Sprinklers provide best frost protection for almonds

When freezing temperatures are in the forecast, sprinklers are virtually the only tool most almond growers can use to protect their crops.

Wetting the orchard floor with sprinklers, or even drip irrigation, allows the ground to absorb more heat during the day before a nighttime freeze, and continuing to apply water during the coldest hours provides additional protection.

"Sprinklers are our main frost protection because as the water freezes it releases heat," said Joe Connell, University of California Cooperative Extension farm advisor emeritus in Butte County. "As long as you keep it wet, ice stays at 32 degrees and does not get colder. If you have ice at 32 degrees, it radiates heat at 32 degrees that travels up and warms the buds."

Connell made his remarks to a group of Sacramento Valley almond growers who had recently endured one of the most challenging frost winters in recent memory.

"The beginning of 2018 was a difficult one for frost," he said. "We had more days below freezing than I can remember around Chico. We had nine days below 32 degrees."

The critical temperatures for frost damage depend on both the variety and the stage of nut development.

For Nonpareil almonds, when the temperature drops to 24 degrees and the flowers are showing pink, a grower can expect 40 percent damage, but even a freeze at 27 degrees will damage half the flowers at full bloom, or 90 percent of the small nuts.

UC researchers tested almond varieties with artificial freezes in the 1990s, and found that Nonpareil is the most frost tolerant among the mid-blooming varieties, while Carmel is the most sensitive and Price is intermediate.

Among the late-blooming varieties, Mission is the most sensitive, while Padre and Butte are similar, with Butte just slightly more sensitive.

There are microclimates in areas a few miles apart within the Central Valley great enough to make a difference in frost danger.

"The side canyons can have local cold air that moves in," Connell said. "We have radiation frost on cold, clear nights, and we have advection frost when cold air moves in from Alaska."

The advection frosts are usually more damaging and difficult to manage with sprinklers because they involve wind from the north that constantly brings cold air moving across the orchard floor.

Even temperature differences between spots a few feet away from each other within the same canopy can affect the extent to which a freeze damages the crop.

"The younger orchards don't have as many nuts after a freeze because the canopy is lower and closer to the cold air," Connell said. "In some areas, you will see nuts low on the tree close to the wet soil and higher up in the tree from the warmer air, and a band with no nuts in between."

When water is too scarce to run the sprinklers all night during a freeze, it can be possible to use them selectively and still be of some help.

"If you have enough water to run every fourth line it might help some, but the air between those lines will be colder," Connell said.

The volume capacity of microsprinklers will determine how effective they can be in warming the orchard during a freeze.

"Microsprinklers don't usually have enough volume to heat the air," Connell said. "We found at lower volumes with microsprinklers you get less warming. We have microsprinklers at 30 or more gallons per hour that will give you warming, but at 15 gallons they won't."

Drip irrigation is not likely to put out enough water to keep the temperature of ice on the orchard floor at 32 degrees, but it may help to capture more heat in the ground during the day before a nighttime frost.

"We're not going to get much heat from drip irrigation, but if you run it during the day to wet the surface it will help a little," Connell said. "You may want to make sure the orchard surface is moist so it can store heat from the day."

Cover crop management can also help, because any vegetation on the orchard floor taller than four inches tends to make the ground colder.

The topography in nearly all of the almond production areas of the state makes fans generally ineffective for frost protection.

"In a big valley like the Sacramento or San Joaquin, the ceiling is usually too high for wind machines to be of much good," Connell said. "The citrus area of the San Joaquin Valley is at a few hundred feet elevation, so the wind machines can work. Napa is a narrower valley, so they work better. You have to know the height of your ceiling if you're going to use wind machines. To measure the ceiling, you have to put up a pole or tower with a temperature sensor."

Chapter 23 of the University of California Almond Production Manual, available at http://anrcatalog.ucanr.edu, is devoted to frost protection.

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)