Grapefruit is one of the easier citrus fruits to grow from seeds. Keep in mind that while many varieties of grapefruit will grow true to form from seeds, some hybrid varieties will not. Also, because of genetic diversity (the mixing of pollen from different trees), not all seeds will produce trees that grow fruit as sweet or as large as the grapefruit from which you extracted your seeds. Grapefruit grow best in a subtropical environment, in hardiness zones 9 through 11. Collect your seeds and begin the indoor growing process in late winter.
Cut open a healthy, ripe grapefruit and remove the seeds. Wash them off and set them on a paper towel. Place a damp paper towel over them to keep them from drying out.
Fill growing pots (with drainage holes) with potting soil. Make a hole in the soil approximately 1/4 inch deep and place a grapefruit seed into the hole. Cover with potting soil. Water to make the potting soil damp but not soggy.
Place the growing pot inside a plastic bag and set in a warm, sunny location out of direct sunlight. Keep the growing pot warm--approximately 70 to 75 degrees Fahrenheit during the day and no lower than 60 degrees F at night--for 14 to 21 days, until you see one to two leaves form on the sprout. Remove the plastic bag at that time.
Keep your young trees in a warm and sunny location with soil that is damp but not soggy. Transplant your trees to a larger growing pot filled with potting soil when they are 4 inches tall. Keep soil damp but not soggy, allowing it to partially dry between waterings.
Plant your trees outside in a sunny location with well-drained soil after one year. Plant in spring, after all possibility of frost in your area has passed. Trees should be planted at least 20 feet apart. Dig your hole one and a half times as wide as your root ball but no deeper than the root ball. Place your tree in the center of the hole and cover the roots with soil, covering the root ball no more than 1 inch. Too much soil around the base of the tree can result in a condition known as foot rot and kill the tree.
Build a water ring 5 to 6 inches high around the tree just a little wider than the planting hole you dug. Fill the ring with water and allow it to soak into the soil. After the water has soaked in, add more soil around the tree if necessary to keep the root ball covered. Keep the ground moist but not soggy around your young tree for four weeks, until the tree shows signs of growth.
Spread 1 cup of 8 to 13 percent nitrogen fertilizer at least 1 foot from the trunk of the tree in October and water it in thoroughly. No other fertilizer should be added the first year.