🌡️ Enter Your Conditions
📊 Your VPD Reading
VPD Heatmap Chart
Find the cell matching your temperature and humidity to see your VPD zone at a glance. Green = optimal for veg. Yellow-orange = optimal for flower.
Leaf VPD by Temperature & Humidity (kPa)
Leaf temp assumed 2°F below airStage-by-Stage VPD Reference Table
Target ranges for temperature, humidity, and VPD at each growth stage. Both Imperial (°F) and Metric (°C) shown.
| Stage | Temp °F | Temp °C | RH% | Target VPD (kPa) | Priority | Notes |
|---|---|---|---|---|---|---|
| Seedling / Clone | 70–77°F | 21–25°C | 65–80% | 0.4 – 0.8 | Low transpiration | Roots not yet established; stomata must stay open |
| Early Veg (wk 1–3) | 72–80°F | 22–27°C | 60–70% | 0.8 – 1.0 | Active growth | Begin ramping up airflow and nutrient uptake |
| Late Veg (wk 4–6) | 75–82°F | 24–28°C | 55–65% | 1.0 – 1.2 | Peak growth | High transpiration drives rapid nutrient delivery |
| Early Flower (wk 1–4) | 75–82°F | 24–28°C | 50–60% | 1.0 – 1.3 | Transition | Begin reducing humidity to lower mold risk |
| Peak Flower (wk 5–9) | 75–82°F | 24–28°C | 45–55% | 1.2 – 1.5 | Dense buds | Balanced between resin production and stress |
| Late Flower / Pre-Harvest | 72–78°F | 22–26°C | 35–45% | 1.4 – 1.6 | Mold prevention | Lower temp + low RH minimizes botrytis risk |
How VPD Is Calculated
The mathematics behind vapor pressure deficit — and why leaf temperature matters more than air temperature.
The Formula
VPD is calculated using the Buck equation to find saturation vapor pressure (SVP), then multiplying by (1 − RH/100) to get the actual deficit:
SVP = 0.61078 × e^(17.269 × T / (T + 237.3))
VPD = SVP × (1 − RH / 100)
Leaf VPD = SVP(T_leaf) × (1 − RH / 100)
Why Leaf VPD Matters
Plant stomata respond to the vapor pressure difference at the leaf surface, not in the ambient air. Because transpiration cools the leaf, leaf surface temperature is typically 1–3°C lower than air temperature.
Using air VPD overestimates the actual driving force plants experience. Leaf VPD gives a more precise reading — especially important when dialing in precision grows.
For beginners: Air VPD is close enough and requires no extra equipment.
For precision growing: Use an infrared thermometer to measure actual leaf surface temperature, then use that value instead of air temperature.
- Leaf temp typically 2°F (1°C) below air under LED
- Leaf temp can be 4–5°F below air under HPS (more radiant heat)
- Stressed or wilting plants may have higher leaf temps than air
VPD Troubleshooting Guide
Common VPD problems and exactly how to fix them in your grow room or tent.
VPD Too Low (<0.4 kPa)
Symptoms: Stomata closed, slow growth, droopy leaves, yellowing, poor nutrient uptake, increased disease risk.
Causes: Humidity too high (above 80%), temperature too low, or both.
Fixes:
- Add a dehumidifier — target 55–65% RH for veg
- Increase temperature by 2–4°F
- Improve air circulation with clip fans
- Check for cold spots near floor or corners
- Reduce watering frequency (over-watering raises ambient RH)
VPD Too High (>1.6 kPa)
Symptoms: Wilting, leaf curling/cupping upward, brown leaf tips, stunted growth, heat stress, nutrient lockout.
Causes: Temperature too high (above 85°F/29°C), humidity too low, or both.
Fixes:
- Add AC or upgrade cooling — keep under 82°F (28°C)
- Add a humidifier — target 50–65% RH for veg
- Increase intake fan speed for fresh cooler air
- Consider silica supplements — increases plant heat tolerance
- Raise lights if LEDs are causing canopy heat
VPD Swings Between Day/Night
Problem: Lights-off causes temperature to drop and RH to rise, swinging VPD from 1.2 kPa (lights on) to 0.5 kPa (lights off). Wide swings stress plants and increase mold risk.
Fixes:
- Maintain lights-off temp within 5–8°F of lights-on temp
- Use a humidity/temperature controller (e.g., Inkbird, Ranco) to maintain stable conditions
- Run a small circulation fan during dark period
- Monitor with a data logger to see overnight drift
Automating VPD Control
Manual VPD management is time-consuming. Environmental controllers automate this by running HVAC, humidifiers, and dehumidifiers based on real-time sensor data.
Budget tier: Inkbird humidity controller (~$30) + plug-in dehumidifier
Mid tier: Govee or Inkbird with AC + humidifier + dehumidifier on separate outlets
Pro tier: TrolMaster or Pulse Pro with full HVAC integration and historical data logging
GrowAI tier: Real-time VPD monitoring with instant alerts when conditions drift out of target range — all on your phone.
VPD Frequently Asked Questions
The most common questions about VPD in grow rooms.
Monitor VPD Automatically — 24/7
Stop manually checking temperature and humidity. GrowAI monitors your grow room in real-time and alerts you the moment VPD drifts out of your target range.
Start Monitoring Free →