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• From the UF/IFAS Extension Miami-Dade

• Subject: WARNING - Cold Weather Forecast
  
  

This is a follow-up message about the preparation for potential cold damage, and additional instructions from Dr. Crane on running irrigation to mitigate cold damage, which is helpful for nursery crops as well.

FROM: Jonathan Crane UF/IFAS TREC 
Looks like an advective-windy (15-30 mph) freeze or near freeze on Sunday morning around sunrise and on early Monday morning (~3-4AM) a radiation type (or slight breeze, ~8 mph) freeze. Due to the lower %RH on daytime Monday the temperature may drop fast after sundown. If the NWS forecast temperatures are off for Homestead then we may be colder and colder longer than their model looks.
Comments
You should be prepared ASAP – irrigation systems working and plenty of fuel. 
The windy freeze on Saturday night/Sunday morning means you must monitor wind speeds. If the winds are >10 mph you should probably not turn on your system until the wind speeds are 10 mph or less even if you have a high volume (rainbird or spinner) type irrigation system that can put out 0.25 inches of water per acre per hour. However, if you have a mature/large tree grove where wind speeds are generally less because of the multiple large tree canopies and you have a high volume under tree irrigation system you may be able to turn on with wind speeds at 10-12 mph. However, if you have young trees or a high volume over tree system I would suggest you don’t turn on if winds are above 10 mph. On Sunday night/Monday morning – at least now, looks more like a radiation (no wind and clouds) type freeze – but again you need to observe the wind speeds, if windspeeds are low <10 mph you can irrigate.I saw some growers have completely covered their shade house to keep the plants warm. If you cannot do this, blocking/covering the north side, if you can, may make a lot of difference since we will have strong cold wind this time.  Or you can cover the most cold sensitive plants inside a shade house.
Regards,
Qing

Hi, All:
As you may be aware of the low temperature this weekend (see the message below), and here are two options you may consider to provide some protection to your plants (outdoor in particular) with irrigation:
1, Saturate the soil in the container in the afternoon before the night with potentially damaging temperatures, and here is the reference:Irrigation Management (https://edis.ifas.ufl.edu/publication/EP429)Soils and growing media serve as sinks for energy during the day and sources of heat at night. Heat is transferred through conduction. Dry soils and media have more air spaces, which limit heat retention and inhibit heat transfer (Rosenberg, Blad, and Verma 1983). In addition, wet soils and media are usually darker, resulting in increased absorption of solar radiation. Therefore, watering plants to field or container capacity before cold events can help protect the plants by increasing heat absorption and storage.2, Run overhead irrigation to avoid damaging temperatures, particularly freezing temperatures, occur to the plants. You may need to monitor the temperature and be aware of its limitations:  Running overhead irrigation protects plants from freezing by exploiting the latent heat of fusion: as water turns to ice on plant surfaces, it releases heat, keeping the plant tissue temperature at or near 32F. This method requires continuous water application to maintain the freezing process, forming a protective ice layer that insulates plants from lower air temperatures.  Requirements: 

• Continuous Operation: The irrigation must run constantly throughout the freezing event. If the water stops or is insufficient, evaporation can cause further cooling, causing more damage than not using water at all.
• System Limitations: This method is effective for radiation frosts, but generally not recommended if wind speeds exceed 5 mph.
  
• Hopefully, this information helps, particularly for those who are new in the business. Please let me know if you have any questions. Take care and stay warm.
  Regards,
  Qing
  Qingchun Liu, Ph.D
  Extension Agent I for Commercial Ag/Ornamentals
  UF/IFAS Extension Miami-Dade
  Department of Parks, Recreation & Open Spaces
  18710 SW 288th ST
  Homestead, FL33030
  Office: 305-248-3311 ext 241 / 305-679-0241
  Cell: 305-968-9601
  Fax: 305-246-2932
  http://sfyl.ifas.ufl.edu/miami-dade/ From: Miami Dade Chapter FNGLA <dadefngla-gmail.com@shared1.ccsend.com>
  
  The coldest night of the season thus far could potentially be Saturday night into early Sunday morning.
  
• Temperature graphics are available 24/7 at weather.gov/miami/coldweather and weather.gov/miami/probabilistic
• Please see below forecast for Saturday Night/Sunday Morning - Apparent temperature is the "feels like" temperature and includes windchill. 
  
  
  Charles LaPradd
  Agriculture Manager
  Miami-Dade County
  
  Miami Dade Chapter FNGLA | 18710 SW 288th St. | Homestead, FL 33030 US.  Have a question about our products or services? Contact us today and one of our team members will be happy to help.