Generally speaking, trees and shrubs only need to be watered when they are planted, and while they adapt to their new homes. Once established, rain will provide all the water necessary. But since Mother Nature can be unpredictable, you will do yourself and your trees and shrubs a big favor if you continue to water them during the summer months for the first couple of years after you plant them.
Water when they’re young
Be sure to give trees and shrubs a thorough watering to establish deep roots. Most of a tree’s feeder roots are located within 12 inches of the surface. A shallow root system will result in an unhealthy tree or shrub that will not be able to tolerate drought and will be susceptible to pests. Water the entire area out to the drip line of the widest branch extending from the plant, so that the roots will grow out and get nutrients from the soil in a wide area around the plant. Do not allow the soil around the plant to dry out, and continue to water thoroughly for several weeks after planting to be sure the roots are well established.
If a shrub has been well chosen to suit its site, then it demands little care beyond watering during the first few months to one year after planting. Make sure new shrubs receive sufficient amounts of water during their crucial first summer in your yard. Established shrubs seldom need to be watered, except in arid parts of the country or during an extreme drought. When you do water, remember to water slowly over a long period of time to allow the water to soak deep into the soil. Avoid frequent light watering as this leads to a shallow, weak root system.
How much water is available to a particular tree depends on the depth and spread of its roots. Watch trees closely to determine when they need water. Signs of water stress include wilting, a change in leaf color (from shiny to dull, or from dark green to gray-green) and premature leaf fall.
Fruit trees that are actively growing generally need 1 inch of water once a week, or about 2 gallons of water per square foot of root space once a week. (The roots generally spread out somewhat farther than the top canopy of the trees.) A newly planted tree would have a root spread of up to 2 square feet and, therefore, would need 2–4 gallons of water a week. Adjustments can be made for rainfall and soil type. Your tree may need water quite often in very sandy soil, less often in heavier soil. Always dig down a few inches into the soil first to see if watering is necessary. Trees in a lawn area should have a deep soaking about twice a summer in addition to normal lawn watering.
Choose your method
There are a number of ways to water efficiently: basins, furrows, sprinklers, soakers or drip systems. The most important goals are to eliminate runoff, to confine water inside the drip-line of branches, and to apply water uniformly.