When natural rainfall drops below that amount, start giving the tree some water. Don't just rely on lawn sprinkling to provide adequate water to your young trees - it usually does not water deeply enough. In central Texas, heavy clay soil is very common. It is possible to over-water trees and other plants growing in clay soil. Before giving water, dig down into the soil three or four inches to see if it is dry.
Water quality is another concern.
Water that is high in sodium can burn the foliage, and minerals can build up in the soil if there has been little rain. Water your trees deeply, but less frequently than you would turf. Avoid spraying water on the leaves if your water contains excessive amounts of sodium. Mulch will prevent competition between your tree's roots and the roots of grass or weeds. Mulch prevents summer soil temperatures from becoming lethal to tree roots.