The following information comes from the Oregon State Extension:
Trees need to be watered regularly during their first three summers after planting to survive. Water newly planted trees with 10-15 gallons of water once a week throughout the summer and during other dry spells. Water slowly so that moisture soaks deeply into the soil to reach the root zone. A slow-drip watering system is ideal, but you can also use the five-gallon drilled bucket technique, to water your trees more effectively.
Use mulch to reduce evaporation, deliver organic nutrients, and helps prevent the growth of weeds. Mulching prevents lawn
mowers and string trimmers from getting close to the tree and damaging the trunk. Next to watering, mulching is important to the health of newly planted trees. Be sure to pull the mulch away from the bark of the tree in a three-inch radius to prevent fungus growth or infection. Remember the 3-3-3 Rule: 3 inches away from the trunk of the tree, 3 inches deep, in approximately a 3-foot radius. 

Keep weeds, grass, and other plants 18″ from the trunk of your new trees. Weeds and grass in your tree’s root zone absorb water and nutrients that should be reaching the tree’s roots.