The short version: Water in the morning when you can. Leaves dry during the day, so leaf diseases get less of a chance, and the plant meets the heat with a full tank. Evening is second best if the water goes on the soil, not the leaves. Midday is wasteful rather than harmful. Water only when the soil is dry a finger deep.
Why morning wins
- Plants lose most of their water during the day, so a morning soak is in the root zone before demand peaks.
- Many leaf diseases, downy mildew and leaf spots among them, need hours of wet leaf to take hold. Leaves wetted in the morning usually have hours of drying time ahead; leaves wetted at dusk often stay wet until morning.
- Powdery mildew is the odd one out. It infects dry leaves in humid air, so wet foliage is not what starts it; crowded plants with little airflow are.
- Slugs and snails move at night and love a damp surface. Evening watering lays out the welcome mat.
- Morning air is cool and still, so less of the water evaporates before it soaks in.
If mornings are not possible
The hour matters most when water lands on the leaves. Drip lines, a soaker hose or a can at the base keep foliage dry, and with those the evening is as good as the morning. Some water utilities also set the hours you may water, so check yours.
- Water in the early evening rather than at night, so the surface has some drying time.
- Put the water on the soil at the base of each plant with a can or a wand, not over the top with a sprinkler.
- Mulch the bed. A mulched surface stays damp longer, so you can water less often and the timing matters less.
- In a heatwave, check again in the early evening and water only if the soil is dry 5 cm down. Pots in particular may need a second drink in a day.
What about the middle of the day
Water droplets do not scorch the leaves of most vegetables; that fear is larger than the evidence. The real cost of midday watering is evaporation: a good share of what leaves the hose is gone before it reaches the roots. The exception is a plant that is flagging at two in the afternoon. Push a finger 5 cm into the soil, and if it is dry at that depth, water the soil at once rather than waiting for evening. Run a hose that has been lying in the sun until the water comes out cool first. A wilted plant is losing more than you are.
The garden is at its best early in the morning anyway, and the leaves will be dry long before dark.
Related reading
A good watering habit is the cheapest start for anything from the vegetable seed range.