Transpiration is water loss from plant surfaces as vapour, almost all through stomata. As water evaporates inside the leaf, tension pulls a continuous water column up the xylem from the roots. That flow also carries calcium and other ions that move with water, and it cools the leaf.
In practice
The main driver is vapour pressure deficit (VPD): how much drier the air is than the inside of the leaf. Higher light, warmer leaves and drier air raise transpiration; still, humid air lowers it.
What that means in the grow: - Too low (humid, still air, VPD below about 0.4-0.5 kPa): slow water and calcium movement, guttation, and conditions for mould. - Too high (hot, dry air, VPD above about 1.6 kPa): stomata close, leaves taco or droop, and growth slows. - Calcium-related tip problems on fast new growth often reflect low transpiration, not low calcium in the feed.
A large plant in flower can transpire litres per day. That is why rooms gain humidity with canopy size.
Don't confuse with
- Evaporation from the medium: water lost from the pot surface, not through the plant.
- Guttation: liquid droplets pushed out at leaf edges by root pressure.
- Relative humidity: one input to VPD, not the plant's water loss.
