Both "sweet" and "bitter" orange trees (Citrus sinensis and Citrus aurantiaum, respectively) exist, but most people only know sweet orange trees and their sugary, tastier fruits. Orange trees were first introduced into subtropical North America in the 1500s by the Portuguese and Spanish but didn't become an important U.S. crop in Florida until the 1820s and in California in the 1870s. Orange trees grow well in U.S. Department of Agriculture hardiness zones 9 through 11.
The sweet orange is the most widely grown tree fruit crop in the world, according to "Economic Botany: Plants in Our World." The wild ancestors to modern oranges are extinct, and botanists can only speculate as to the origins. Two camps exist: some believe oranges came from a now extinct tree once native to southern China and northeastern India, while others speculate that the orange is a hybrid between a tangerine and a pummelo. Trade routes from China to Persia found oranges being distributed. It was the Islamic Moors that first brought orange trees into Spain.
Subtropical in origin, orange trees can tolerate only limited exposure to subfreezing temperatures and are not well-suited to tropical regions where heat, humidity and rainfall are constant year round. According to "Tropical Look," orange trees begin to sustain damage when temperatures reach 25 degrees Fahrenheit; damage becomes severe with leaf drop and branch die-back at 20 degrees, and below 15 degrees will fully kill the tree. All modern varieties of oranges like Valencia, Washington Navel or Hamlin are grafted and grow upon the disease-resistant roots of other citrus trees like sour orange trees (Citrus aurantium) or rough lemon trees (Citrus jambhiri), as mentioned by "Tropical and Subtropical Trees."
Orange trees are evergreen and typically grow 25 feet tall, attaining a rounded canopy with slightly pendent branch tips. Very old trees that have not been afflicted by root nematodes and soil fungus diseases can reach mature heights of 50 feet and 35 feet wide, according to "The Tropical Look." Stout spines typically occur on orange trees' youngest twigs. The five-petaled waxy white flowers are deliciously fragrant and yield minimal pollen. The fruits are in one of three classifications. Normal oranges are rounded and have a sweet, seeded fleshy filled with juice; blood oranges have a dark red and orange-mottled flesh, and navel oranges peel more easily, are often seedless and have a "belly button" protrusion caused by the early abortion of the flower's second ovary.
The fruit formed by orange trees is botanically classified as a berry with a leathery skin, the combination of two skin layers that contain pockets of aromatic oils that release the familiar fragrance of citrus when crushed. Orange fruits and other citrus fruits are specifically a fruit type called a hesperidium. Interestingly, this specific name comes from Greek mythology: when Hera married Zeus, she was given orange seeds that she grew in the garden of Hesperides. Ripe orange fruits do not need to be the color orange. In subtropical regions that never have cool nighttime temperatures--below 50 degrees Fahrenheit--ripe oranges may remain green according to "Economic Botany: Plants in Our World."
Purdue University shares that the oils throughout the tissues of orange trees can result in allergic reactions for some humans. Respiratory troubles can result if exposed to the sweet fragrance of orange blossoms and the sawdust of the wood of orange trees, once used for polishing jewelry, has caused asthma. Excessive contact with the volatile oils in orange peel can produce dermatitis. People who suck oranges often suffer skin irritation around the mouth. Those who peel many oranges may develop rash and blisters between the fingers. This oil is harvested from leaves, twigs and immature fruits to yield Petitgrain oil, according to "Tropical and Subtropical Trees."