the appetizer:

The big chill in California may translate to big prices on produce. Freeze affects oranges, lemons, broccoli, cauliflower, celery, strawberries, and lettuce, among others.

Kate Heyhoe  

Holy Guacamole, Batman!
Lemons and Avocados Attacked
Farmers Fight Air and Ground War

by Kate Heyhoe


Arctic air punched California in her green belt last month, severely rupturing the vital citrus veins that run from the Central Valley to Ventura and down to Southern California. Some oranges, lemons, and tangerines were harvested early, as were avocados, but enough turned to slush to cause economic pains in grocery bins across the nation. This year's shortage of farm workers meant more fruit than usual was left hanging when the front blew in.

Frozen Orange

I See Orange

Artichokes took a big hit. You could see the lifeless spheres drooping from thick stalks. To keep from suffering a total loss, farmers paid the added price of dispatching field workers to whack off the decaying chokes; an attempt to salvage what they could of the still-growing stalks. Field workers also early harvested whatever avocados they could find. Young carrots just beginning to sprout succumbed, though more established carrot crops were expected to be fine.

The tipping point of disaster varies from crop to crop. Lemons start to tank at 29 degrees F. Avocados perish in the mid 20s. When the air falls below 34 degrees, strawberries are just a breath away from turning to mush. Broccoli and cauliflower may be winter vegetables, but they're still not comfortable in severe cold. Brief periods of freeze may do little more than maim a crop or leave minor surface wounds, but persistent chill strikes without mercy. And that's what happened last month, to the tune of $1 billion in estimated damage, rivaling the disaster of 1990, when five mornings of 18 degree temperatures killed not just vegetables and fruit, but whole citrus trees.

When arctic air strikes, it's interesting to see the many ways farmers rally to action. Not all tactics work for all plants, and protective frost and freeze measures can range from low-tech and desperate to high-tech and expensive. None are guaranteed.

Water, surprisingly, comes to the rescue in times of deep freeze. And water, precious as it is, is still cheaper than heating stations which burn fossil fuels.

Frantic efforts to lay down irrigation lines typically precede arctic whammies. Watering over a freezing field can create a blanket of ice that functions as an insulator. And irrigation from micro-sprinkler systems can also protect crops from frost and freeze.

Sound implausible? Here's how it works: As 1 gram of water freezes, 80 calories of heat energy are released. So while water turns to ice, the heat it generates helps displace the crop's natural heat, which is being constantly released into the environment.

But here's the tricky part: As water evaporates, it also releases heat into the environment. Specifically, the same 1 gram of water that gives off 80 calories as it freezes can also disperse 600 calories of heat energy into the environment as it evaporates. So there's a balancing act: you need more than seven times the amount of water to freeze and offset the evaporation effects in order to get any beneficial heat to the crop. Furthermore, when the ice on a plant begins to evaporate, it sucks away heat from the plant. So as long as the ice is forming, the plant receives heat, but as soon as evaporation (especially from winds over 5 mph) sets in, the plant's temperature takes a dive. Once begun, irrigation must continue until the ice is loose and melting, which means until the sun comes up and air temperatures rise. Hence the need for a secure electrical source, and no brown outs. As you can see, keeping the irrigation flowing in adequate amounts and long enough is critical to success.

Some farmers fight freeze with wind. Wind actually helps keep temperatures up. When the wind stops, cold settles in and pours deep into soil. Wind machines are high-powered fans with blades that span 16 feet and spin some 30 feet above ground. They're used to force warmer air down to crop level. Sometimes helicopters function as wind machines, hovering for a spell then moving on to the next area, and returning periodically to rewarm each spot. This works fine with the more common radiation frosts or when heaters generate warm air, but if the air above is as cold as an arctic blast, as it was last month, wind machines don't help. (Such conditions are known as advective freezes.)

Smudge pots, tarps, and anti-ice sprays can be called into action, too. Sometimes these methods work, but sometimes it's just too cold for too long for anything to do any good. At best, farmers can manipulate the temperature by three to five degrees, but when subfreezing temperatures settle in for a week, there's not much anyone can do.


Copyright © 2007, Kate Heyhoe. All rights reserved.

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This page created January 2007