VPD Explained: The Metric That Controls Plant Transpiration
Temperature and humidity get all the attention. But it's the relationship between them — Vapor Pressure Deficit — that actually determines how fast your plants transpire and absorb nutrients.
What is VPD?
VPD stands for Vapor Pressure Deficit. It measures the difference between how much moisture the air could hold (at 100% relative humidity) and how much it currently holds. The unit is kilopascals (kPa).
Think of it as a measure of the air's "thirst" for water. High VPD means the air is dry and eager to pull moisture from any source — including your plants' leaves. Low VPD means the air is already saturated and water evaporates slowly.
This matters because healthy leaves are essentially at 100% humidity internally. VPD tells you how aggressively moisture will escape from those leaves into the surrounding air.
Same Humidity, Different VPD
At a fixed relative humidity, VPD changes with temperature:
| Temperature | Relative Humidity | VPD |
|---|---|---|
| 18°C (65°F) | 60% | 0.84 kPa |
| 24°C (75°F) | 60% | 1.19 kPa |
| 29°C (85°F) | 60% | 1.64 kPa |
Same 60% humidity — but evaporation rate nearly doubles as temperature rises.
Why VPD Matters
Plants pull water and nutrients up from their roots through a process called transpiration — water evaporating from the leaves creates suction that draws more water up from below. VPD controls the rate of this process.
VPD Too Low
Water can't evaporate from leaves fast enough. Transpiration slows. Nutrient uptake suffers. Plants become sluggish, prone to calcium and magnesium deficiencies, and susceptible to mold and mildew in the stagnant, humid conditions.
VPD Too High
Water evaporates faster than the plant can replace it. Stomata close to prevent desiccation. Photosynthesis slows. In extreme cases, you'll see wilting, leaf curl, tip burn, and nutrient lockout from over-concentration.
The goal is to stay in the sweet spot where transpiration is active but not excessive — pulling nutrients efficiently without stressing the plant.
Ideal VPD by Growth Stage
Plants can handle — and benefit from — higher VPD as they mature. Young plants with undeveloped root systems can't replace water fast enough to keep up with aggressive transpiration. Mature flowering plants can handle more.
| Growth Stage | Target VPD (kPa) | Notes |
|---|---|---|
| Clones & Seedlings | 0.4 - 0.6 | Fragile roots, minimal transpiration capacity |
| Vegetative | 0.8 - 1.0 | Established roots, active growth |
| Early Flower | 1.0 - 1.2 | Transition period, stretch phase |
| Mid/Late Flower | 1.2 - 1.5 | Maximize nutrient uptake, lower humidity prevents mold |
Make Changes Gradually
Plants don't like sudden environmental shifts. When adjusting VPD between growth stages, make changes over 2-3 days rather than all at once. Shock can set back growth more than the "wrong" VPD would.
The Leaf Temperature Problem
Here's where VPD gets complicated. The VPD that matters is the pressure deficit between the leaf surface and the surrounding air — not just the air temperature and humidity you measure with a sensor.
The problem? Leaf temperature is almost never the same as air temperature. And it varies dramatically across a single plant.
What Affects Leaf Temperature
- • Transpiration itself — evaporating water cools leaves, just like sweat cools skin
- • Light intensity — leaves closer to lights absorb more energy and heat up
- • Light spectrum — LEDs produce less infrared heat than HPS, so leaves run cooler under LEDs
- • Airflow — moving air carries away heat and affects evaporation rate
- • Leaf size and thickness — larger, thicker leaves retain more heat
- • Position in canopy — top leaves are warmer than shaded lower leaves
Thermal imaging of a single plant can show a 10-12°C temperature range across different leaves. The top of the canopy might be 30°C while shaded lower leaves are 22°C — same plant, same moment, wildly different VPD at the leaf surface.
Why Infrared Thermometers Fall Short
The obvious solution seems to be measuring leaf temperature directly with an infrared thermometer. Point, click, done. But this approach has serious limitations.
An IR thermometer gives you the temperature at one point — not the average leaf temperature across your canopy. If you measure a sun-scorched top leaf, you'll get a high reading. Measure a shaded lower leaf, you'll get a low one. Neither represents the "true" average leaf temperature your plants are experiencing.
Even if you take multiple readings and average them, you're likely overweighting the upper canopy (which is more visible and accessible) while underweighting the much larger surface area of mid and lower leaves.
The Practical Solution: Use an Offset
Rather than obsessively measuring leaf temperature, use a consistent leaf temperature offset when calculating VPD. This is the typical difference between air temperature and average leaf temperature for your specific setup.
LED Grows
Leaves typically run 1-3°C cooler than air temperature
HPS Grows
Leaves may run close to air temperature or slightly warmer due to infrared
At very high PPFD (1500+ µmol), the offset decreases as light energy warms the leaves more.
OMGROW Handles This Automatically
The OMGROW system has a default leaf temperature offset built in when calculating VPD — you don't need to configure anything. Better yet, our light sensors track your day/night cycles automatically, and the system adjusts the offset accordingly. During lights-on, a larger offset is applied; when lights go off and stomata close, the offset reduces toward zero. No manual switching required.
Don't Chase "Perfect" VPD
We've seen growers obsess over hitting exact VPD targets — adjusting humidity and temperature multiple times per day based on spot measurements. This usually does more harm than good.
Problems with Over-Optimization
- ✗ Constant environmental changes stress plants more than a slightly "off" but stable VPD
- ✗ Your leaf temperature measurements are probably wrong anyway
- ✗ You may push other parameters into dangerous territory (too humid, too cold) just to hit a VPD number
A better approach: set your environment to reasonable temperature and humidity ranges for the growth stage, apply a consistent leaf temperature offset when thinking about VPD, and leave it alone. Plants are more resilient than the charts suggest.
VPD at Night
When lights go off, air temperature drops — but you still want to maintain roughly the same VPD to avoid shocking the plants. This means humidity needs to drop proportionally.
There's another factor: most plants close their stomata at night, drastically reducing transpiration. Without evaporative cooling, leaf temperature rises closer to air temperature. At night, you can assume a leaf temperature offset closer to 0°C to -1°C rather than the larger offset you'd use during the day.
Night-Time Rule of Thumb
If your day temperature is 28°C at 55% RH, and night drops to 22°C, you'll need to reduce humidity to around 45-50% to maintain similar VPD. A dehumidifier running at night is common practice.
VPD is Not Everything
It's possible to hit "perfect" VPD while running an environment that's terrible for your plants. VPD charts don't override common sense.
Temperature Still Matters
Running 18°C with 40% humidity might give you a decent VPD — but it's too cold for optimal metabolism. Plants will be sluggish regardless of what the VPD says.
Humidity Has Limits
If your VPD chart says you need 75% humidity during flower, something is wrong. That's an invitation for Botrytis (bud rot) regardless of what the math says about transpiration.
Use VPD as one tool among many. Keep temperature in the 25-30°C range for LED grows. Keep flowering humidity under 60%. Hit the right PPFD for the growth stage. Then worry about fine-tuning VPD within those constraints — not the other way around.
Calculating VPD
For those who want to do the math themselves, VPD can be calculated using the Tetens equation for saturation vapor pressure:
SVP = 0.61078 × e(17.27 × T / (T + 237.3))
VPD = SVP × (1 - RH / 100)
Where T = temperature in °C, RH = relative humidity in %, e = Euler's number (~2.718)
For leaf VPD (which is what actually matters), you calculate SVP at the leaf temperature and subtract the actual vapor pressure at air conditions:
Leaf VPD = SVPleaf - (SVPair × RH / 100)
In practice, VPD charts and calculators handle this for you. The key is using the right leaf temperature offset for your lighting setup.
Summary
- ✓ VPD measures the air's "thirst" — how aggressively it pulls moisture from leaves
- ✓ Target 0.4-0.6 kPa for seedlings, 0.8-1.0 for veg, 1.0-1.5 for flower
- ✓ Leaf temperature ≠ air temperature — use a consistent offset (typically -1 to -3°C for LEDs)
- ✓ IR thermometer spot checks won't give you accurate average leaf temp
- ✓ Don't chase perfect VPD — stable conditions beat constant adjustments
- ✓ VPD is one parameter — don't sacrifice safe temp/humidity ranges just to hit a number
VPD Calculated Automatically
Our sensors report temperature, humidity, and leaf-adjusted VPD to your dashboard — with the offset applied automatically based on your lights-on/lights-off cycle. No configuration, no guesswork.
See Temperature Sensors