Watering at the hour that guarantees wet leaves overnight

Six in the evening. The heat has gone, the plants look grateful, nothing is evaporating away before it reaches the roots. It is the considerate hour and it feels like the right one.

For the soil it is. For the leaves it is close to the worst possible time.

What you have actually done

The foliage is now wet, with perhaps two hours of drying weather left before the light goes.

Then the temperature falls, surfaces cool below the dew point, and dew arrives to top up whatever has not dried. The leaves stay wet through the night and into the morning until the sun clears them.

Five extra hours, every single time you water. Over a season that is the difference between mildew establishing and not, because leaf wetness duration is the variable spores actually respond to.

Why morning is better on both counts

The leaves dry. A morning watering has the whole day to evaporate off the foliage, so the overnight dew starts from a dry leaf.

The roots get it when it matters. Water in the soil before the day's heat is water available during the hours the plant is transpiring hardest.

The usual objection is evaporation loss, and it is real but smaller than it feels — most of what you apply in the early morning reaches the soil, because the sun is low and the air still cool.

If evening is the only option

Water at the base. Soaker hose, drip line, or a can with the rose taken off, aimed at the soil.

This removes the problem completely. The soil gets its water, the leaves never get wet, and the only moisture on the foliage overnight is the dew that would have arrived anyway.

Overhead sprinklers in the evening are the specific combination to avoid.

The midday myth

Watering in bright sun does not scorch leaves. The droplet-as-lens effect is far too weak on the flat leaves of ordinary garden plants to cause burning, and the studies that looked for it did not find it outside of hairy-leaved species.

Midday watering is simply inefficient — a good deal evaporates before it reaches the roots. That is a reason to avoid it, but not the dramatic one usually given.

Reading the evening

How much drying time an evening watering actually gets depends on when the dew arrives, and that depends on the dew point spread:

• Wide spread, breezy — dew late or not at all. Evening watering is much less costly.
• Narrow spread, clear, still — dew soon after dark. Evening watering buys almost no drying time at all.

The forecast shows both lines, so on the nights when it genuinely does not matter you can water when it suits you — and on the nights when it does, aim at the soil.