Faced with a crowd of almond growers who knew as well as anyone how hard the frost was for some growers, Joseph Connell, an Extension Advisor Emeritus of the University of California, minced no words.

“It was a rough year,” he said.

Then he shared some of the more complex details of this year’s frost, and advised growers on how to protect their crops in future years.

“You can have localized microclimate affects,” he told the crowd. “We measured air temperatures dropping six degrees in 30 minutes.”

That sharp drop took place in a study orchard when a wall of air funneled out of a canyon onto the orchard.

The table above shows estimated percentages of cold injury to Nonpareil fruit buds and small nuts exposed for 30 minutes to cited temperatures at indicated fruit stages, according to a handout Connell gave growers.

“Ground cover on the orchard floor is very important in frost protection,” Connell said, adding that even half an inch of dry crust cuts heat storage during the day.

He offered a number of tips for keeping soil heat storage potential up.

“It would be beneficial to run irrigation during the day, just a quarter inch to wet that soil so it radiates at night,” he told the growers.

Four inches of grass is a relatively cold surface, Connell told growers, pointing out that trimming it to two inches or shorter would capture more heat at the ground level. Likewise, loose fluffy soil doesn’t store much energy.

There are two types of protection growers can use as temperatures start to drop, he said: radiation heating and convection heating.

Radiation heating is like a person standing next to a heater, while convection heating is like central heat ducted through a house.

“We get radiation heating coming off microsprinklers,” he told growers, adding that ice on trees isn’t necessarily a worst case scenario. “As long as there’s liquid water on the ice, it’s 32 degrees, and it’s radiating 32-degree temperatures up through the trees.”

Connell said solid set sprinklers applying 40 gallons of water per minute per acre will provide frost protection under most conditions seen in California.

While micro-sprinklers may not alter air temperatures much, UC research shows they can change exposed temperatures, those of the buds and flowers on almond trees.

Aside from sprinklers, forced air can also heat trees, whether produced by wind machines or helicopters.

If there is a strong inversion pattern of warm air just above the orchard, with a low ceiling, helicopters can be effective, he said.

A visual example of an inversion would be a layer of chimney smoke hanging along a horizontal plane over Chico in the winter.

“Wind machines work best in narrow valleys,” he said.

Helicopters are more sophisticated at warming orchards: they can detect warm air with sensors, follow it, and then use their weight to displace it downwards. A helicopter loaded with water is heavier and displaces more air with its thrust, he said.

For a recap of the freeze damage and resources available to growers, see the blog “After the frost: Almond outlook from the UC, FSA, Farm Bureau, and more.”

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