The water budget

Transpiration is water exiting the plant as vapor through the stomata. Most of the water a plant takes up leaves again. Estimates of how much stays vary: one extension source puts the share kept for growth at under 5 percent (Purcell, 2021), another puts the share used in photosynthesis and growth together at about 10 percent (Whiting et al., 2016). Only about 1 to 2 percent of the water a plant transpires goes into photosynthesis itself (Michaels et al., 2022). The overwhelming majority evaporates out of the leaves. Why bother? It pulls the water-and-nutrient column up (the engine from Lesson 5), it cools the leaves, and it's largely an unavoidable side effect of opening stomata for CO₂.

What follows for care

Humidity: dry air increases the vapor gap between leaf and air, so water leaves faster while the stomata are open. Push it far enough and the leaf starts closing them to save water (Lesson 6). Heated indoor air in winter is often dry (University of New Hampshire Extension, 2025).

Air movement: still air saturates near the leaf and slows transpiration; moving air carries that layer away and speeds it up.

Damaged roots can't keep the column supplied, so leaves wilt despite wet substrate. That's how overwatering does its harm, too: the lack of air damages the fine roots (University of Maryland Extension, 2023).

Evapotranspiration: the whole-pot budget

Zoom out to the whole pot. Water leaves as vapor in two main ways: transpiration from the plant and evaporation from the surface of the mix. Their sum is evapotranspiration, a term defined for field crops (Allen et al., 1998).

Fig. 10. Evapotranspiration: transpiration from the plant plus evaporation from the surface of the mix. Both rise with heat, dryness, and air movement.

Footnote on guttation: drops on a leaf can be guttation rather than dew. Dew is moisture from the air that condenses on objects near the ground once they cool below the dew point (National Weather Service, n.d.); guttation is water pushed out through the leaf by root pressure, in drops that look like dew (Michaels et al., 2022). On humid nights with cool air and warm soil, the stomata are shut and transpiration has stopped, so root pressure can push water up into the leaves, where structures called hydathodes (HY-duh-thodes) on the tips and margins of the blade release it (Starbuck, 2009). Not every plant does it, because root pressure itself happens only in some plants (Michaels et al., 2022). The droplets carry dissolved minerals and sugars, but what the plant loses this way is inconsequential (Starbuck, 2009). It can look worrying the first time. Now you know what it is.