Why the calculated metrics matter
Temperature and humidity alone will mislead you.
The numbers that actually predict growth aren't raw readings - they're what you derive from
them. Here's what VPD, PPFD and DLI tell you, and the bands to hold.
VPD
Vapor Pressure Deficit
kilopascals · kPa
The true driver of transpiration. Temperature and humidity in isolation don't tell you
whether a plant can move water - VPD combines them into the single number that governs
stomata, CO₂ uptake and nutrient flow. Too high and leaves lose water faster than roots
resupply, so stomata close and photosynthesis stalls; too low and transpiration slows,
stunting nutrient uptake and inviting mould and disease.
target Greenhouse sweet spot ≈ 0.45-1.25 kPa (~0.85
ideal)
PPFD
Photosynthetic Photon Flux Density
µmol/m²/s
How many usable light photons land on a square metre of canopy every second - the
intensity that actually drives photosynthesis, not lux or watts. Too dim and growth
stalls; too bright and you bleach leaves and waste energy and money. Targets are
crop-specific, so CropWatch shows the live reading against the band for the crop you're
growing.
target Crop-specific · lettuce 200-400 → tomato
700-1,200
DLI
Daily Light Integral
mol/m²/day
If PPFD is the rate, DLI is the dose - the total light delivered to the canopy over a full
day. It's the number that predicts yield, quality and timing, and it lets you trade
intensity against hours: a gentler PPFD over a longer photoperiod can reach the same DLI.
Track it and you stop guessing whether the crop got enough light today.
target Most greenhouse crops ≈ 12-30 mol/m²/day
