What VPD tells you
Plants take up water through the roots and release it through tiny pores in the leaves. The vapor pressure deficit (VPD) measures how hard the air pulls on that water. In the right range, plants keep their pores open, take up nutrients with the water and grow fast. When VPD is too low, the air is so humid that little water moves and mold has an easy time. When it is too high, plants close their pores to save water and growth stalls.
Temperature and humidity alone do not tell you this, because warm air can hold much more water than cool air. 60% humidity is humid at 18 °C but quite dry at 30 °C. VPD combines both into one number you can steer by.
Target VPD by growth stage
| Stage | VPD | Why |
|---|---|---|
| Seedlings and clones | 0.4 to 0.8 kPa | Few or no roots yet, so humid air keeps them from drying out. |
| Vegetative | 0.8 to 1.2 kPa | Strong roots move more water and nutrients for leaf growth. |
| Flowering and fruiting | 1.2 to 1.6 kPa | Drier air keeps dense flowers and fruit free of mold. |
These ranges are a starting point used by many growers and greenhouses. Some plants, like tomatoes or orchids, have their own preferences, so watch how your plants respond.
How to use the VPD calculator
- 1Measure air temperature and relative humidity at plant height, not near the lamp or the floor.
- 2Set the leaf offset. Measure the top leaves with an infrared thermometer, or keep the default of 2 °C cooler.
- 3Pick the growth stage. The calculator shows whether your VPD fits and the humidity that would put you in the middle of the range. The chart shows which temperature and humidity pairs work.
Frequently asked questions
What is VPD?
Vapor pressure deficit is the difference between how much water vapor the air could hold at its temperature and how much it actually holds. It is measured in kilopascals (kPa) and tells you how strongly the air pulls water out of the leaves. Low VPD means humid, sluggish air; high VPD means dry air and stressed plants.
Why does leaf temperature matter?
The water evaporates from inside the leaf, so the vapor pressure in the leaf depends on the leaf temperature, not the air. Leaves are usually 1 to 3 °C (2 to 5 °F) cooler than the air because evaporation cools them. Under strong HPS lamps or in sun they can be warmer. An infrared thermometer pointed at the top leaves gives the real offset.
What is the best VPD for seedlings, vegetative and flowering plants?
A common guide is 0.4 to 0.8 kPa for seedlings and clones, 0.8 to 1.2 kPa for vegetative growth and 1.2 to 1.6 kPa for flowering and fruiting. Young plants with few roots need humid air; mature plants can move more water and like drier air, which also lowers the risk of mold in dense flowers.
How do I raise or lower VPD?
To raise VPD, lower the humidity with a dehumidifier or more fresh air, or raise the temperature. To lower VPD, add humidity with a humidifier or lower the temperature. Change one thing at a time and wait an hour before you measure again.
Which formula does the calculator use?
Saturation vapor pressure comes from the Tetens equation, 0.61078 × e^(17.27 × T ÷ (T + 237.3)) in kPa with T in °C. VPD is the saturation pressure at leaf temperature minus the actual vapor pressure of the air, which is the saturation pressure at air temperature times the relative humidity.