Apple trees need water to produce their fruit, but are essentially hardy trees that grow well in many climates. As long as they are planted in a fairly rich, well-drained soil, and mulched well at the base, you shouldn't have to water them frequently during the growing season. Nature usually provides enough rain for apple trees to grow well on their own in temperate and northern climates. If the season has been dry and you aren't seeing the type of growth or fruiting you want, water them occasionally. In other regions, such as southern or hot climates, water is essential during the growing season.
Mulch the trees at the base. Spread a thin layer of mulch, no more than an inch deep, in a circle around the base of the apple tree. The circle should spread out as far as the branches of the tree reach, or at the "drip line." Leave some breathing room right at the trunk; mulch crammed up against it can encourage rot and pests.
Watch for rain. Your trees should not go more than three weeks without water. If there is no rain within three weeks, water them or risk disease and low production of apples.
Move the sprinkler over to the trees. Use the lawn sprinkler you regularly use on your lawn or garden, and simply sprinkle the apple trees once a week. Mature trees need no more water than this.
Water young trees more often. Run a garden hose with a sprayer attachment out to their location if your trees are young, or in an orchard setting that does not get watered as frequently. Young trees need about an inch of water per week.
Water using the sprayer to simulate the natural fall of rain. Avoid soaking branches and trunks, and direct the spray into the ground or occasionally onto the leaves of the apple trees.