Citrus is a subtropical crop. Frost hazard is the most limiting factor related to the suitability of a given location in the southern San Joaquin Valley to produce citrus.
Citrus, generally, in the southern San Joaquin Valley is planted in a zone or ‘belt’ approximately 500 to 950 feet above sea level on the eastern side of the Valley. The reason that citrus is grown here in the low foothills is that cold air runs downhill and any prospective orchard site should have good cold air drainage. During the winter, on windless frosty nights, an inversion layer forms several hundred feet above the valley floor. An inversion layer is a sandwich of warm air trapped in-between layers of cold air above and below.
At the higher elevations of the citrus belt, citrus trees may actually be within the warm inversion layer. On a frosty night, trees in the inversion layer may be in air that is above freezing, while air in the coldest regions of the Valley may be close to 20 degrees.
Relative topography is important, as well, even if the orchard will be located in the area generally known as the citrus belt. Citrus should not be grown in low areas where cold air is trapped by natural topography, vegetation, or by manufactured obstacles like aqueduct walls, raised roads, or dams.
For a general overview of the physics of freeze events see Dr. R. Snyder's (Department of Land, Air and Water Resources, University of California) Web site at http://biomet.ucdavis.edu.
Winters have been relative warm in the southern San Joaquin Valley of California since the last significant frost that occurred during the winter of 1998/1999, and even that frost event was mild compared to the previous tree-killing big freeze of 1990/1991.
Miss citrus belt
Anyone who has suffered the economic consequences of a severe freeze is very conscious of the freeze hazard associated with planting citrus, but many new to the citrus industry are not. In the time intervals between severe freezes, such as the interval we are currently in, citrus plantings tend to proliferate into areas down hill from the true citrus belt.
As a general rule of thumb in the San Joaquin Valley, a new citrus planting located to the west of the current most westerly citrus grove in a given area, is probably in harms way, especially if this next nearest grove is out of sight. Insurance companies, that provide additional frost insurance to growers, have actuarial tables based on maps and information developed by the United States Department of Agriculture Risk Management Agency that divide counties into areas of relative frost hazard. New growers are well advised to consult with insurance companies to help assess the relative frost hazard of any new prospective planting.
Severe freezes usually move the citrus belt back to the east toward higher ground when fruit is not only frozen, but the trees are killed as well. The eastern edge and center of the citrus belt remains stable; it is the western edge that ebbs uphill under the onslaught of the freezing storms that occasionally explode out of Alaska and flows downhill with the warm winters that our Mediterranean climate often provides.
The observation that homeowners in Delano, Arvin or southwest Bakersfield have orange trees does not mean that these areas are commercial citrus growing areas. Replanting a single orange tree is much less expensive than replanting a grove.
Some growers plant early-maturing citrus varieties in more marginal areas with the knowledge that the fruit will be harvested before the coldest periods of the year. Navel oranges such as Fukumoto, Beck, Newhall, TI, Fisher and some mandarin varieties like the Satsuma and Clementine are picked in October, November and early December and are not usually in danger from the worst frost periods in late December and January.
Lemons and a new crop of Valencia oranges may begin to freeze at temperatures as high as 28 degrees or 27 degrees, but the wood of even baby trees can usually withstand temperatures to 22 degrees and much colder if the wood is older.
Wind machines are not heaters. The further out a particular orchard is on the valley floor, the less effective wind machines are. The warm air of the inversion layer is a lot further up in the sky if the orchard is located at 400 feet above sea level compared to one at 700 feet. If the inversion layer is too high, stirring up the air with a rapidly rotating propeller is not going to bring much warm air down to freezing fruit.
Orchard heaters, such as those burning propane, are almost prohibitively expensive to operate, and those burning some other fuels can run afoul of air pollution regulations. Frozen fruit is difficult to separate from unfrozen, so even partial freezing of the outer fruit on a tree canopy can seriously downgrade the whole harvest.
The San Joaquin Valley climate is such that hundreds of different crops are grown successfully here. A site that is only marginally suited to citrus will likely be perfect for one or more of these many alternate and potentially profitable crop choices. Planting a frost tolerant crop on a cold piece of land will avoid having to bring in the New Year by starting wind machines or waking up worrying if the oranges froze last night or not.