Should I water the leaves or the soil?

This is one of the few pieces of gardening advice that is nearly universal, and the reasons behind it are worth knowing because they tell you when to break it.

Step by step

1. Aim at the ground, not the plant.Take the rose off the can or set the hose to a gentle flow and put the water at the base of the plant, on the soil, as close to the ground as you can manage without washing soil away.
2. Water slowly enough that it soaks in.Water applied faster than the soil can take it runs off sideways and takes soil with it. A slow trickle at the base outperforms a fast blast every time, particularly on dry or sloping ground.
3. Build a shallow basin around anything important.A low rim of soil around a newly planted tree or shrub, or a shallow saucer-shaped depression, holds the water where you put it until it soaks in instead of letting it run away.
4. Use a sunken pipe or pot for deep-rooted plants.A length of pipe or an upturned plant pot sunk beside a tomato, courgette or newly planted shrub delivers water straight down past the surface, where it cannot evaporate and cannot run off.
5. Water in the morning if you can.Any splash onto foliage then dries within the hour. The same splash in the evening keeps leaves wet all night, which is exactly the condition fungal disease needs.
6. Know the exceptions.Damping down a greenhouse floor in heat to raise humidity and cool the air; hosing dust or aphids off foliage as a control measure; misting a few humidity-loving plants. None of these is watering the plant and none replaces watering the soil.

Tips

• Overhead watering is the standard route by which potato and tomato blight, rose black spot and courgette mildew move around a garden — the splash carries spores from soil and leaf to leaf.
• Squash, courgette, tomato, potato and rose are the plants where the difference shows most clearly. If you have persistent mildew or blight, changing how you water is a free intervention.
• A sprinkler on a border is overhead watering by another name. On a lawn it is the appropriate tool; on planting it wets foliage, misses roots and loses a great deal to evaporation.
• Wetting the leaves of a wilting plant makes it look better for ten minutes without doing anything for it. The wilt returns because the roots still have nothing.

Common mistakes

• Spraying over a bed with a hose on a fine setting — It wets the foliage, wets the top centimetre of soil, and evaporates. It feels like watering and delivers almost nothing to roots.
• Watering in the evening onto leaves in still humid weather — The foliage stays wet all night. That is the condition mildew, botrytis and blight need, and it is entirely avoidable.
• Watering the trunk of an established tree or shrub — The absorbing roots are near the edge of the canopy, not at the stem. Water at the trunk mostly runs down the outside of the root ball.

Questions people ask