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Farmers fight Jack Frost

April 23, 2011

You may have already heard the low, insistent hum treading stealthily into your early morning dreams. Perhaps you know from whence it comes - or perhaps, like so many non-farming residents, you are still wondering.

No, it's not the arrival of extraterrestrial invaders.

The sound you are beginning to hear, which is following the footsteps of a tricky Valley visitor, is the thrum of orchard fans working like foot patrolmen combating a silent killer - otherwise known as Jack Frost.

With more than 14,000 acres of tree fruit in the Valley and emergent buds in development, frost is one of the biggest threats orchardists are facing at this time of year.

"There are eight or nine different stages of fruit bud development," said Steve Castagnoli, OSU Extension horitculture agent for Hood River County. "Different fruit buds are affected by critical temperatures at different points."

Here's the bottom line: When frost forms on a bud, it can, to put it bluntly, kill it dead - leaving devastated crop harvests for the summer and fall, fewer fruits for our tables and significant local economic impacts.

Somehow that familiar hum now begins to take on a more complex tone, perhaps appealing to more ears than farmers' alone.

Frost fans work in a very effective manner - particularly on nights with clear skies, calm wind, freezing ground temperatures and warmer above-ground air. These conditions are known as radiation frosts with cold inversion.

Fans are turned on as orchardists receive alerts from the daily frost report - provided by a Seattle-based meteorologist whose services are contracted for by Columbia Gorge Fruit Growers.

When engaged, the fans push warmer air toward the ground, creating a mixture of warmer and freezing temperature air and preventing the formation of frost.

When there is no inversion (warm air above freezing air) orchardists must resort to the use of orchard heaters to interfere with frost formation.

According to Jean Godfrey, executive director of Columbia Gorge Fruit Growers, some of the first frost fans went on this year in Odell and the upper Valley on Monday, April 18. More fan nights are expected this week as well.

The length of run time for the fans depends entirely on weather conditions and the location of the orchard, with various micro-climates spread across the varying geography of the valley.

"Last year, we had to start fans a couple of times as early as midnight," said Godfrey, referring to a few treacherous nights during last year's growing season.

Fans will run until temperatures and conditions change enough to ensure a frost danger has passed.

"Most growers have alarms in their homes which go off when temperatures reach one degree above the danger zone," said Godfrey. "That's when they get to work protecting the buds."

On average, one fan can effectively serve between 8 and 10 acres of trees, according to Castagnoli, who estimates that about two-thirds of area orchardists use fans.

"There are areas on the East Side slopes and around Oak Grove that are without fans," noted Castagnoli.

Orchard heaters add a secondary line of defense against frost for orchardists. Newer models typically use propane fuel to burn for heat. Older versions still rely on diesel. Both models heat the air within the orchard, providing that critical frost-preventing mixture of warmth and cold.

"It is pretty uncommon here," said Castagnoli, "but there are also some farmers who use sprinklers to manage frost as well."

Any form of sprinkling (under tree or over tree) will work to prevent frost formation by utilizing the thermal energy (heat) of the water in combating lower air temperatures.

Unfrozen water, by definition, exists at a temperature above freezing. Think of sprinkler use on orchard soil or tree branches as a little like letting the water run through your pipes to prevent a freeze-up.

To simplify the concepts: Temperatures on buds will be prevented from dropping below 32 degrees while 33 degree or higher temperature water is warming colder air temperatures or while water is present and undergoing the process of turning into ice.

For the geeks out there, this second phenomenon is described under the scientific phrase known as the latent heat of freezing - wherein a small amount of heat is released as the water transitions between its liquid and solid states.

Orchardists use these phenomena by continually applying water in a sprinkling method to maintain above-freezing temperatures until conditions improve.

The science may be complex, but the application of the science is executed in simple hard work.

Skilled, vigilant farmers prepare for the battles in every season, understand their land and trees, learn their science and apply the interventions and manpower needed to bring in a successful harvest.

Tonight, your neighbor may be out there in the wee hours ready to ignite heaters or switch on the fans in time to beat the next dangerous single-digit drop of the mercury.

Most of the rest of us are lucky enough to lie in bed simply wondering where the humming comes from.

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