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Gardening glossary

Evapotranspiration

Evapotranspiration combines two processes that gardeners usually treat separately. The first is evaporation — water lost directly from wet soil, leaves, and any other surface into the air. The second is transpiration — water pulled out of the soil by plant roots, drawn up through the xylem, and released as vapour through stomata in the leaves.

Together, ET tells you the total water demand of a planted area in millimetres per day. On a hot, dry, windy summer afternoon in the UK, mature plants can transpire 5–8 mm of water (5–8 litres per square metre) per day. On a cool, cloudy, still autumn day, the same plants might lose less than 1 mm.

Four climate variables drive ET up or down:

1. **Temperature** — warmer air can hold more moisture and pulls water out of leaves faster. 2. **Humidity** — drier air increases the vapour-pressure gradient that drives transpiration. 3. **Wind** — moving air sweeps away the saturated layer right next to a leaf, refreshing the gradient. 4. **Solar radiation** — sunlight provides the energy that drives both evaporation and stomatal opening.

Plant-related variables matter too. Leafy mature crops transpire far more than seedlings. Crops in full sun transpire more than those in partial shade. Sandy soils evaporate more freely than clay or well-mulched soils.

How to use ET in practice:

- **Free local ET data.** The UK Met Office and many smart irrigation controllers publish daily ET totals for your postcode. A typical British summer week often runs 25–35 mm of ET. - **Replace what is lost.** If ET for the week is 30 mm and natural rainfall was 10 mm, your garden has lost roughly 20 mm net — about 20 litres per square metre to irrigate. - **Mulch slashes evaporation** by 60–80%, leaving transpiration as the main remaining loss. - **Container plants** lose more relatively because they have higher surface-to-volume ratios; this is why a 10-litre pot can dry out in 24 hours.

For most home gardeners, an evapotranspiration mindset replaces the calendar-based "water every Tuesday" habit with a much better question: how much water has my garden lost since I last watered?

Where this comes up in our guides

Related terms

Capillary action

Capillary action is the upward movement of water against gravity through narrow spaces in a porous material — soil pores, paper towels, or plant xylem. It is the physical principle behind bottom watering, self-watering pots, and the rise of water from roots to leaves.

Wicking

Wicking is the use of a porous strip — usually cotton or synthetic cord — to move water from a reservoir into a pot via capillary action. The plant draws what it needs and stops, eliminating the over- and under-watering swings of conventional watering.

Soak and drain

Soak and drain is a top-watering technique where you water thoroughly until liquid runs freely from the drainage holes, then let the substrate dry to the appropriate depth before watering again — never on a fixed schedule.

Overwatering

Overwatering is the chronic condition where soil stays saturated for too long, suffocating roots and triggering rot. It is usually caused by watering too frequently, poor drainage, or oversized pots — not by giving too much water in a single session.

Underwatering

Underwatering is when a plant goes too long without enough moisture, causing leaves to wilt, crisp, and curl as cells lose turgor. Mild drought stress is recoverable; chronic underwatering shrinks root systems and stunts growth long-term.