🌱 Welcome to a series: The Grower’s Guide to Calculated Metrics
We’re starting a short series to break down key calculated metrics that greenhouse growers use every day – what they are, why they matter, and how to use them effectively.
Why? Because greenhouse sensors collect a lot of data, but data is only valuable when it’s easy to interpret and act on.
Episode 1: VPD (Vapor Pressure Deficit)
What it is:
Vapor Pressure Deficit (VPD) is a measure of the difference between how much moisture the air can hold when saturated and how much it is actually holding.
- Expressed in kilopascal (kPa).
- Tells you how “thirsty” the air is for water vapor, which drives plant transpiration.
Why it matters in plants:
Plants lose water through transpiration (via stomata in their leaves). The rate of this water loss depends strongly on VPD:
- Low VPD (humid air, close to saturation):
- The air is nearly full of moisture
- Transpiration slows, stomata may close → less CO₂ intake → reduced photosynthesis
- Higher risk of fungal diseases (Botrytis, mildew) due to dampness
- High VPD (dry air, far from saturation):
- Air is very “thirsty”
- Plants transpire heavily → risk of water stress and nutrient imbalances
- Stomata may close to prevent excess water loss, again limiting photosynthesis
- Optimal VPD (the “sweet spot”):
- Balanced transpiration
- Good nutrient flow, gas exchange, and steady growth
- Lower stress and minimized disease risk
Typical ranges:
- For most greenhouse crops: 0.5 – 1.2 kPa
- Seedlings → often benefit from lower VPD (more humid)
- Mature fruiting crops → may thrive at slightly higher VPD
How it’s calculated:
VPD = es(T) × (1 – RH/100)
Where:
- es(T) = saturation vapor pressure at temperature T
- RH = relative humidity (%)
How growers use it:
- Climate control rules (heating, venting, misting)
- Irrigation timing and amount
- Crop steering to control vegetative vs. generative growth
📊 With DataPilot:
VPD can’t be measured directly – it’s calculated.
With DataPilot, growers only need to input air temp, leaf temp, and humidity; the system automatically calculates and visualizes VPD trends through custom dashboards, making it easier to track and act on.