Orange Trees
Native to China and India, oranges have been cultivated in the United States since the early 1700s. Valued for their fragrant white flowers in the spring and nutritious fruit in late fall and winter, orange trees are suitable for U.S. Department of Agriculture hardiness zones 9 through 11. With good care, the tree grows to be approximately 20 to 30 feet tall and has a naturally well-rounded shape. Orange trees need full sun and well-drained soil to thrive.
Water
Newly planted trees benefit from the addition of a watering basin around the tree. A ridge of soil creates a berm to hold water and allow it to soak into the soil immediately surrounding the tree. Newly planted trees need to be watered once or twice a week to keep the soil moist but not wet. Established trees need water every week or two, depending on the weather and soil type. Hot weather and sandy soils require frequent watering. Wet, soggy soils and standing water can kill the tree.
Fertilizer
Orange trees need light fertilizing with a high nitrogen fertilizer during the first year. The University of California Cooperative Extension recommends applying 1 tablespoon of ammonium sulfate monthly during the spring and summer. Older trees need 1/2 cup of ammonium sulfate per application for every year of tree age, up to a maximum of 4 cups. Four applications, spread four to six weeks apart, are needed. A citrus formula fertilizer can also be used according to label directions.
Mineral Deficiencies
Iron or zinc deficiencies can be a problem in areas with alkaline soils. Symptoms of deficiency include yellow leaves with green veins. Chelated zinc or iron sprays applied directly to the foliage provide the missing minerals.
Pruning
Citrus trees need pruning only to remove dead branches and limbs that cross or touch the ground. Removing interior branches allows light to penetrate the tree and promotes better air circulation. Suckers or "water sprouts" need to be removed, especially those arising from the roots or below the tree graft. Pruning can be done at any time, but pruning after fruit harvest prevents crop reductions.
Harvesting Fruit
Citrus fruit begins to turn orange in the fall, but they are not fully ripe until later in the winter, depending on the variety. Color is not the best indicator of when to pick fruit, as ripe citrus often have green spots. Citrus ripens over the winter and benefits from remaining on the tree until full sweetness is attained. Taste is the best indicator of ripeness. Fruit left on the tree too long over-ripens and dries out. Freezing weather also damages fruit.
Protection From Sun
Young trees are susceptible to sun damage until the foliage grows in enough to protect the trunk. A thin coating of whitewash or interior latex paint thinned with an equal quantity of water protects the tree from sun damage. Commercial tree wraps are also available.
Cold Protection
Sustained temperatures below freezing damage young trees, flowers and fruit quickly; but mature orange trees are cold hardy to 25 degrees Fahrenheit or lower. During periods of sustained freezing weather, trees can be protected by covering them with a tarp or blankets. The addition of a few strings of outdoor lights adds warmth under nonflammable tarps or plastic coverings. The covers should be peeled back or removed immediately once the weather warms.
