Friday, February 7, 2014

Baby, It’s Cold Outside: How We Protect Our Citrus Trees From Freezing

Have you ever left a tender houseplant on the porch or patio during a cold snap? What about that time there was an early frost and you forgot to cover the tomatoes? If you remember how frustrated you felt at finding your plants killed or damaged by the chill, can you imagine what it would be like for a citrus grower to see this?

After effects of a killing freeze.
Yes, that’s thousands of ruined oranges on the ground after a killing freeze (a long time ago, fortunately). In the past, citrus growers often lost their whole livelihood in a single episode of below-freezing weather. It’s happened many times in Florida’s history.

Various ways citrus trees were protected from cold weather.
Over the years, growers have tried all kinds of methods to protect Florida’s citrus crop from cold weather. Sand and hay have been piled around trunks. Wooden and canvas tents have been built—around each individual tree! And some growers have even tried building a roof over their groves

Later, in-grove heaters, which kept the air warm around the trees, and wind machines, which “mixed” ground-level cold air with cooler air higher up, became very popular. But today, Florida growers overwhelmingly use a method called micro-irrigation to protect their groves.

You might have seen a photo of oranges after micro-irrigation. It’s eerily beautiful, but it doesn’t really look like it would be good for the tree, does it?

Surprisingly, though, all that ice is actually keeping the tree warm! When water freezes and changes its state from a liquid to a solid, that process actually gives off some heat.  This is called “heat of fusion”--and as long as irrigation machines continue to apply a fine spray of water, it continues, keeping the plants warm. However, growers do have to watch out for wind. That’ll make the ice evaporate...which will lower the temperature again.  (Here's a Wikipedia article for the science-minded who’d like to read more about “heat of fusion”.)

If this sounds kind of complicated, it is. Growers use some pretty high-tech equipment to monitor conditions and make sure the trees don’t accidentally get damaged, while also conserving water as much as possible. Growers often don’t get much sleep on cold nights. But at least we don’t have to build a little shed around each tree anymore!

If you happen to have your own citrus trees at home, you may now be wondering if you should be covering them with ice when the mercury drops. The answer is no. This method is too fiddly and too risky for the home grower.

A tiny tent protecting a small orange tree from the cold.
So what should you do? It depends in part on what type of trees you have, since some types of citrus are much more tender than others. In general, though, you probably don’t need to be concerned about your trees or their fruit till temperatures drop to 28 degrees or lower for four hours or more.

If this is in your forecast, there are a few things you can try. First, water your trees well. It will help keep them warm, and reduce their stress. If your tree is small enough, drape it with a fabric cover; if it’s too big for this, wrap its trunk with cloth.  You can also try putting strings of Christmas lights or another electric light on the trees to generate a bit of heat. And if the forecast looks really dire and your fruit is ripe, do pick it.

Yes, it’s true; growing citrus in Florida, especially its more northern areas, can get a little complicated sometimes. But we’re sure you’ll agree that the sweet, juicy fruit we harvest makes it all worthwhile.

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