Though the main concerns with apples trees are pests and diseases, there may be times when it is necessary to worry about watering an apple tree or irrigating an apple orchard. In most cases, established apple trees need little water other than that provided through natural means. In exceptionally dry years, additional watering may be required. New plantings also require more water than established trees. Therefore, make sure you understand the age of your trees and how long they have been in the ground.
Monitor the weather and use a rain gauge to keep track of how many inches of water you receive each week. While there may be some, even many, weeks when rain water is adequate, at other times supplemental watering may be needed.
Note of the time of the year. Apple trees require more watering during the summer, the height of the growing season, than at other times of the year.
Prepare ahead by placing sprinklers in the orchard to ensure good coverage of all trees. To do this, test the radius of each sprinkler, multiply that distance by two, and place the sprinklers a little closer together than that distance would indicate. Remember, water pressure could be lower when all sprinklers are online.
Use a rain gauge to see how much water the sprinklers put out each hour. This will be important when it comes time for you to water apple trees, showing you how much water to provide.
Water apple trees at least once a week when the natural water sources need to be supplemented. From a full canopy to harvest, a rule of thumb is that apples need 1.8 inches of water every 7 days. At blooming time, apples only need 20 percent of this amount. Gradually increase the water between these amounts each week until the 1.8 inches is reached, probably near the beginning of June.