San Joaquin Valley citrus braces for first frost

San Joaquin Valley citrus growers are bracing for frost conditions starting Dec. 4 after the National Weather Service issued a freeze watch.

"Wednesday night and Thursday morning are going to be our first critical test for the frost season," said Joel Nelsen, president of Exeter-based California Citrus Mutual. "Thursday, Friday and Saturday mornings could be critical."

It"s not clear yet how serious a threat mandarins or navels may face. Forecasts call for lows around 25 degrees in the coldest parts of the valley. Citrus isn"t planted in the coldest locations, but there"s reason for concern. Duration, for example, isn"t clear from the forecasts.

"It"s going to be critical for our mandarin producers," Nelsen said. "Their threshold temperature is something below 32 degrees for about four hours. The threshold for (navels) is four hours or more at 28 degrees. We could get some rain in on Tuesday night and Wednesday morning, which would help."

To prepare, citrus growers will run water to moisten and warm the ground. As temperatures approach frost levels, wind machines will be turned on to circulate the ground-level warm air.

Under the right conditions that can boost grove temperatures at least three or four degrees, Nelsen said.