Young trees, in their first two years of life, have shallower roots and are growing rapidly, just like young children. They need more watering than older trees, especially in in the summer, because their roots are closer to the surface, where soil is more susceptible to evaporation. Without being able to dig deep, young trees will depend on outside watering. Of course you’ll want to water a tree as soon as it’s planted. After that, the key is to encourage deep root growth by keeping the soil moist all the way down past the roots. Proper mulching will help to cut down on evaporation and keep the soil moist.
Mature trees need less water. In the hottest months you’ll want to water anywhere from once per month to a few times a month based on rainfall conditions and the size of your tree.
Remember, don’t over-water. Soil around your trees should be moist, but not soggy or overly wet, and should dry briefly during the day so oxygen can permeate the soil. A good way to see if it’s time to water is to check the soil about 2” down by creating a small trench with a trowel or spade. If the soil is moist to the touch, you can wait before watering again.