When to Water
Water in the morning or in the evening, but never during the heat of the day, during which time you waste the water because it evaporates quickly.
Water in the morning if you garden in a humid climate or are watering plants that are prone to foliage diseases. If you water in the evening, the soil and the plants will stay wet most of the night, promoting disease and fungal growth. This is especially important if you use overhead watering (like a sprinkler), which sprays water right on the plant.
Water at night if (1) you have no disease problems in the garden; (2) you live in an arid climate; or (3) you use a drip-irrigation system that waters the soil, not the plant leaves.
How Often and How Much
The best guideline is to water your garden when about half the available water in the soil is depleted. (Don't wait until a lot less is available; big fluctuations between wet and dry harm plants.) To figure out when that is, dig down 4 to 12 inches and feel the soil. Squeeze a handful of soil into a ball and see what happens.
If you have sandy soil, your sample should stick together slightly or form a weak ball under pressure. If it doesn't, you need to water.
For loamy soils, your sample should form a loose ball under pressure. If the soil looks dry and won't form a ball, it's time to water.
Clay soil should form a ball easily and ribbon out between your thumb and forefinger when you squeeze it. If you have to apply even a little pressure to form a ball, your garden needs some water.
But how much water should you apply? Always wet the soil at least 1 foot deep. Otherwise, plant roots will stay in the top 4 inches of soil, causing plants to be more vulnerable to water stress. Deep watering encourages deep roots, which allow plants to better withstand drought.