Dwarf oranges (Citrus spp.) like the dwarf Washington navel orange (Citrus sinensis "Dwarf Washington Navel"), which survives outdoors year round in U.S. Department of Agriculture plant hardiness zones 9 to 10, and other dwarf citrus trees grown in containers, generally produce fruit less prolifically than their in-ground counterparts. However, container cultivation allows growers in cooler areas to enjoy these dwarf plants, which make attractive, unique ornamentals, with their fragrant white flowers and edible, attractive fruits. A healthy, vigorous dwarf orange tree will periodically outgrow its container, experiencing leaf drop or discoloration and dieback not caused by drought stress. At this point, the plant requires either root pruning to keep it in the same container or transplanting into a larger container.
Water the bonsai orange tree in its current container deeply about a day before you repot it to make the plant's root mass easier to remove from the container and minimize stress to the plant.
Select a container with ample drain holes that is about 25 percent larger than the orange tree's current container and fit a piece of wire mesh in the bottom to keep potting medium from falling through the holes. As a general rule, a year-old dwarf orange tree only requires a container with a diameter of 8 inches and two- or three-year-old trees require a container with a diameter of 10 to 12 inches. The ideal container size for long-term dwarf citrus tree cultivation is 16 to 20 gallons, so gradually work up to that size.
Place a few inches of high-quality, well-drained potting soil in the bottom of the container. As an alternative, use an appropriate homemade rooting medium that could contain, for example, a blend of equal parts sand, peat moss and perlite or vermiculite. Add enough soil so that when you set the bonsai orange's root mass in the new container the soil surface will be 1 to 4 inches below the container's lip, depending on the container size.
Tip the orange's current container onto its side, lifting it if it is not too heavy, and slide the root mass out of the container. If the root mass does not slide out easily, tap the lip of the container on the edge of a counter, using one hand to catch the plant by cradling the soil surface.
Inspect the orange's root mass. Use a sharp, clean knife to cut off any dead, diseased or damaged roots, leaving only healthy roots, which look firm and white. If the plant is pot-bound, with roots growing in a tight circle around the edge of the root mass, use the sharp knife to make four evenly spaced vertical cuts each 1 inch deep around the circumference of the root mass. Also, cut an "X" into the bottom of the root mass.
Set the root mass in the center of the prepared container atop the growing medium. Make sure the soil surface of the root mass is 1 to 4 inches below the top of the container and add or remove soil under the root mass, as needed. Fill in the space around the roots with soil or medium, gently firming it down as you go to reduce later settling.
Water the soil around the orange roots slowly and deeply, stopping only when excess water begins to exit out of the container's drain holes. Add more soil around the root mass, if needed, to make up for any settling.
Trim off any branches on the orange tree that were injured during the repotting. Make any cuts below the bottom of an injured section and just above a branch junction, leaf node or bud facing in the desired direction.