Young trees, like young people, need a little extra care to get off to a good start. Newly planted trees have a small root system. Providing supplemental water is very important to help maintain the tree until it can grow its roots. It is important to concentrate watering efforts on the "root ball" area. It is also helpful to water the immediately surrounding area to encourage new root growth beyond the root ball.
When natural rainfall equals an inch of water every week, additional water may not be needed. However, we typically have hot, dry summers and young trees need additional water. Don't just rely on turf watering systems, as they usually do not water deeply enough. A good rule of thumb to follow is to provide five (5) gallons of water per week for every inch of stem caliper (trunk diameter measured six  inches above the ground on young trees). There are several techniques for proper "deep-watering" of trees including:
Providing extra water should be done for several years. A general rule is to water one year for each inch in diameter (measured 6” above the ground) of size, plus one more year. For instance, a tree with 2” of diameter should be provided supplemental water for three years.
Did you know that some of the signs and symptoms of drought and over-irrigation are the same? Over watering will cause leaves to turn yellow or fall off and can cause the tree to fail. Just like drought, the plant begins to wilt. Roots need oxygen to survive; too much water drives all oxygen out of the soil. Other indications of overwatering include yellow leaves; growth of algae and mushrooms, wilted shoots and the soil is constantly damp. Indications of under watering include dry soil, wilted or curled leaves and older leaves turn brown and drop off.
A good method of preserving soil moisture is to provide a 2" to 3" layer of mulch over the root area. Taper the depth of mulch so that there is no mulch touching the tree trunk. Do not mulch deeper than 3” as this can cause problems. Mulch also provides nutrients and keeps the root area cool in the hot summer months.
Watering is especially important during hot summer months when rainfall is limited. However, providing supplemental water in the spring and fall is just as important for optimum growth and overall good health.