Morning is best, evening is fine, and a deep soak beats a daily sprinkle

Water in the morning if you can. The soil is cool, the plant has the day to take the water up, and any damp on the leaves dries quickly. Evening is the next best time. The water soaks in with less evaporation, and the plant has the night to recover. Midday is the worst of the three, but it is not a disaster.

The bigger decision is how much and how often.

A soak, not a sprinkle

Water thoroughly, then leave the bed alone for a week or so. A daily light watering wets the top centimetre, keeps roots near the surface and wastes water. A deep soak reaches the root zone and encourages roots to go down after it.

A useful guide is about 20 to 25 litres per square metre every 7 to 10 days in dry weather. That is roughly two to two and a half full watering cans on a square metre, or a hose left running long enough to soak to a spade’s depth. Sandy soil holds less, so it may need half the amount twice as often. Clay holds more but takes water slowly, so apply it in stages and let it soak in.

Rain counts. One millimetre of rain on one square metre is one litre. A 10 mm shower gives 10 litres per square metre, which is a proper soak. A 2 mm shower barely reaches the roots.

Check before you water

Push a trowel or a spade into the soil and look at it a spade’s depth down. If it is damp, wait. If it is dry, water.

The feel of the soil can mislead. Clay feels damp even when the water left in it is no longer available to roots. Sand feels dry even when there is some moisture. Dig, squeeze a handful, and look at whether it holds together. After a few seasons you will know what your soil is telling you.

What to water first

If water is short, spend it where it changes the crop:

• Seedlings and transplants. They have small roots and no reserve. Water them at planting and for the first week or two.
• Leafy crops. Lettuce, spinach and cabbage need steady moisture. A dry spell makes them bolt or stop growing.
• Flowering and fruiting crops. Peas, broad beans, runner beans, courgettes, tomatoes and sweetcorn need water when flowers and fruit are forming.
• Potatoes. Water every 10 to 14 days once the tubers are marble-sized.
• Carrots, beetroot and parsnips. Water before the soil dries out, for example after 14 days without rain. They will survive longer, but the roots get woody.

Onions, garlic, leeks and established herbs need the least. Water them at establishment and in very dry spells, then leave them.

How to water

Water at the base of the plant, not over the leaves. Make a shallow basin around the stem on a sloping bed so the water stays long enough to soak in. Use a rose on the can for seedlings so you do not wash them out.

Water in two passes if the ground is very dry. The first pass wets the surface and breaks the crust; the second soaks in. On clay, this matters more than the total volume.

A mulch of 5 to 7.5 cm of compost or straw after watering will hold that moisture for days longer. See how to mulch a vegetable bed.

Signs you are getting it wrong

Wilting at midday is normal. Many plants lose turgor in the heat and recover by evening. Wilting at dusk, or first thing in the morning, means the soil is genuinely dry.

Leaf curl on tomatoes, a dull grey-green colour on brassicas and a pause in growth all point to water stress. So does a bed where the surface is damp but a trowel comes up dry.

If you are on a hosepipe ban, the same rules apply with a can. Hosepipe bans and allotments explains what is allowed.