A common mistake of novice gardeners is to water often, but superficially. Many in fact water every day, rapidly spraying the soil with water, then moving on. The result of this shallow watering is that only the surface soil is moistened. Unless there is good rainfall to compensate, plants grown this way react by producing mostly superficial roots, in the upper 2 inches (5 cm) of soil, a situation that leaves plants in risk of serious drought stress, especially if the weather becomes hot and dry.
Ideally, you’d water less often, but for a longer period, enough to soak the soil thoroughly. When rainfall is absent, watering once a week may be enough where temperatures are moderate and the soil holds water well, while a twice-weekly watering is more likely to be required in a hot, dry climate or where the soil retains little water. These infrequent but abundant waterings ensure that the water penetrates far into the soil, which stimulates the plants to also produce roots that extend to all levels, even fairly deep into the ground. That way, if the upper part of the soil starts to dry out, the plant can still find water and won’t suffer.
Please not that this tip is designed for people gardening in the ground. Plants grown in pots are in a very different situation. True, they still need deep watering every time, but they also dry out far more quickly than in-ground plants. In many cases, it may be necessary to water container plants daily, especially if the pot is small, because their root system is limited to the soil inside the pot and has nowhere to go for water in case of drought.
But how do you know if you’ve watered enough? There are so many factors that come into play—type of plant, planting density, presence or absence of weeds, presence or absence of mulch, rain in recent days, type of soil (dense or light, clay or sand), etc.—that it really isn’t possible to tell just like that.
To get a better idea, though, there is a simple test you can do.
About an hour after carrying out what you thought was a “thorough watering”, dig a small hole in the soil. If the soil is moist to a depth of at least 6 inches (15 cm), all is well. You can tell the soil is moist, because it will be darker than dry soil. However, if the soil is only moist to a depth of 2 to 4 inches (5 or 10 cm), the plants didn’t receive enough water. Water again. And water for a longer period next time.
Over time, most gardeners pick up the habit of watering thoroughly without wasting water, but in the beginning, it’s better to check first-hand to see if you’re doing it right!
Generous but infrequent watering also promotes deeper dispersion of roots by trees that are near pavement, where deeper roots are desired. (Actually, deeply dispersed roots are an asset to any tree, but are just more important for those who are likely to damage pavement.) Chaparral plants, like so many of our natives, are very sensitive to rot if watered too frequently. However, they need to be watered frequently until they get established and disperse their roots. That is not easy for newly installed chaparral plants that want regular watering, and also do NOT want regular watering.