Mr. Weedy says: How much water do your trees and shrubs actually need? Each week trees & shrubs need one gallon of water per foot of height. Purchase a rain gauge and test your current irrigation system to gauge the amount of water and output from your sprinkler heads. Remember, it’s better to water and longer intervals than shorter ones. i.e. once a week for 45 min. per zone versus three times a week at 15 min. per zone.