Climate

VPD for growers

Vapor pressure deficit, or VPD, is how hard the air pulls water out of a leaf. Relative humidity on its own does not tell you that, because warm air holds far more water than cool air. This guide covers what VPD measures, why the leaf's temperature matters, what to aim for, and what happens to it after lights-off. The math is the same as the VPD Chart & Calculator.

Updated October 2026 10 min read By Connor Schmitz

Vapor pressure

Air can hold only so much water vapor before it is full and water starts condensing out. That ceiling is the saturation vapor pressure, and it climbs steeply with temperature. Air at 80 °F can hold about twice as much water as air at 60 °F.

Relative humidity (RH) is how full the air is, as a percent of that ceiling. VPD is the room left before it is full, in kilopascals (kPa). It is what the air could hold minus what it does hold:

VPD = saturation vapor pressure × (1 − RH ÷ 100)

The inside of a leaf is close to saturated. Water moves out through the stomata toward the drier air, and the bigger the gap, the faster it goes. That outflow is transpiration. It cools the leaf, pulls water and nutrients up from the roots, and is how calcium reaches new growth.

Both rooms are at 60% RH. The gold lines are VPD in kPa. At 60 °F the gap is 0.71 kPa. At 86 °F it is 1.70 kPa, so the pull on the leaves in the warm room is more than twice as strong at the same humidity reading.

The same thing works the other way. Michigan State University Extension gives an example where VPD holds steady at 1.38 kPa while the room warms from 60 to 90 °F, and the RH reading climbs from 22 to 71% to keep it there. A grower who chases one RH number all day is giving the plants a different VPD every time the temperature moves.

Quick check

Room A is 65 °F at 50% RH. Room B is 85 °F at 70% RH. Which room dries leaves out faster?

Answer: Room B. Room A is 1.05 kPa and Room B is 1.23 kPa. Air at 85 °F has so much more room for water that 70% full still leaves a bigger gap than 50% full at 65 °F.

VPD chart

A VPD chart lays this out as a grid, with temperature down the side, RH across the top and VPD in each cell. Find the room's two readings and read across. The colors below are kPa bands, so the eye can follow how the same RH changes color as you move down the temperature rows.

Air °F	40% RH	50%	60%	70%	80%
65	1.26	1.05	0.84	0.63	0.42
70	1.50	1.25	1.00	0.75	0.50
75	1.78	1.48	1.19	0.89	0.59
80	2.10	1.75	1.40	1.05	0.70
85	2.47	2.05	1.64	1.23	0.82

Under 0.5 kPa 0.5 to 1.2 1.2 to 1.6 Over 1.6

Air VPD in kPa, which assumes the leaves are at air temperature. The next section covers when they are not.

Leaf temperature

A chart like the one above gives air VPD. The plant feels leaf VPD: the inside of the leaf is saturated at the leaf's own temperature, which is often a few degrees off the air. A leaf that runs cooler than the air holds less vapor inside, so the pull on it is weaker than the air VPD says.

How far off? In a study at Utah State University, well-watered leaves stayed within about 2 °C (3.6 °F) of air temperature under sunlight, LED and high pressure sodium (HPS) lights. A water-stressed leaf in still air stops cooling itself, and the model put leaves 8 °C above air under LEDs and 12 °C above under HPS in that worst case. Under typical indoor conditions, LED leaves ran about 1.3 °C cooler than HPS leaves.

Leaf 4 °F warmer 1.71 kPa A dry or crowded plant close to a hot fixture. The leaf is pulled harder than the room reading says.

Leaf at air temp 1.27 kPa Air VPD, what a plain temperature and humidity sensor reports.

Leaf 2 °F cooler 1.06 kPa A healthy, transpiring canopy under LEDs. Common indoors.

Leaf 4 °F cooler 0.87 kPa Fast transpiration and good airflow. The plant feels a much damper room than the sensor shows.

One room, 77 °F and 60% RH, four canopies.

To get leaf temperature, point an infrared thermometer at the top of the canopy, at leaves in the light, or use a leaf temperature sensor. Take the difference from air temperature and put it in the calculator's leaf setting.

Most common mistake

Running a room to an air VPD target with a cool canopy. If the chart says 1.0 kPa and the leaves are 3 to 4 °F below the air, the leaves are sitting near 0.6 to 0.7. Check the canopy temperature before changing the dehumidifier setpoint.

Targets

Young plants with few roots need a low VPD so they do not dry out faster than they can drink. MSU Extension suggests about 0.3 kPa while rooting cuttings, and Greenhouse Management gives 0.3 to 0.7 kPa once roots start. For finishing crops, MSU advises keeping VPD above 0.5 kPa so plants keep transpiring and foliar disease stays down, especially in a dense canopy.

Past that, published targets for each stage are thin. The vegetative and flowering bands below are Bizon estimates, the same ones the calculator uses, and they shift with the crop, the light level and CO₂. Two studies help set the edges. In tomato, 0.8 kPa day and night gave more photosynthesis and fruit than 0.5. At 2.2 kPa, tomatoes showed stress and cracked fruit.

Try it

Set the room and the canopy

Air temperature77 °F Relative humidity60% Leaf vs air2 °F cooler

Leaf VPD, kPa1.06

Air VPD, kPa1.27

Dew point62 °F

Between bands.

Propagation 0.3 to 0.7

Vegetative (estimate) 0.8 to 1.0

Flowering and fruiting (estimate) 1.2 to 1.6

Scale 00.51.01.52.02.5 kPa

The marker is leaf VPD. Propagation is from Greenhouse Management; the other two bands are Bizon estimates, as in the VPD Chart & Calculator.

Running too damp costs yield as well as inviting disease. In a 2025 study, cannabis flowered at 0.25 to 0.62 kPa made 71% less flower by weight than plants grown drier, and flowered three weeks late.

VPD Chart & Calculator Leaf and air VPD from temperature, humidity and leaf offset, a full chart colored for the stage, and the dew point. No account needed.

Open the calculator

Lights-off

VPD can fall fast once the lights go off. The room cools while the water vapor in the air stays put. Here is a room that ran 78 °F and 60% RH all day, with nothing removing moisture as it cools:

Lights on 78 °F, 60% 1.31 kPa.

Cooled to 72 73% RH 0.72 kPa, with the same water in the air.

Cooled to 68 84% RH 0.37 kPa. Past the 85% RH line, MSU notes, water lost from leaves stays on them and fungal pathogens do well.

Dew point 63 °F Any leaf, wall or window colder than this gets wet. Under a clear sky, leaves radiate heat away and run cooler than the air, so they reach it first.

When RH stays high, the water leaves give off lingers on their surfaces, and MSU Extension notes that this favors fungal pathogens. Dew on the canopy makes it worse. Run the dehumidifier ahead of lights-off and let the temperature fall slowly, so moisture comes out of the air before the room cools to its dew point.

Quick check

The night temperature drops from 75 to 65 °F and nothing else changes. What happens to VPD?

Answer: It falls. Cooling lowers the ceiling while the water stays put, so RH climbs and the gap shrinks. Cool the room far enough and the gap closes at the dew point.

Calcium

Calcium moves through a plant almost entirely in the xylem, carried by the water that transpiration pulls up from the roots. It barely moves in the phloem, so a leaf or fruit cannot borrow calcium from older tissue. Whatever transpires the most gets the most calcium. That ties two common calcium disorders straight to VPD.

Tipburn VPD too low Young inner leaves of lettuce and other leafy crops transpire little. In damp air, as in fog, transpiration slows further and their margins brown, even with plenty of calcium in the root zone.

Blossom-end rot Leaves win Fruit transpires far less than leaves. In hot weather, leaves draw more of the water and the fruit gets less calcium. High humidity cuts calcium flow into fruit further. Leaf tests often read normal while the fruit runs short.

Adding calcium to the feed does not fix either one if VPD and transpiration are the problem. Keep VPD in range, keep air moving across the canopy so the inner leaves transpire, and keep root-zone water steady, since breaks in water uptake, the kind that show as wilting, raise the risk. Foliar calcium has a narrow role. UC IPM notes it has reduced tipburn in leaf lettuce trials but does not reach the inside of a lettuce head in time, and University of Delaware Extension notes fruit absorbs little through its skin. A calcium spray is a separate application and its ppm does not count toward the feed.

Adjusting VPD

VPD has two levers, temperature and the water in the air. Moving either one moves VPD, and so does the leaf temperature.

To raise VPD Drier Dehumidify, raise the air temperature at the same moisture, or vent when the outside air holds less water than the room.

To lower VPD Damper Humidify or fog, or lower the air temperature at the same moisture.

Leaf side Airflow Moving air thins the still layer around each leaf, helps it transpire and pulls its temperature toward the air's.

Winter in a cold climate pushes the other way. Outside air at 20 °F and 80% RH, heated to 75 °F, ends up near 10% RH and 2.67 kPa, drier than any target on this page. Summer greenhouses can be worse. In one study, midday VPD in a summer tomato greenhouse ran 4 to 5 kPa. Humidifying brought it to about 1.6, and fruit yield per plant rose 22.7%.

Very dry air does not keep pushing water out faster forever. Across many species, stomata close as VPD climbs, which saves water but cuts photosynthesis and growth. So a VPD that runs too high shows up as slow growth as well as wilting.

Summary

VPD is the gap between what the air can hold and what it holds. It drives transpiration; RH alone does not tell you the gap.
The same RH means a much higher VPD in a warm room than a cool one.
Leaves are rarely at air temperature. Measure the canopy and use leaf VPD.
About 0.3 kPa for rooting cuttings, above 0.5 for finished crops, and stay clear of 2.2 and up.
Calcium follows transpiration. Tipburn and blossom-end rot are VPD problems first.

Sources

University extension services and peer-reviewed research, plus the ones the VPD Chart & Calculator uses. The kPa figures in this guide are worked from the FAO formula.

Allen, R.G. et al. (1998), FAO Irrigation and Drainage Paper 56, chapter 3, "Meteorological data" ↗. Saturation vapor pressure (equation 11), vapor pressure deficit and dew point.
Wollaeger, H. and Runkle, E., Michigan State University Extension (2015), "Why should greenhouse growers pay attention to vapor-pressure deficit and not relative humidity?" ↗. Air holds twice the water at 80 as at 60 °F, the 1.38 kPa example, 0.3 kPa for rooting, above 0.5 for finishing, RH over 85% and fungal pathogens, and the tomato studies at 0.8 and 2.2 kPa.
Greenhouse Management, chapter 11, "Managing Vapor Pressure Deficit in Greenhouses" ↗. 0.3 to 0.7 kPa during propagation.
Nelson, J.A. and Bugbee, B. (2015), PLoS ONE 10(10): e0138930 ↗. Leaf temperature under sunlight, HPS and LEDs, and leaves cooled by a clear sky.
Corredor-Perilla, I.C., Kwon, T.-H. and Park, S.-H. (2025), Frontiers in Plant Science 16 ↗. Cannabis flowered at 0.25 to 0.62 kPa.
UC Statewide IPM Program, "Tipburn," Lettuce Pest Management Guidelines ↗. Calcium moving in the transpiration stream, low ET, and foliar calcium.
Ernest, E.G. and Johnson, G., University of Delaware Extension (2018), "Blossom End Rot Revisited" ↗. Leaf and fruit competition for calcium, humidity, and foliar sprays.
Ho, L.C. and White, P.J. (2005), Annals of Botany 95(4): 571–581 ↗. Blossom-end rot and calcium in tomato fruit.
Zhang, D. et al. (2017), Scientific Reports 7: 43461 ↗. Lowering midday VPD in a summer tomato greenhouse.
Grossiord, C. et al. (2020), New Phytologist 226(6): 1550–1566 ↗. Stomatal closure and lower photosynthesis as VPD rises.

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