Orange trees are tropical trees that adapt well to containers. Because of this, oranges can be grown indoors or in greenhouses in USDA hardiness zones where they would freeze outdoors. An orange tree can be grown from seed, but because orange trees may take up to 15 years to mature, most gardeners prefer to grow their trees from grafted varieties of orange tree. Popular orange tree varieties include sweet oranges, which include naval oranges, blood oranges, round oranges and acidless oranges. These fruits are often grafted onto the roots of sour orange trees.
Plant oranges in the ground in USDA hardiness zones 9 through 11. Grow orange trees in containers in all other zones. Orange trees do not survive when temperatures drop below 20 degrees.
Select a container for oranges that is slightly larger than the orange tree's rootball. Plant the orange in loamy, well-drained soil and fill the soil to the rim of the container to prevent overwatering the tree. Transplant orange trees into larger containers only as the tree becomes rootbound. Placing an orange tree in a container that is too large can lead to root rot if the container becomes waterlogged from holding too much water. In the ground, plant orange trees on the south side of a home, wall or building structure. Place the tree's root ball slightly higher than the soil line to assist in drainage.
Water the orange tree once weekly as the soil dries out. Water just enough so that the soil becomes as wet as a wrung-out sponge. Allow the soil to dry between waterings. Never overwater orange trees.
Prune indoor trees as the branches become long and spindly from low light conditions. Prune outdoor trees only to maintain height or rejuvenate the tree by cutting it back by 1/3. Orange trees have a naturally round shape and typically need very little pruning. Prune in between an orange tree's fruiting and the next bloom cycle.
Feed orange trees with a nitrogen-based, granulated fertilizer (21-0-0) up to four times yearly. Use ¼ cup four times yearly the first year, ½ cup four times yearly the second year. Use 2/3 of a cup four times yearly from the third year on. Scatter the granulated fertilizer evenly over the root zone of the tree. For container trees, use a liquid fertilizer formulated for citrus according to the package directions. Directions will vary among fertilizer brands. Cultivate shallowly around trees in the ground to prevent weeds and grasses from stealing the tree's water and nutrients.
Move container orange trees outdoors in warm weather so that the trees receive at least eight hours of sunlight daily.