Roots depend on a balanced ratio of soil, air and moisture within the ground. If you overwater your tree, its roots cannot access oxygen from soil particles. Water pushes air out and surrounds the roots. As a result, the roots stop absorbing moisture and oxygen, and leaves droop or wilt. In general, a tree requires at least two deep watering sessions each week during its main growing season, spring and summer. As cooler weather sets in, most trees enter dormancy, which is a resting period. Although the soil should be moist during dormancy, most trees need only one irrigation session every few weeks during fall and winter. Observing the weather and the tree's soil moisture level prevents overwatering that can lead to root suffocation.
Water moves soluble elements from soil to a tree's vascular system for dispersal to stems and leaves. Overwatering, however, reduces or stops this nutrient flow. Drooping leaves may appear brown or yellow from nutrient deficiencies. Even if roots could absorb water in soggy conditions, the wet soil causes nutrients to run off. With nutrients effectively removed from the area, leaf drooping is even more widespread.
As a tree's roots sit in soggy soil, they become stressed and weakened. Thriving in wet conditions, pathogens such as bacteria and fungi attack the struggling roots. Healthy tree roots turn into wet, brown masses that are perfect for pathogen reproduction. The roots slowly die back as the tree succumbs to nutrient and moisture loss. Leaves droop and eventually drop from the tree. Reducing your watering frequency helps the tree recover, but widespread root rot may cause significant die-back across the entire tree and not leave much hope for it to have a healthy growing season.
Most trees need damp soil to a 20-inch depth. Insert a shovel's blade carefully into the soil about 6 inches from your tree's trunk. As you remove the shovel blade from the soil, verify that the soil attached to the blade is moist but not muddy. Do not water your tree's soil if muddy conditions are revealed by the soil check, but irrigate deeply if the soil shows a dry consistency. A deep watering allows the tree roots to stretch downward into the soil, producing better root establishment as well as nutrient and moisture absorption success.