Citrus 101

How Citrus Trees are Grown

Citrus GraftingPlant a citrus seed from a Valencia orange and what do you get? A Valencia orange tree just like the one your seed came from? Nope -- you can never be sure exactly what you'll get because plants grown from seed are all slightly different. (There are, however, some exceptions. Read on.) Trees vary in individual size and in the size and sweetness of their fruit. And, some trees may be more resistant to diseases.

Naturally, citrus growers, to ensure the best possible crop, want to make sure all their trees are of the same high quality. To do this, each new tree they plant is grown, or propagated, not from a seed but by grafting or budding. (More on this later.) In most citrus trees, the scion, or top of the tree, is a different variety from the roots or rootstock of the tree. Citrus growers plant trees whose tops will grow Washington navel oranges or Eureka lemons on a rootstock that has special characteristics like disease resistance, quick to bear fruit or restricting tree size for easier harvesting.

Nurserymen begin the process of propagating a new citrus tree by planting a seed for the rootstock. Most citrus have an unusual characteristic -- they can produce trees that are genetically identical to the parent through a natural process in seed development called nucellar embryony. While many rootstocks are produced by seed, because of nucellar embryony, they are all the same.

After the rootstock is a year old, a single bud is taken from a branch of the desired scion variety and inserted into the bark of the young seedling. This bud grows into the top of the tree that produces the fruit.

Fruit Development

Citrus flower buds begin to form in early winter and develop through late winter and spring. Most flowers don't result in the formation of fruit because more than 99% of them usually fall off. Because the number that do become fruit depends largely on temperature and moisture, adequate water is very important during and immediately after flowering.

Orange BlossomsMost pollination is done by insects, but due to a rather interesting process natural to some citrus called parthenocarpy, fruit can develop without pollination. Some varieties, such as the Clementine Mandarin, require cross pollination with another citrus variety.

Why do some citrus, like Valencia oranges, have seeds, and others, like Washington navels, generally have none? Parthenocarpy is the main reason. If parthenocarpic flowers are not pollinated, they generally don't produce seeds. Some, like the Washington navel, don't produce viable pollen.

After bloom, fruit develops from five to 18 months, depending on the variety and growing area. Unlike many other types of fruit, most citrus can be left on the tree without becoming overripe.

Soil Type

Citrus will grow in most soils from sandy to adobe clay, provided it drains well. Sandy soils must be watered and fertilized more frequently than soils with a higher clay content and growers can add organic matter such as manure or compost to improve water and nutrient holding abilities.

Irrigating and Fertilizing

Water quality is very important. Water high in salt content, common in some desert regions, can cause injury to leaves, burning leaf tips and margins. Lower levels of salts can cause the tree to grow poorly or to produce fewer or smaller fruit.

Nutrients that citrus needs in relatively large amounts are nitrogen, phosphorous, potassium, sulfur and calcium. In lesser amounts, citrus requires iron, magnesium, copper, zinc, manganese, molybdenum, boron and chlorine. In California and Arizona soils, usually only nitrogen must be added, most often in late winter to meet the demands of spring bloom.

Pruning

Unlike most other fruit trees, citrus trees don't require regular pruning. Commercial growers trim tree tops to keep them smaller so they are easier, safer and less expensive to pick, and "hedge" the sides to let more sunlight into the trees to improve yields. Hand pruning opens up the trees to allow more sunlight into the center.

Frost Control

Wind MachineProtection from frost is critical. Most citrus will freeze when fruit temperature drops to 27-28°F. The main methods of frost protection in California and Arizona are by wind and water. Wind machines -- large fans on poles about fifty feet above the grove -- are turned on when the temperatures near freezing. The fans mix the slightly warmer air above the grove with the colder air near the ground, which warms the air around the tree.

By applying water, the heat built up in the soil during the day is lost more slowly, and air temperatures around the fruit stay warm a little longer. A few growers still use oil-burning orchard heaters, but this once common method is seldom used now because of the cost.

Packing Citrus

Getting fruit from the tree to the table is complicated. A citrus fruit is a living organism which must be protected from injury, decay and water loss on its journey across the country or across the ocean.

GradingFruit are carefully picked by hand and put into bins which hold about 900 pounds. The bins are hauled by truck to a packinghouse, where the fruit is pre-graded to eliminate the obvious culls (bad fruit) and washed to clean away field dirt and dust. The fruit's natural wax, removed during the washing phase, is replaced by a food grade, non-animal source wax to restrict moisture loss and extend shelf life.

The fruit is then graded, most often by electronic camera systems. After grading, the fruit is divided by size and the first grade fruit is stamped or labeled "Sunkist." An unmarked second, or choice, grade is also packed. The remaining fruit is sent to the juice plant.

Most oranges and grapefruit are packed with robotic carton packing machines in 40 pound cartons. Lemons and most tangerine cartons are volume-filled with a set number of fruit. The cartons are stacked on pallets and placed in a pre-cooler before being loaded on trucks or rail cars for transportation to markets.