on leaf VPD.
If leaf VPD is zero or negative, the leaf is at or below the dew point and water can condense on it.
Typical VPD ranges by stage
| Stage | Typical range | Why |
|---|---|---|
| Cuttings and germination | about 0.2–0.6 kPa | Limit water loss before roots form |
| Seedlings and young plants | about 0.4–0.8 kPa | Gentle transpiration |
| Vegetative growth | about 0.8–1.2 kPa | Active transpiration and nutrient uptake |
| Mature, flowering or fruiting crops | about 1.0–1.5 kPa, by crop | Strong transpiration; drier air reduces disease |
| Night | keep above about 0.2 kPa | Avoid leaf wetness and condensation |
These are typical published starting points; your crop, cultivar and grower protocol set the actual target. Michigan State University Extension explains why VPD predicts plant water loss better than RH (MSU Extension). Crop-specific guidance: tomatoes, lettuce, propagation.
How to change VPD
| To | Do this | Note |
|---|---|---|
| Raise VPD (air too humid) | Dehumidify; ventilate if outside air is drier; raise temperature slightly | Dehumidifying lowers the actual vapour pressure without changing temperature |
| Lower VPD (air too dry) | Humidify (fog, mist, wet floors); lower temperature; shade | Avoid wetting leaves late in the day |
| Keep leaf VPD close to air VPD | Air movement across the canopy | Keeps leaves near air temperature |
At night, when crops still transpire and vents are closed, VPD often falls below 0.2 kPa. That is when greenhouse dehumidifiers and air movement prevent leaf wetness. For enclosed rooms, see grow room dehumidifier sizing.
Common mistakes reading a VPD chart
- Using a sensor in the sun or near a heater: the temperature, and so the VPD, is wrong.
- Mixing air VPD and leaf VPD targets.
- Assuming one "ideal zone" fits every crop and stage.
- Changing only RH when temperature also changed.
- Reading the room sensor instead of the canopy.
FAQ
Frequently asked questions
What does VPD stand for?
Vapour pressure deficit: the gap between saturated and actual vapour pressure of the air, in kPa.
What does a VPD of 1.0 kPa mean?
The air could hold 1.0 kPa more water vapour before saturating; for many crops in active growth that is a comfortable drying power.
How do I use a VPD chart in °F?
Convert the temperature (°C = (°F − 32) ÷ 1.8) and read the same table; the °F values are shown next to each row.
What is a good VPD?
For many crops in active growth, about 0.8–1.2 kPa; lower for propagation and seedlings, higher for some mature crops.
How do I read a VPD chart?
Find the air temperature row and the RH column; the value at the intersection is VPD in kPa.
Is high or low VPD worse?
Both cause problems. Low VPD slows transpiration and favours disease; high VPD stresses plants and can close stomata.
How do I lower VPD?
Raise humidity or lower the temperature.
How do I raise VPD?
Dehumidify, ventilate with drier air, or raise the temperature slightly.
Sources
- FAO Irrigation and Drainage Paper 56, Chapter 3: Meteorological data.
- Michigan State University Extension, Vapor pressure deficit for greenhouse growers.
