Near-record low temperatures expected in all San Diego areas tonight have forced some growers into the unusual position of staving off frost damage by picking crops such as oranges and avocados months before their normal harvesting time.
In the past, protective measures such as irrigation and wind machines have been enough to save crops in San Diego from sustaining serious damage. But National Weather Service forecaster Wilbur Shigehara said it is the first time in recent memory that growers have actually had to remove crops from the trees because of the cold temperatures.
"I simply can't remember any time the growers have picked their crops because of the (frost) warning," Shigehara said. "It's not just the low temperatures, but the duration of the cold that is worrying us. Temperatures will fall extremely fast (tonight), reaching freezing by 7:30 in the coldest orchards. That could mean about 13 hours of below-freezing temperatures in those areas."
The two major crops grown in San Diego County, oranges and avocados, are usually not harvested before late spring, said Vincent Lazaneo, horticulture adviser with the University of California Cooperative Extension. While some varieties of oranges are ripe enough to remove from the trees now and won't suffer in quality, avocados are another story.
"Many avocados are not ripe enough to meet commercial standards, and growers don't have a choice," Lazaneo said. "They have to provide frost protection instead of harvesting. Oranges can tolerate temperatures as low as 26 degrees because their sugar content works as a sort of antifreeze. Avocados, however, will be damaged if temperatures go below 30 for any length of time."
The lowest minimum temperature in San Diego's agricultural areas Thursday night was 23 degrees at Valley Center, Shigehara said. It had only climbed to 28 by 8 a.m. Friday. Nighttime temperatures in the low 20s are expected in most agricultural areas through at least Tuesday.
"Growers should be prepared for a period of prolonged dangerously low temperatures," Shigehara said. "There will be no relief through early next week, though we do expect (tonight) to be the coldest of the cold."
The high Friday at Lindbergh Field tied the 99-year-old record for lowest maximum high temperature when it struggled to reach 53 degrees. The low temperature was 37, just three degrees shy of the record, also set in 1888.
Undoubtedly some crops will be damaged by the unusually low temperatures, Lazaneo said.
"It is apparent that some damage will occur, but to what extent can be controlled by the growers," Lazaneo said. "The two major preventive measures are irrigation and wind machines. The wind machines are effective only in a short duration, so most growers are resorting to irrigation.
"Water contains heat in it, and by watering the orchard prior to the onset of freezing temperatures, that heat will be stored in the soil and seep out at night, offering some frost protection."