Global warming means less rainfall. So it is important to water your plants without wasting drinking water.
Of course, it all depends on the weather, the type of soil and your plants’ specific needs. In addition, the slope and orientation of your property will also affect how quickly your soil dries out. A south-facing or steeply sloped bed requires more frequent watering than a flat one that gets only morning sun. In addition, some types of soil, especially those containing large amounts of clay, dry out more slowly than others.
Generally speaking, you can judge by how dry the soil is. Before watering a bed, dig a hole about 10 cm deep with a trowel. If the bottom of the hole feels warm and dry, it is time to water; if the soil is cool and damp, it is moist enough. Beds with recently planted annuals or perennials should be watered when the soil is dry to a depth of about 5 cm. Lawns may be watered when the ground is dry down to their roots, or about 7 or 8 cm.
When and how?
Avoid watering at midday on warm days, because up to 50% of the water will evaporate before it has a chance to soak into the soil. Whenever possible, water your plants in the morning or, failing that, early evening. Just be sure that the foliage has a chance to dry before nighttime, to reduce the risk of fungal disease.
Don’t water just the top few centimetres of the soil. The water needs to penetrate deeply to encourage the growth of long roots to carry plants through the next drought. To thoroughly soak the ground, it is best to water slowly, with a fine spray, particularly on clay soil or steep slopes, which are unable to absorb much water at a time. If the water starts to run off, turn off the hose and start again a bit later.