The water budget

Why the same plant dries out faster in winter, and faster in a wide pot.

Transpiration is water exiting the plant as vapor through the stomata. Almost none of the water a plant drinks stays in it: less than 5% is retained for growth, and only about 1 to 2 percent of the water moving through is actually consumed in photosynthesis to build sugar (Purcell, 2021; 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. The same plant dries out faster in winter heated air than in summer humidity. It has nothing to do with how much the plant is using and everything to do with how fast it's losing.

Air movement: still air saturates near the leaf and slows transpiration; moving air carries that layer away and speeds it up. A grow-room fan is half disease prevention, half driving the engine.

Damaged roots can't keep the column supplied, so leaves wilt despite wet substrate. Same outcome as overwatering, reached by a different path.

Evapotranspiration: the whole-pot budget

Zoom out to plant + pot + substrate and water leaves by two routes: transpiration from the plant and evaporation from the substrate surface. Their sum is evapotranspiration. This is why a wide shallow pot dries faster than a tall narrow one of equal volume (more exposed surface), and why mulching the surface extends time between waterings.

Evapotranspiration: vapor from leaves and substrate surface.

Fig. 10. Total water loss = transpiration from the plant + evaporation from the substrate surface. Both rise with heat, dryness, and air movement.

Footnote on guttation: those beads at the leaf tips first thing in the morning aren't dew. Dew condenses onto a leaf out of the air; this comes out of the leaf. Overnight, when transpiration is low, the roots keep pumping salts into the xylem and water follows, building pressure that pushes sap out through permanent pores called hydathodes at the tips and margins (Singh, 2016). It's a sign the roots are working, and mostly nothing to worry about. Two small caveats: the droplets carry dissolved minerals and sugars, so as they dry they leave a pale crust on the leaf tip and can spot a windowsill; and because hydathodes are permanently open pores, they're one route bacteria use to get into a leaf, so it's worth not leaving foliage sitting wet for long stretches (Paauw et al., 2023). New keepers panic. You now know what it is.

From memory Tier 1

Strengthen your understanding

Try these in your head first, then open to see.

Same plant, same pot: why does it need water far more often in January than in July?

Heated winter air is drier, which widens the vapor gap between leaf and air, so the plant transpires faster, independent of how much water it's actually using.

Two identical plants, equal soil volume: one in a wide shallow pot, one tall and narrow. Which dries out faster, and why?

The wide shallow one: more exposed substrate surface means more direct evaporation, on top of transpiration. That combined loss is evapotranspiration.

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