Answer: Citrus are subtropical plants, and grow more or less all the time in subtropical regions. They are perfectly adapted to growing indoors. In fact, most indoor environments are warm enough to keep citrus growing nearly all the time. Indoors, it is not unusual for citrus trees to bear three or four crops a year.
Grow citrus in a soil mixture of equal parts loam, peat moss, and sand. When watering the plant, soak it thoroughly and then allow the top inch of soil to become somewhat dry before watering again.
Citrus trees are heavy feeders and should be fertilized once a month with a chelated mix of manganese, iron and zinc (most multipurpose fertilizers contain these minerals. Citrus trees also love humidity. You can add moisture to the air with a humidifier, by misting the plants frequently, or by placing them in a tray filled with pebbles with water added to the top of the pebbles.
Don't be surprised if your citrus sheds a lot of immature fruit after blooming. Like many fruits, citrus produce many more fruits than the plants can support. So there is nothing much to worry about if your plant drops surplus fruit, provided it is otherwise healthy. To help increase the number of fruit-bearing blossoms on your tree, you can transfer pollen from blossom to blossom with a small paintbrush.
During the warm summer months, your tree will benefit from being outdoors. Be sure to expose it to the brighter sunlight gradually, by placing it outdoors in a shady spot for a few days before moving it into direct sunlight. As your tree grows larger, you might consider placing it on a decorative platform with casters to make moving it easier.
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