A citrus tree is a member of a group of tropical evergreens cultivated around the world for their fruits. They are members of the genus Citrus, which has about 16 species. Well-known citrus tree types include the orange, lime, and lemon. Recognized for their mild to sour acidic flavor, citrus fruits have pulpy flesh enclosed in a tough rind. Due to their tropical origin, citrus trees can only be grown in regions that are nearly frost-free year-round.
The genus Citrus is thought to have originated in Southeast Asia. Most varieties are thorny or spiny, and are cultivated as large shrubs or small trees. Usually each citrus tree bears small, fragrant white flowers from which the various fruits grow. Citrus tree species passed through India before reaching Europe in the 13th and 14th centuries. They were then introduced to the Americas by Europeans, beginning a long history of cultivation in tropical and subtropical regions there.
The orange tree, Citrus sinensis (C. sinensis), is a bush tree that bears sweet orange fruits that range from about 2 1/2 to 4 inches (5-15 cm) in diameter. C. limon, more commonly known as the lemon tree, bears small, tangy yellow fruits with a size of about 2 to 3 inches (5-8 cm) long. Lime trees, or C. aurantifolia, are closely related to lemons. The fruit of the lime tree has green flesh, and is slightly smaller but more acidic than a lemon. Grapefruits, from the C. paradisis tree, are slightly larger than oranges and have tarter flesh that ranges from pale yellow to pink.
Sweet, small orange fruits including clementines, tangerines and mandarins come from the citrus tree variety C. reticulata. Each of these fruits is about 1 1/2 to 3 inches wide (4-8 cm) and has a less acidic, sweeter flavor than an orange. The kumquat is one example of a "citrus" tree or fruit that is not actually a member of the genus Citrus.
Citrus trees begin to bear fruit after a period of seven or eight years, with the fruits taking a year to mature. Basic growing requirements include neutral to acidic soil and long periods of sunlight. Some types of citrus trees, for instance certain lemons, can withstand periods of temperatures as low as 15 degrees Fahrenheit (-9°C). In areas with cold winters though, citrus trees can only be grown indoors or in a greenhouse.