Cold and freeze protection of

blueberries in Florida

Dr. Dorota Z. Haman Agricultural and Biological Dept. University of Florida

Frost versus Freeze Frost is a local condition which occurs in your area

on a still night, temperatures usually go no lower than 29-30 degrees F, and it warms up again the next day.

A freeze involves an entire region, has significantly lower temperatures, and may last for several days.

Definition of frost/freeze warnings issued by National Weather Service.

Warning

Wind Speed

Air temperature

Frost

Below 10 MPH

Above 32oF

Frost/freeze

Below 10 MPH

Below 32oF

Freeze

Above 10 MPH

Below 32oF

Irrigation and cold protection

When used properly, water can provide partial or complete crop cold protection.

Improper use of water can increase cooling or ice loading and cause greater damage than if no water were used at all.

It is important to know what principles are involved in using water for cold protection.

Irrigation systems Surface Sprinkler Microirrigation

This stage can tolerate 15 - 20 F temperatures.

This stage can tolerate 25 to 28 F.

Heat transfer conceptsConduction Heat is transferred through the material,

i.e., through the molecules. Convection Transfer of heat by the movement of

masses of heated liquid or gas. Radiation - Transfer of heat from one object to

another without the need for a connecting medium.

Latent Heat Transfer heat transfer due to a phase

change.

Latent heat transfer Heat of fusion Heat that is released when liquid

water freezes to ice; For each gram of water frozen, 80 calories are released;

Heat of vaporization Heat lost when water changes from liquid to vapor. At 32 F, the amount lost is 596 calories for each gram of water.

Dry bulb temperature and wet bulb temperature

Dry bulb - the temperature of the ambient air. The same thing as the normal air temperature read with a normal thermometer.

Wet bulb the temperature of a wet surface under the same condition the temperature is lower due to the evaporation from the wet surface

Dew point temperature temperature at which water condenses is the air usually, lower than wet bulb temperature.

, 2

Relative humidity (RH)

Percentage or ratio of water vapor in the air in relation to the amount needed to saturate the air at the same temperature.

Relative humidity depends on the air temperature. Warm air holds more water vapor than cool air.

Rate temperature fall depends on relative humidity.

Dew point in cold protection Water vapor in the air absorbs infrared radiation

and can slow the rate of temperature fall.

Fog can provide significant frost protection High dew point reduces radiant heat losses from a

plant and the temperature falls slowly.

Low dew point is associated with rapid reduction in temperature.

Psychrometer - A psychrometer is a device used to determine atmospheric humidity by the reading of two thermometers, the wet bulb and dry bulb thermometers. The wet bulb thermometer is kept wet by a moistened sleeve. With a psychrometer, one determines how much cooler the wet bulb is than the dry bulb and then calculates humidity by using appropriate graphs or tables.

Psychrometer

From R. L. Snyder

Crop temperature A typical method of estimating crop temperature is

using the air temperature and adding a "safety factor" of several degrees.

This causes systems to be started before they actually need to be, resulting in excess water and energy use.

Knowing exactly when to start and be able to wait may allow to avoid protection completely.

Crop temperature measurements

Thermocouples are temperature-measuring devices small enough to be inserted into buds, blossoms or small fruit.

This stage can tolerate 15 - 20 F temperatures.

This stage can tolerate 25 to 28 F.

Inversion Happens on clear nights Temperatures drop significantly at the surface due

to radiation. The temperature in the lower atmosphere inverts The temperature increases with altitude to the top

of the air layer. The warm air in an inversion is important for some

frost protection

Radiation Frost Advective Freeze Winds less than 5 MPH Winds higher that 5 MPH

Clear sky May be cloudy

Cold air mass 30 to 200 ft Cold air mass 500 to 5,000 ft Inversion develops

Cold air in the low spots

White or black frost damage

Easier to protect Difficult to protect

When to start? To avoid damage under low dew point conditions, sprinklers should be started at:

1.1C (34F) if the dew point is -4.4C (24F) or

above 1.7C (35F) if the dew point is -6.7 to -5.0C (20-23F) 2.2C (36F) if the dew point is -9.4 to -7.2C (15-19F)

This recommendation should only be followed when a frost is predicted. Sprinklers may be turned off when the wet bulb temperature has risen to 1.1C (34F).

Rules of freeze protection with water

Apply water uniformly

Apply fast enough to keep ice wet all the time

Apply enough water to protect the plant

Uniformity High system uniformity is necessary for effective frost

protection.

Sprinkler overlapping The overlap is extremely important for frost/

freeze protection Higher winds require closer spacing of

sprinklers

Ice forming indicator

The temperature of a plant covered in ice will drop below a dry plant if the ice dries.

Ice forming indicator

Wind speed greatly reduces effectiveness of sprinkler irrigation for freeze protection.

Ice forming indicator The color of the ice forming on plants is very important. If the system is properly working, the ice will be clear.

L20VH 1/2" Brass Impact Sprinkler

Primary Applications: 1/2" (13 mm) Full Circle, Brass, Wedge Drive Impact Sprinkler Features and Benefits:

Patented, self-flushing wedge drive Durable brass die-cast arm Stainless steel springs and fulcrum pin Chemically resistant washers Two-year warranty Wedge drive runs on smaller nozzles and lower pressures Corrosion and grit resistant Self-flushing design reduces wear from grit Built to last

Models: L20VH

Specifications: Bearing: 1/2" Male NPT, Plastic Trajectory Angle: 10 degrees Operating Range: 25-80 psi(1.7-5.5 bar) Flow Rate: .56-2.98 GPM(0.13-0.64 m 3 /h) Radius: 22-32ft.(6.71-9.9 m)

Used primarily in undertree permanent systems. Excellent frost protection sprinkler giving superior distribution patterns. Exceptionally strong drive due to the patented self-flushing wedge drive arm which allows for low precipitation rates required for tight and problem soils.

Automatic frost monitoring and alarm systems

Provide early warning by: text message to your mobile activation of: sirens, strobe lights and irrigation systems

Be ready Reinstall suction lines and check primers on the pumps. Test and service the pumping unit Replace filters and have spare filters available. Treat diesel tanks for water and algae. Check lines and sprinklers in the field for leaks and clogged

nozzles and proper rotation. Check water pressure on ends of distant lines. Make sure drainage in and around fields is adequate. Make sure

roadways around and through the field will withstand traffic at night during irrigation ( the soil will be wet!).

Have a high-intensity spotlight ready to plug into the truck to check sprinkler operation.

Be ready Have rain suits and boots available for everyone

responsible for checking the irrigation system. Have wires or drill bits available to unclog nozzles. Have tools and replacement parts that are necessary to

exchange nozzles and/or sprinklers. Put shielded minimum thermometers in cold, average,

and warm areas of fields. Hang some ribbons on trees or poles around fields to

detect slight breezes.

Be ready Identify a good source of agricultural weather

information and watch it closely. Consider subscribing to a weather service that issues

freeze warnings. Consider purchasing a monitor that calls you when

the temperature gets low. Consider purchasing a hand-held wind meter or

anemometer to measure wind speed. Consider purchasing a sling psychrometer to

measure wet bulb and dry bulb temperatures, relative humidity, and dew point.

Thank you for your attention