Thursday, January 13, 2011

Protecting Plants from Freeze Damage - Understanding Irrigation for Freeze Protection

What the Public Needs to know about Protecting Strawberries from Freeze Damage by Craig Chandler and Vance Whitaker (Gulf Coast Research and Education Center - http://gcrec.ifas.ufl.edu/)

(Although this article speaks about strawberry freeze protection specifically, the information is appropriate for any fruit, vegetable or ornamental plant in the garden or landscape.)

As you talk to people about strawberry freeze protection, here are some facts you may want to keep in mind:

Understanding that this was an extremely rare January (2010) can be of great help to the public. Typically, there are no more than a few freezes each winter in the Plant City/Dover area (Hillsborough County – Central Florida – USDA Hardiness Zone 9b), and these are often of short duration (3-4 hours or less). A period of 9 days in which the minimum daily air temperature is below 32 ° F and the maximum air temperature is ≤ 60 °F, as was the case during the first half of January this year, is a once in a lifetime occurrence. According to FAWN (Florida Automated Weather Network – http://fawn.ifas.ufl.edu/ ), the weather station at Dover typically experiences a total of 96 hours below 45 °F in January. This year (2010) it was 264 hours.

When a freeze is the result of clear skies and calm conditions, sprinkler irrigation is an efficient and cost effective method to protect strawberry flowers and fruit (in a commercial setting). Eighty calories of energy (in the form of heat) are generated for each gram of water that freezes. This heat can keep plants at 32 °F, but water has to be applied continuously and in sufficient quantities.

Newspaper and TV reporters often state that a coating or blanket of ice insulates flowers and fruit, protecting them from air temperatures below 32 °F.  To the casual viewer or reader this sounds like a logical explanation, but if it were true, sprinkler systems could just be run until the plants were covered with ice and then turned off. What protects strawberry flowers and fruit is the heat of the liquid water (which comes out of the ground at about 72 °F) and the heat that is generated as the irrigation water changes from liquid to solid (a chemical property known as the heat of fusion).

Damage to strawberry flowers and fruit can start to occur when tissue temperature reaches 30 °F. A period of very warm weather preceding the freeze may result in tissue being damaged at a higher temperature. Conversely, a period of cold weather preceding the freeze may condition tissue such that it can withstand temperatures below 30 °F before damage occurs.

When there is little or no wind, sprinkler systems will generally not be turned on until the air temperature is 31 °F. However, when an advective (windy) freeze is expected, it is common to turn the sprinkler system on when the air temperature is 34 °F. This is because the temperature of wet plant tissue exposed to wind will initially drop due to evaporative cooling. Then as water begins to freeze, the heat of fusion will counteract the heat loss due to evaporation and the temperature will stabilize at about 32 °F.

Wind can result in evaporative cooling that lowers tissue temperatures 5-6 °F below the air temperature. This is why, if it’s windy, the air temperature may need to be 36 °F or higher before it is safe to turn off the sprinklers.

No comments:

Post a Comment