Citrus growers prepare for frost

It may be fall where you are, but citrus growers are anticipating winter, the time they must protect their crops from freezing temperatures.

Most of them will rely on powerful wind machines which tower above their orchards as silent sentinels for most of the year. But on December and January’s chilly nights and early mornings they leap into action in a symphony of warming drafts.

Wind machines don’t create warm air, but they find it above the temperature inversions typical of winter, especially in Central California’s San Joaquin Valley. Their huge fiberglass propellers pull it down, and push it through the orchards.

The temperature increase they create may be only three to five degrees, but it is usually enough to prevent delicate citrus fruits from freezing as they ripen on the trees, exposed to a suddenly hostile environment.

As well as the downward force the 20-foot propellers exert by being slightly tilted, they rotate steadily 360 degrees around their support towers to direct the warming blasts to all the trees that surround them.

Power for each propeller (often called a fan) is supplied by a heavy duty industrial engine on the ground, transferring its power upward through a series of bearings and shafts. The current favorite is a V-10 engine, and the most popular fuel is propane, with some operating on natural gas, others on diesel fuel. Early day machines that operated on electrical power have mostly been replaced, as has gasoline.

The rule of thumb has been that each wind machine can protect 10 acres of trees, but engineers and designers are confident they can expand the coverage to 15 acres in the near future.  With costs to a grower of $24,000 to $34,000 for each machine, expanded coverage will be welcomed.

In extreme cases, where temperatures are unusually low (in the low 20s or teens), or remain low for a long period (all night) growers supplement the effect of the wind machines by running water through their orchards. No matter what the air temperature is, water pumped from the ground exits at about 60 degrees, more than 30 degrees above the thermometer reading.  Even if icicles form, the water has a measurable warming effect.
Since citrus trees were first planted commercially in Southern California in the late 1800s, protecting the fruit from frost and freezing temperatures has been a serious concern for growers and marketers of oranges. The popularity and expanded acreage of the thin-skinned and more vulnerable mandarin varieties has exacerbated the need for protection of the fruit from severe winter temperatures.

Older growers can remember the early and primitive attempts to provide heat in their orchards by surrounding them with air-polluting smudge pots. Those were simply heavy metal pans with two or three-foot smoke stacks. Filling the pan with diesel or other fuel and igniting it provided smoky heat.

The labor requirement of filling the fuel chambers and maintaining them through extended cold periods was enormous. Some contend that the smokier the atmosphere became the better the protection from frost it provided. Smudge pots didn’t originate smog, but they did their part in maintaining it.

Now, with wind machines, labor saving steps are opening in many directions. Where starting each machine individually used to be standard, automatic, temperature-actuated start mechanisms are now in use. Or growers, alerted by frost alarms in their homes, can have access to electronic devices to start some or all of their machines remotely.

Refueling the machines has evolved from tending a tank at each machine to locating large tanks centrally with underground lines serving several machines.  Some large supply tanks are now located at roadside, or centrally within orchards, simplifying delivery by fuel suppliers.

Frost protection for citrus fruit is as essential today as it was 100 years ago. Figures on thermometers haven’t changed, but those on the calendar have, bringing warmly welcomed changes and much cleaner air.

Don Curlee is an agriculture consultant in the Valley. His column appears each Monday in The Recorder.