What every grower should know.
VPD - Vapor Pressure Deficit
The difference between how much moisture is in the air versus how much it can hold. Optimal VPD ranges change as plants grow:
- Vegetative: 0.8-1.2 kPa
- Flowering: 1.0-1.5 kPa
DLI - Daily Light Integral
Total photosynthetically active radiation (PAR) delivered in a day, measured in mol/m²/day:
- Leafy greens: 12-17 mol
- Fruiting plants: 20-30 mol
- Flowers: 25-40 mol
Photosynthesis
Plants convert light energy into chemical energy using CO₂ and water:
6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
Transpiration
Water movement through plants, cooling leaves and pulling nutrients. Affected by:
- Temperature
- Humidity
- Light intensity
Temperature Ranges
Optimal leaf temperature for most plants:
- Day: 75-85°F (24-29°C)
- Night: 65-75°F (18-24°C)
- RTWC: 65-70°F (18-21°C)
Humidity Control
Relative humidity by growth stage:
- Clones: 70-80%
- Vegetative: 50-70%
- Flowering: 40-50%
What can go wrong?
Temperature Too High
Leaves curl upward, edges brown, growth slows. Open stomata close to conserve water.
Solutions
- Increase exhaust fan runtime
- Add intake fan for air exchange
- Move lights further from canopy
Temperature Too Low
Leaves droop, growth stalls, roots struggle. Nutrient uptake slows significantly.
Solutions
- Reduce exhaust fan runtime
- Add heat lamps or pad heaters
- Insulate grow area
Humidity Too High
Mold and mildew risk increases, leaves stay wet. Pests thrive in damp conditions.
Solutions
- Increase air circulation
- Reduce watering frequency
- Add dehumidifier or dry air source
Humidity Too Low
Plants lose water too fast, leaves curl downward, tips brown. Stressed plants.
Solutions
- Use humidifier or misting system
- Reduce exhaust fan runtime
- Group plants together