To care for a grapefruit tree, provide young trees with 3 to 6 inches of water weekly, prune the tree as needed and apply nitrogen to the soil to fertilize it. Wrap the tree trunk to protect it from sun damage, and harvest ripe grapefruit during autumn and winter.
Plant grapefruit trees in an area with full sun and loose, well-draining soil, and after the first year, water the tree once every seven to 14 days. To help the tree preserve moisture, place a 3-inch-thick layer of mulch around the tree beginning 1 foot away from the base of the tree trunk.
Prune grapefruit trees by removing excessive growth during the first year, and remove diseased or damaged branches as needed throughout the year. To encourage growth, apply 1 to 1 1/2 pounds of nitrogen fertilizer to mature trees six to eight weeks before blooms appear, or apply 1/4 pound of nitrogen to the tree four times per year.
Prevent sun damage by wrapping the tree trunk in a protective covering during the hottest months of the year, or paint the bottom of the tree trunk with white, flat, non-latex paint diluted with an equal amount of water. Harvest grapefruits during the autumn and winter by cutting the fruit off the tree with pruning shears.