Grapefruit trees (Citrus paradisi) hail from the Caribbean but are now grown throughout the U.S. in USDA hardiness zones 9 through 11. The trees are hardy and relatively drought-resistant once they're established. Watering is still necessary to encourage a bountiful production of the tree's tart fruit. The plant's watering needs vary according to the tree's stage of life, with more water being needed at the time of planting.
Build a ring of soil around the base of a freshly planted grapefruit tree. Use a spade and pile the soil's perimeter 2 inches high and 3 to 4 inches wide. The ring itself should be approximately 24 inches in diameter.
Fill the ring with water. The water will slowly soak into the soil adjacent to the grapefruit tree.
Repeat every three to four days. After the grapefruit tree has been planted for two weeks, decrease watering to once every 10 days. The soil ring will slowly dissipate over the course of several months, after which the tree will be established, according to Texas A&M University.
Water the established tree once every two to three weeks. Apply enough water to moisten the dirt around the tree to a depth of 12 to 18 inches.