Tree watering is a key part of tree care, but it is difficult to recommend an exact amount due to the variety of climates. A few guidelines will help you to water your trees properly.
Watering Newly Planted Trees
For new trees, water immediately after you plant a tree. Usually 30 seconds with a steady stream of water from a garden hose w/ a diffuser nozzle per tree seedling is sufficient.
Watering Trees During First Two Years
During the first couple growing seasons, your newly planted tree is expending a lot of energy trying to get its roots established in the soil. Especially during the first few summers of your new trees life, it will have a difficult time dealing with heat and drought. You can make this easier by providing water and covering the soil with wood-chip mulch. Deep watering can help speed the root establishment. Deep water consists of keeping the soil moist to a depth that includes all the roots.
How Much Water and When
Not enough water is harmful for the tree, but too much water is bad as well. Over-watering is a common tree care mistake. Please note that moist is different than soggy, and you can judge this by feel. A damp soil that dries for a short period will allow adequate oxygen to permeate the soil.
You can check soil moisture by using a garden trowel and inserting it into the ground to a depth of 2", and then move the blade of the trowel back and forth to create a small narrow trench. Then use your finger to touch the soil. If it is moist to the touch, then they do not need water.