What Is VPD?
VPD stands for Vapor Pressure Deficit. In simple terms, it measures how much "drying power" the air has — how aggressively the air is pulling moisture from your plants' leaves.
Think of it this way: your plant breathes through tiny pores on its leaves called stomata. When the air around the leaf is dry, water evaporates quickly through these pores. When the air is humid, evaporation slows down. VPD puts a number on this difference.
A higher VPD means the air is pulling water from leaves faster. A lower VPD means the air is nearly saturated and evaporation is slow. Both extremes cause problems. The goal is to find the sweet spot where your plants transpire at a healthy, productive rate.
Why VPD Matters More Than Temperature or Humidity Alone
Most growers track temperature and humidity separately. The problem is that the same humidity percentage means very different things at different temperatures.
For example, 60% relative humidity at 20°C is a completely different environment than 60% relative humidity at 30°C. At 30°C, the air can hold far more water, so 60% RH still means the air has strong drying power. At 20°C, 60% RH is more gentle.
VPD captures this relationship in a single number. Instead of juggling two variables and trying to remember the right combination for each stage, you can monitor one value that tells you exactly how your plants experience the climate.
How VPD Is Calculated
The formula behind VPD is straightforward once you understand the components:
- Saturation Vapor Pressure (SVP) — The maximum amount of water vapor the air can hold at a given temperature. Warmer air holds more. This is calculated at the leaf surface temperature.
- Actual Vapor Pressure (AVP) — The amount of water vapor currently in the air. This is derived from the air temperature and relative humidity.
VPD is the difference between these two values:
VPD = SVP(leaf temperature) - AVP(air temperature, humidity)
GrowVPD Pro handles all of this math automatically. You enter temperature and humidity, and the app displays your VPD value instantly.
Leaf Temperature vs. Air Temperature
This is a detail many growers overlook. The VPD that matters is calculated at the leaf surface, not at your thermometer hanging on the wall.
Leaves are typically 1–3°C cooler than the surrounding air due to transpiration (evaporative cooling). Under intense lights, the offset can be smaller. In low-light conditions, it can be larger.
In GrowVPD Pro, you can adjust the leaf temperature offset in the VPD chart settings. If you have an infrared thermometer, measure a few leaves to find your actual offset. Otherwise, the default 2°C offset is a solid starting point.
Tip: If you have a smart temperature sensor placed at canopy level, you will get readings much closer to actual leaf conditions than a sensor mounted on the wall or near the exhaust.
VPD Zones Explained
VPD is measured in kilopascals (kPa). Here are the zones and what they mean for your plants:
| Zone | VPD Range | What Happens |
|---|---|---|
| Under-transpiration | < 0.4 kPa | Air nearly saturated. Stomata close, nutrient transport slows, mold and mildew risk increases significantly. |
| Seedling / Clone | 0.4 – 0.8 kPa | Gentle transpiration. Ideal for seedlings, fresh clones, and transplants that have limited root systems. |
| Vegetative | 0.8 – 1.2 kPa | Strong, healthy transpiration. Plants grow vigorously, nutrient uptake is efficient, stems strengthen. |
| Flowering | 1.0 – 1.5 kPa | Optimal for flower development. Slightly drier air encourages resin production and reduces bud rot risk. |
| Over-transpiration | > 1.5 kPa | Plants lose water faster than roots can supply. Stomata close defensively, causing nutrient lockout, wilting, and heat stress. |
Important: These ranges are guidelines, not hard rules. Healthy, well-established plants with strong root systems can handle slightly higher VPD. Stressed plants or plants under intense light may need the lower end of each range.
How to Read the VPD Chart in GrowVPD Pro
The VPD tab in the app displays an interactive heatmap. The horizontal axis shows temperature, the vertical axis shows humidity, and each cell is colored based on the resulting VPD value.
- Blue — VPD too low. Air is too humid for the temperature.
- Green — Optimal VPD. This is your target zone.
- Yellow — VPD getting high. Plants are starting to work hard.
- Orange / Red — VPD too high. Plant stress is likely.
Use the stage selector at the top to switch between seedling, vegetative, and flowering targets. The green zone shifts accordingly, because younger plants need lower VPD and flowering plants tolerate (and benefit from) higher VPD.
Tap any cell to see its exact values. If you have a connected smart sensor or Bluetooth sensor, the chart highlights your current position with a marker so you can see at a glance where you stand.
How to Adjust VPD
VPD Too High (Plants Wilting, Leaves Curling Up)
Your air is too dry relative to the temperature. To bring VPD down:
- Increase humidity — Run a humidifier, place wet towels in the tent, or mist the walls (not the plants directly during flower).
- Lower temperature — Increase exhaust, raise lights slightly, or reduce light intensity.
- Reduce airflow directly on plants — Oscillating fans pointed slightly above the canopy instead of directly at leaves.
VPD Too Low (Risk of Mold, Slow Growth)
Your air is too humid for the temperature. To bring VPD up:
- Decrease humidity — Run a dehumidifier, increase exhaust fan speed, improve air exchange.
- Increase temperature — Run lights at full power, reduce exhaust slightly, add a small heater if needed.
- Increase airflow — More air movement helps evaporate moisture from leaf surfaces.
Pro tip: If you use GrowVPD Pro with smart automation, you can set rules like "If VPD drops below 0.7 kPa, turn on the dehumidifier" or "If VPD exceeds 1.4 kPa, activate the humidifier." The app handles this automatically through the Automation tab.
Night vs. Day VPD
When lights go off, temperature drops but humidity often stays the same or rises (since plants continue to release moisture for a while). This means VPD drops at night — sometimes dramatically.
A VPD that was perfect during lights-on can become dangerously low during lights-off, leading to condensation on leaves and bud rot. This is why many experienced growers:
- Run the dehumidifier harder during dark periods
- Keep a small temperature differential between day and night (no more than 5–8°C drop)
- Maintain good airflow even when lights are off
In GrowVPD Pro, you can log both day and night temperature in your diary entries to track how your VPD changes across the full 24-hour cycle.
Common VPD Mistakes
- Ignoring leaf temperature. Using air temperature alone overestimates VPD. Account for the 1–3°C leaf offset.
- Not adjusting for growth stage. Seedlings cannot handle the same VPD as flowering plants. Always match your target to the current stage.
- Measuring in the wrong spot. A sensor at the top of the tent reads different from one at canopy level. Place your sensor where the plants are.
- Chasing a single perfect number. Consistency matters more than precision. Staying within a 0.2 kPa range consistently beats hitting 1.0 kPa once and swinging wildly.
- Forgetting nighttime. VPD management does not stop when the lights go off. Night is when mold strikes.
Using VPD with Smart Automation
GrowVPD Pro's Automation tab lets you create VPD-based rules that control your smart devices automatically. For example:
- If VPD exceeds 1.3 kPa, turn on the humidifier
- If VPD drops below 0.6 kPa, increase exhaust fan speed
- If temperature rises above 30°C, dim lights to 80%
The automation engine includes hysteresis (so devices do not flip on and off rapidly), time-based overrides, and safety limits. Check out our Tuya and Mars Hydro connection guides to get your devices set up.