Freeze Protection with Irrigation

I have been asked by one of my avid readers to explain why agricultural crops are often protected from freezing weather by spraying them with water.  That is a good question since if you were to spray a warm body, like a person or animal , with water, they would be very cold.  Plants are not warm bodies and are not affected by windchill in the classical sense. 

First, I must say that there are many factors that go into making a plant winter hardy.  The rootstock, the variety of fruit, the nutrient level of the plant the previous summer, the weather prior to the freeze, the fruit load,  the soil type and the topography of the plantation all impact cold hardiness. 

Assuming all other facters are equal, what is the deal with spraying water all over them?  The short answer is the heat of fusion.  When water changes phases from a liquid to a solid, it emits a burst of energy as heat.  As I recall, it is very small, but something like 2-3 calories per gram of water freezing is emitted.  That burst of heat, of course melts the ice and turns it minutely back into water liquid.  If a liitle drop of water begins to freeze, emits it’s heat, and cold continues to be around it, of course, the cold overcomes the tiny little heat and just freezes it.  In other words, there is a little bit of resistance on water’s part to turning phses and becoming ice.  If all the water on the trees were just frozen, the little heat would be dissipated and the tree would freeze.   If, however, you keep putting water on the tree from an irrigation sprinkler, there is always liquid water coming in.  By its definition, liquid water must be warmer than 32 degrees, plus, it keeps making that heat of fusion burst a few calories to keep it on the knife edge. 

It seems like this would be very minimal and it would not really work that well.  It is true, we are talking saving the trees a few degrees in order to just barely keep them alive and not frozen.  A citrus tree, with all its internal sugars and juices will exprience freeze damage if it is exposed to 28 degrees for more than 4 hours. This is without the irrigation water on it.   So if the water can keep the treess to a minimal level of warmth, it will save the trees.

There is another aspect of irrigating to preserve trees, and that is evaporation.  This is usually a minor point.  Imagine if you sprayed water on a warm body like a human and set that person outside.  The evaporation of the water would cool him even further.  So in order to not have the heat loss of evaporation, there must be about 100 percent humidity.   The math behind it is this.  You must have 7.5 gallons of water freezing and changing phases to produce the heat necessary to counteract evaporation i n a normal situation, say 75% humidity.  That is usually easily done since the method of adding water to the trees is by overhead irrigation and you can pour lots of water on them.  I just mention this because in certain weather conditions such as low humidity, wind, etc. it can become a factor.  Usually by the time all this overhead water pours down, the humidity near the tree’s branches is near 100% anyway.

It is a myth that the ice you see covering trees in the new’s pictures is serving as an insulator and keeping the buds warm.  Ice is a very poor insulator.  Although I think it would block evaporation.  heh-heh

I hope  I will not see any examples of this on my upcoming trip to Florida. Right now I have no pictures to show you.  If you want to read more about this, try this link:

http://edis.ifas.ufl.edu/ch182

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