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 32 o F Frost/freeze Below 10 MPH Below 32 o F Freeze Above 10 MPH Below 32 o F
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 25 to 28 F. This stage can tolerate 15-20 F temperatures.
Heat transfer concepts Conduction 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 ºF, 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 25 to 28 F. This stage can tolerate 15-20 F temperatures.
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 Winds less than 5 MPH Clear sky Cold air mass 30 to 200 ft Inversion develops Advective Freeze Winds higher that 5 MPH May be cloudy Cold air mass 500 to 5,000 ft 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: 1.1 C (34 F) if the dew point is -4.4 C (24 F) or above 1.7 C (35 F) if the dew point is -6.7 to -5.0 C (20-23 F) 2.2 C (36 F) if the dew point is -9.4 to -7.2 C (15-19 F) This recommendation should only be followed when a frost is predicted. Sprinklers may be turned off when the wet bulb temperature has risen to 1.1 C (34 F).
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 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. 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)
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