The amount of water a tree needs depends on many factors, including the age and species of the tree, the time of year, weather and soil type. As a rule, newly planted and young trees require more frequent watering than older, well-established trees. But during extended periods of drought, all trees benefit from supplemental watering.
For the first several months after planting, most of the tree's roots are still within the original root ball.
Watering Newly Planted Trees
For the first several months after planting, most of a tree's roots are still within the original root ball, with some roots beginning to grow beyond this area. The root ball and the surrounding soil should be kept evenly moist to encourage healthy root growth. After a few months, expand the watering zone to cover the entire area under the canopy. It can take two or more growing seasons for a tree to become established — for roots to venture into the soil well beyond the planting hole. It's vital to provide supplemental moisture in those early years, if nature doesn't provide regular soaking rains. During hot, dry weather, new trees may require water as often as three times per week to ensure that the root ball doesn't dry out.
Once a tree is established, apply water in a wide band around the outer reaches of the tree's canopy, called the dripline.
Watering Established Trees
It's a common misconception that a tree's roots are a mirror image of the aboveground canopy. In reality, an established tree's roots usually extend well beyond the edge of the canopy, or drip line. Although some anchor roots may reach deep into the soil, most tree roots are concentrated in the upper 12" to 18" of soil. When watering established trees, provide a deep, soaking irrigation to the entire area beneath the tree canopy and extending several feet beyond the drip line. Ideally, you should moisten the soil to a depth of 10" each time you water. To prevent rot, don't apply water to the area directly around the trunk.
Know When to Water
The easiest way to check soil moisture is to take a long (8"-plus) screwdriver and poke it into the soil. It will pass easily into moist soil, but be difficult to push into dry soil. If you can't poke it in at least 6", it's time to water. This technique works best in clay and loam soils.