# Transpiration

> The evaporation of water from a plant, mainly through open stomata, which pulls water and dissolved nutrients up from the roots.

- Canonical: https://genetixbay.com/knowledge/glossary/transpiration.html
- Also written: transpire, transpiring, evapotranspiration, leaf water loss
- Glossary category: Plant biology · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Transpiration is water loss from plant surfaces as vapour, almost all through [stomata](https://genetixbay.com/knowledge/glossary/stomata.md). As water evaporates inside the leaf, tension pulls a continuous water column up the [xylem](https://genetixbay.com/knowledge/glossary/xylem.md) 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](https://genetixbay.com/knowledge/glossary/vpd.md) (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.

## Related terms

[Stomata](https://genetixbay.com/knowledge/glossary/stomata.md), [Xylem](https://genetixbay.com/knowledge/glossary/xylem.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [Relative humidity](https://genetixbay.com/knowledge/glossary/relative-humidity.md)

## Read more

- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)
- [Humidity by stage: RH targets from clone to cure](https://genetixbay.com/knowledge/environment/humidity-by-stage.md)

## Sources

1. Taiz L, Zeiger E, Møller IM, Murphy A. 2015. Plant Physiology and Development, 6th edition. Sinauer Associates.