A VPD chart for weed shows how strongly the air pulls water out of the leaves: VPD (vapour pressure deficit) combines temperature and humidity into a single number in kPa. The usual target values, calculated at the leaf, are 0.4–0.8 kPa for seedlings and rooted cuttings, 0.8–1.2 kPa in the vegetative stage and 1.0–1.5 kPa in flowering. With the leaf 2 °C cooler than the air, that means roughly 53–65% humidity in veg (air at 26 °C), 45–55% in flower (24–26 °C) and 62–75% for seedlings (24 °C).
In brief
- Seedlings and rooted cuttings 0.4–0.8 kPa, veg 0.8–1.2 kPa, flowering 1.0–1.5 kPa.
- VPD too low: the plant transpires little and the buds stay damp. Too high: the stomata close and growth slows down.
- The chart assumes the leaf is 2 °C cooler than the air, which is typical under LEDs.
- You correct it mainly through humidity: humidifier, dehumidifier and extractor fan speed.
- It is a guide, not a target to the decimal point: an ordinary hygrometer can be 3–5 points out.
What VPD is, in plain terms
Inside the leaf the air is saturated with water vapour; outside it holds less. VPD is the difference between these two vapour pressures: the larger it is, the faster the leaf loses water through its stomata. This transpiration draws water and nutrients up from the roots and cools the leaf.
Relative humidity alone is not enough, because it depends on temperature: 60% at 20 °C and 60% at 28 °C are two different situations for the plant. VPD puts them on the same scale.
- VPD too low (below the value for the stage): the plant transpires little, moves less calcium to the new leaves and dense buds stay damp, raising the risk of grey mould and powdery mildew.
- VPD too high (above the value for the stage, and in any case above 1.6 kPa): the leaf loses water faster than the roots can supply it; the stomata close, growth slows, the leaves fold up like a canoe and the tips dry out.
VPD values by stage
Seedlings have few roots and want humid air; as the plant grows it tolerates and makes use of a higher VPD; in flowering a high VPD keeps the buds dry. For each stage the table gives an example temperature and the matching humidity. Care in the first weeks is covered in the guide to cannabis seedlings.
| Stage | Leaf VPD | Air, lights on | Relative humidity |
|---|---|---|---|
| Seedlings and rooted cuttings | 0.4–0.8 kPa | 24 °C | 62–75% |
| Vegetative stage | 0.8–1.2 kPa | 26 °C | 53–65% |
| Flowering | 1.0–1.5 kPa | 24–26 °C | 45–55% |
The exception is unrooted cuttings, which stay at 80–90% humidity in a propagator. In the last 2–3 weeks of flowering the air is often cooler (20–24 °C) and humidity drops to 40–50%: at 22–24 °C the VPD stays between 1.0 and 1.45 kPa, at 20 °C it falls to about 0.9–1.1 kPa, which is fine as long as air moves well between the buds; humidity levels for each stage are in the guide to grow tent humidity.
VPD chart: air temperature and humidity
VPD in kPa by air temperature (rows) and relative humidity (columns), with the leaf 2 °C cooler than the air; values calculated with the Tetens formula.
| Air | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% |
|---|---|---|---|---|---|---|---|---|
| 20 °C | 1.13 | 1.01 | 0.89 | 0.78 | 0.66 | 0.54 | 0.43 | 0.31 |
| 22 °C | 1.28 | 1.15 | 1.02 | 0.88 | 0.75 | 0.62 | 0.49 | 0.36 |
| 24 °C | 1.45 | 1.30 | 1.15 | 1.00 | 0.85 | 0.70 | 0.56 | 0.41 |
| 26 °C | 1.64 | 1.47 | 1.30 | 1.14 | 0.97 | 0.80 | 0.63 | 0.46 |
| 28 °C | 1.85 | 1.66 | 1.47 | 1.28 | 1.09 | 0.90 | 0.72 | 0.53 |
| 30 °C | 2.08 | 1.87 | 1.66 | 1.45 | 1.23 | 1.02 | 0.81 | 0.60 |
Find the row for your temperature and the column for your humidity, then compare the value with the ranges for the stage. Above 1.6 kPa the air is too dry for almost every stage, below 0.4 kPa too humid.
If the leaf is at the same temperature as the air, add about 0.3–0.4 kPa to the chart values (between 22 and 28 °C); if it is only 1 °C cooler, add about 0.15–0.2 kPa.
Leaf temperature vs air temperature
The VPD that matters to the plant is calculated at the leaf. Under LEDs the leaf is usually 1–2 °C cooler than the air, because it transpires and LEDs give off little radiant heat; under HPS lamps it can be the same temperature as the air or warmer. That is why, with LEDs, the air can sit in the upper part of the temperature ranges.
The calculation: saturation vapour pressure comes from the Tetens formula, es(T) = 0.6108 × e^(17.27 × T ÷ (T + 237.3)), in kPa. Leaf VPD is es(T leaf) − es(T air) × RH. Example with air at 26 °C, 50% humidity and the leaf at 24 °C: es(24 °C) = 2.98 kPa; vapour in the air = 3.36 × 0.50 = 1.68 kPa; VPD = 2.98 − 1.68 = 1.30 kPa. With the leaf at 26 °C, like the air, VPD would rise to 1.68 kPa.
- Use an infrared thermometer; if the emissivity can be set, choose 0.95, a suitable value for leaves.
- Measure at least an hour after lights-on, aiming from 10–20 cm at upper, well-lit leaves, not at stems or pots.
- Take 4–5 leaves in different parts of the canopy, average them and compare the result with the air temperature at canopy height: the difference tells you whether to use the chart as it is or add the correction.
Adjusting VPD: humidifier, dehumidifier, extraction
VPD shifts when you change humidity and temperature. Humidity is usually the simpler lever: at 26 °C, 5 points of humidity are worth about 0.17 kPa, and one degree more or less at the leaf about 0.17–0.19 kPa.
VPD too high: dry or warm air
- Raise humidity with a humidifier controlled by a humidistat; with ultrasonic models, use demineralised water.
- Turn the extractor fan down, without switching it off, when the incoming air is dry, as in heated indoor spaces in winter.
- Lower the temperature: more extraction if the room is cool, dim the lamp if the tent is warm.
- For seedlings and cuttings, use a dome or a propagator with adjustable vents.
VPD too low: humid or cool air
- Increase extraction, leave it running at reduced speed when the lights are off too and, if needed, raise the temperature slightly while staying below 28 °C.
- Put a dehumidifier in the room if the incoming air is already humid, as in a cellar or in summer.
- Move the air between the plants with circulation fans and empty the saucers. A climate controller with a probe can run the extractor fan and dehumidifier together.
The limits of VPD when growing for personal use
Indoors, the climate in the grow tent depends on the room air. In Ticino the summer air is often warm and humid and it is hard to get below 55% in flowering without a dehumidifier; in winter heated indoor air often drops to 30–40%, too dry for seedlings.
- Instrument accuracy: an ordinary hygrometer can be 3–5 points out, which is about 0.1–0.17 kPa at 26 °C. Chasing the second decimal place makes no sense.
- Differences inside the tent: there can be 2–3 °C and several points of humidity between the canopy and the bottom. Always measure at canopy height.
- Night: with the lights off the leaf is roughly at air temperature and the plant transpires little. In flowering, what matters most is keeping humidity at or below 55–60% with the lights off: many hours above 60% encourage mould.
- Priorities: a temperature between 20 and 28 °C and humidity within the range for the stage come first; how to keep them there is explained in the guide to grow tent temperature. VPD helps you decide which of the two to correct when you are at the limit.
Frequently asked questions
What VPD do you need in flowering?
1.0–1.5 kPa at the leaf: with air at 24–26 °C and the leaf 2 °C cooler, that means about 45–55% humidity. In the final weeks the main thing is to avoid peaks above 60%, which encourage mould.
How do you calculate VPD?
Subtract the vapour in the air, es(T air) × relative humidity, from the saturation vapour pressure at leaf temperature, using the Tetens formula for es. Air at 26 °C, 50% humidity and the leaf at 24 °C give 1.30 kPa.
Do you need to measure leaf temperature?
It is not essential: under LEDs the leaf is usually 1–2 °C cooler than the air, which is what the chart assumes. An infrared thermometer lets you check this now and then, especially if you use HPS lamps.
Does VPD matter for a beginner?
It is useful but not essential. If temperature and humidity stay within the ranges for each stage, VPD is almost always in the right zone already; it becomes valuable when one value is at the limit and you have to choose what to correct.
Hygrometers, fans and climate accessories are in the Air and climate section.
Updated on 13 October 2026.