Not all orange tree varieties will grow well indoors. Choose a dwarf variety that doesn't need to reach a certain size before it will bear fruit. Also, look for varieties that don't require a lot of heat in order for the fruit to ripen. Mandarin orange trees, or Citrus reticulata, are a popular choice for indoor growing because the tree remains small and the fruit does not require extreme heat in order to ripen. Some nurseries also sell grafted orange trees for indoor planting. These are usually a variety of small citrus tree that have had a variety of orange tree grafted on.
Depending on the variety, most dwarf varieties will grow to between 4 and 6 feet tall. They will usually do well in a 20- or 35-gallon pot or planter. You can plant them in regular potting soil, but make sure the pot you choose has good drainage.
Orange trees require a minimum of eight to 12 hours of sunlight each day. A south-facing window is an ideal spot for an indoor orange tree. If your home does not have enough light, there are two possible options. You can purchase grow lights and install them above the tree to provide enough light, or you can put your orange tree outside part of the time. If you put your tree outside part time, watch the temperature closely and bring it indoors if the temperature drops near freezing. Some orange trees do very well being left outside during the summer and brought indoors during the fall and winter.
Although some varieties of oranges require more heat than others in order for fruit to ripen, none will become sweet in cool temperatures. Your indoor orange tree should be placed in a warm location, away from cool drafts and air-conditioning vents. When the fruit is set and ready to ripen, let the room where the orange tree is located get as warm as possible. If it's not too inconvenient to do so, close off any air conditioning vents and keep doors and windows closed so that the room heats up. Provide plenty of water for your orange tree during this process, as it will dry out fast.
Orange trees do not require a lot of care as long as they are protected from freezing temperatures. Over-watering can be more damaging that under-watering. Rather than watering on a regular schedule, check the soil to determine when your indoor orange tree needs water. Don't let the soil dry out completely, but let it get to a point where it is only slightly moist before watering.
Fertilising your orange tree will help keep it healthy and producing fruit. Use fertilisers designed for citrus trees, not for tropical plants. Fertilise up to two times a month. Your indoor tree will also benefit from a minor element spritz twice a year. Minor elements are iron, zinc, manganese and magnesium. Find a liquid fertiliser with these minor elements, mix it with water as specified on the label, and spray it directly on the leaves of your orange tree.
Dwarf trees can be heavy producers, and it is common for some of the immature fruit to fall from the tree to make room for the rest of the oranges to ripen. If the branches on your tree are stressed by too much weight, remove some of the fruit. You will be rewarded with a healthier plant and better fruit production in the long run.
Fruit and Pollination
The size and amount of fruit you are able to harvest will depend upon the variety of tree you choose. Most dwarf trees produce oranges that are smaller than regular varieties. You can expect most dwarf oranges to be about the size of a tennis ball. For grafted plants, the oranges will usually be full-size oranges.
It takes the average dwarf orange tree about three to four years to begin producing fruit. Most nurseries, however, sell trees that are two to three years old, so you don't have to wait so long to begin harvesting fruit. The first two or three years of fruit production might be low, but by about five or six years old, the tree should reach peak production.
Not all orange trees are self-pollinating. Check with a nursery to find out if the variety you choose needs to be pollinated. If it does, simply take a cotton swab and very gently touch it to the inside of the flowers, moving from one flower to another to spread the pollen.