VPD Explained Without the Math
VPD combines temperature and humidity into one number describing how strongly the air pulls water out of a leaf. Getting it in range means the plant transpires steadily, which means it feeds steadily.
The ranges
Seedlings and clones: 0.4-0.8 kPa. Vegetative: 0.8-1.2 kPa. Flower: 1.0-1.5 kPa. Higher VPD means drier, thirstier air.
What goes wrong at each extreme
Too low and the plant barely transpires, calcium and other nutrients that move with water flow slow down, and humidity-loving pathogens thrive. Too high and stomata close defensively, growth stops, and leaves curl and crisp.
The practical shortcut
If you hold the stage temp and humidity targets from a standard chart, your VPD is already in range. VPD becomes worth calculating when you are chasing the last 10% of performance or diagnosing a plant that looks fine but will not grow.
Frequently asked questions
- Should I use leaf temperature?
- Ideally yes — leaf surface usually runs 2-4°F below air temperature under LED, which shifts real VPD. An infrared thermometer makes this easy.