Oranges survive 6 frozen nights

Temperatures in California’s Central San Joaquin Valley citrus growing region dipped below freezing for six straight nights last week.

From Nov. 29 through Dec. 5, temperatures beginning at 10 p.m. to 8 a.m. ranged from 28 to 30 degrees.

This is generally the trigger point for starting up wind machines to take advantage of the inversion layer of warm air which forms 30 or 40 feet above the ground. Growers in historically cold areas were also running water in the orchards to help raise orchard temperatures. Using one or both of these tools growers took advantage of a strong inversion to maintain orchard temperatures above damaging levels.

On Dec. 2, growers reported low temperatures ranging mostly 28 to 29 degrees with the coldest areas at 25 to 26 degrees. Durations below 32 degrees ranged for 6 to 8 hours.

On Dec. 3, conditions were not as uniform ranging from 27 to 30 degrees in the Exeter, Ivanhoe and Lindsay area. Durations below 32 degrees ranged from 8 to12 hours.

There are no reports of freeze of frost damage last night and none are expected. Moisture conditions were dry, eliminating the potential for ice marking which can occur when small droplets of water freeze on the fruit. However, the foliage on some trees is beginning to show the evidence of the frosty mornings.

There may be isolated instances of outside fruit on the perimeter of the coldest orchards that were damaged but this will not be evident for several weeks.

The benefit of these cold nights is that it helps bring out the full color in the fruit and eliminates the need to de-green the fruit in the packinghouse. The result in a naturally colored orange with better shipping quality and longer shelf life.

It is not cheap for growers to protect their crops on these cold nights. With approximately 12,000 wind machines throughout the four county central valley growing area and an average cost of $35 per hour to operate each machine, the industry spent roughly $2,100,000, protecting the crop, during each night, according to California Citrus Mutual.

Wind machines are not heaters. The further out a particular orchard is out 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 the 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.

For the week, growers from Madera to Kern County have spent in excess $10 million running wind machines and water. This is a direct cost to growers that is invested to protect a crop that represents $1 billion in economic value and 15,000 jobs that are tied directly to the harvesting, packing and marketing of the crop.

According to the UC Cooperative Extension, winters have been relatively warm in the South Valley 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.

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 new crop of Valencia oranges may begin to freeze at temperatures as high as 28 or 27 degrees Fahrenheit but the wood of even baby trees can usually withstand temperatures to 22 degrees Fahrenheit and much colder if the wood is older.

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