AWIS Weather Forecast: Cold Protection/Sprinkler/Double Cover

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Dear Strawberry Growers,

This is the time when we experience some of the worst cold events. Today, I have received several phone calls and emails considering the thickness/weight of row covers, the combination of row-covers, double covering, and sprinkler systems.

The predicted minimum AIR temperatures for Wednesday and Thursday morning will drop to the low 20s in wide regions of North Carolina (Figure 1). Please note that the AIR temperature is measured 6 feet above the ground, and the temperature at the plant can be even lower. Those temperatures can cause damage, even with the use of row covers. After consulting with Dr. Barclay Poling, we both came to the conclusion that many might have to use sprinklers or double-covers, depending on your situation and the predicted temperatures.

Wednesday weather radar imageThursday weather radar image

Figure 1: Predicted Minimum Temperatures For North Carolina Wednesday (3/5/19) and Thursday (3/6/19).

Row-Cover and Sprinklers:

Sprinklers on top of a medium weight cover (1.2-1.5 oz, Table 1) should be considered, if temperatures fall below 22-24 F. However, the effectiveness of a cover+sprinkler system cannot be assessed without measuring the temperatures directly at the plant level (Figure 2).

Image of a flower Image of a gage

Figure 2: The temperature reading on a digital thermometer from this bloom can guide all of your critical decision-making when running sprinklers on top of covers. When the blossom stage gets below 27-28°F, there is a good chance the blossom will be damaged. The emerged flower buds may be killed in the mid-20s. By inserting the thermocouple wire into flower buds ahead of the cold event (during the prior day), you can personally monitor the actual blossom temperature at night under the covers

  • It is important to note that it is virtually impossible to determine the actual temperature of the blossom without using a digital thermometer that is connected to a thermocouple wire that has the very end of the thermocouple inserted directly into the receptacle tissue of the open blossom. About 1/3-inch of the thermocouple wire’s Teflon coating must be peeled back so that the end of the thermocouple may be inserted in the blossom – the two wires should be twisted together (three to four twists) before inserting in the bloom.
  • The relationship between air temperature and plant temperature is not constant, and under dry atmospheric conditions, the temperature of the bloom will be colder than the surrounding air. This instrument will eliminate the need to consult dew point temperatures (or wet bulb temperatures) for the purpose of estimating plant tissue temperatures.
  • Make all of your start-up and shutdown decisions using the digital thermometer readings from at least several blossoms in the lowest part of the field.
  • When atmospheric conditions are dry, it is a good precaution to start sprinkling on top of the row covers when the blossom temperature reaches 32 to 33°F. You should not see ice forming right away by starting at a blossom temperature of 32 to 33°F. It is better to waste some water than risk a lower start-up temperature that could produce a “cold jolt” at startup.
  • After you start sprinkling on top of the row covers, you will actually see from your digital thermometer that the bud and blossom temperatures immediately “pop up” (usually to about 38 degrees F). The reason the temperature actually rises this much is that there is a lot of ground heat. When water freezes, a beneficial release of heat occurs (called heat of fusion). As long as you can keep sprinklers turning, you will be able to maintain temperatures of the floral tissues above their critical temperature.
  • In case of serious winds, you will definitely need extra labor to work with you at night to knock ice off nozzles—you want to keep nozzles rotating all night. The problem with nozzles freezing up occurs in frost/freeze and freeze events; it should not be a problem with a black frost event.
  • In case your pump fails, the author has seen growers come through these situations successfully as long as they are able to get their sprinklers “going again” within 1.5 to 2 hours. Keep monitoring bud and blossom temps with your digital thermometer to know how critical the patient may become.
  • Keep running the water on the frozen row cover past sunrise, and never stop running it until the blossoms in the coldest part of the field are registering 32°F. One of the reasons you need to have several blossoms for readings is that the thermocouple wire that was inserted into the blossom the previous day may slip out or lose good contact with the plant tissue.

Double Covering:

Table 1: Weight of row covers and potential protection temperatures

Light Medium Medium Heavy
Weight (oz/yd) .06oz 1.0oz 1.2oz 1.5oz
gsm 20 grams 34 grams 40 grams 51 grams
Protection 27F 24F 22F 20F

If there is enough labor and cover, double covering will have the potential to protect your blooms from temperatures below 22°F (Table 1). However, several things need to be considered:

  • make sure you have good soil moisture before you apply row covers
  • BOTH covers would need to be applied in the early afternoon
  • Often two medium covers may be able to give you protection into the high teens.
  • If you can, monitor still your flower temperatures and have a sprinkler system in place
  • Sometimes running water through the drip line can add some heat to the system as well.

10-day HOURLY forecasts for the Southeast:

North Carolina:

Ahoskie-Tri-Cou Albemarle_AP Andrews Asheboro
Asheville_Munic Beaufort-Michae Boone_NC Burlington-Burl
Cape_Hatteras_AG Chapel_Hill-Hor Charlotte Cherry_Point
Clinton_NC Concord_NC Currituck_NC Edenton-Northea
Elizabeth_City Elizabethtown_NC Erwin-Harnett_C Fayetteville_AP
Fort_Bragg/Simm Franklin/Macon_C Gastonia-Gaston Goldsboro_AFB
Goldsboro_NC Greensboro Greenville_AP_NC Hickory
Jacksonville Jacksonville_(A Jefferson_AP_NC Kenansville-Dup
Kill_Devil_Hills Kinston_AP Lexington-David Lincolnton_AP
Louisburg_NC Lumberton_AP_NC Mackall_AAF Manteo/Dare_Co
Maxton Mcalf_Bogue_Fiel Monroe_AP_NC Morganton-Morga
Mount_Airy-Moun New_Bern North_Wilkesbor Oxford_NC
Piney_Island_Bom Pope_AFB Raleigh-Durham Reidsville_AP
Roanoke_Rap_AP Roanoke_Rapids_R Rockingham_NC Rocky_Mount-Wils
Roxboro_AP Rutherfordton Salisbury-Rowan Sanford_NC
Shelby_AP_NC Smithfield_AP Southern_Pines Southport_NC
Statesville-Sta Tarboro_AP Wadesboro_AP Washington-Warr
Whiteville_NC Wilmington_AP_NC Winston-Salem

South Carolina:

Aiken_AP Anderson_Co_Air Barnwell_AP Beaufort_Cnty_AP
Beaufort_SC Bennettsville_Je Camden_AP Charleston_Exec_
Charleston_Muni Cheraw_AP Chester_AP Clemson-Clemson
Columbia-Columb Columbia/McEnti Columbia_Metro Conway_AP
Darlington Florence_SC Georgetown_AP_SC Greenville_Donal
Greenville_Down Greenville-Spart Greenwood-Green Hartsville_AP
Hilton_Head_AP Kingstree_AP Lancaster-McWhir Laurens_AP
Manning_AP Marion_AP_SC Moncks_Corner_AP Mount_Pleasant_A
Myrtle_Beach Newberry_AP North_Myrtle_Be Orangeburg-Oran
Pickens_AP Rock_Hill Spartanburg_Down Summerville_AP
Sumter_AP Sumter_SC Walterboro_AP Winnsboro_AP


Albany_Municipa Alma/Bacon_Co Americus_AP Athens_Municipa
Atlanta/DeKalb Atlanta/Fulton Atlanta_Intl_Ar Atlanta_Paulding
Augusta/Bush_Fi Augusta_Daniel_F Bainbridge_GA Blakely_AP
Brunswick_GA Brunswick/Glync Canton_GA Carrollton_AP
Cartersville_AP Claxton_AP Columbus_GA Covington_GA_AP
Dalton-Dalton_M Douglas_Muni Dublin/WHBarron Fitzgerald_AP
Fort_Benning Ft_Stewart/Wrig Gainesville_AP_G Greensboro_GA
Homerville_AP Hunter_(AAF) Jesup_AP La_Grange
Lakeland Lawrenceville_AP Macon_AP Marietta
Marietta/Cobb Milledgeville_AP Moultrie_Muni_AP Newnan-Newnan_C
Peachtree_City Rome_GA Savannah Statesboro_GA
Swainsboro_AP Sylvania_AP_GA Thomaston_AP Thomasville_AP
Thomson_GA Tifton_AP Valdosta Valdosta/Moody
Vidalia Warner_Robins_A Washington_GA Washington_GA
Waycross_AP Winder_AP


Abingdon Ashland_VA Charlottesville Chesapeake-Ches
Culpeper Danville Dublin/New_Riv Emporia_VA
Farmville Fentress_Naval_A Fort_Belvoir/Da Fort_Eustis/Fel
Franklin/J_B_Ro Hillsville Hot_Springs/Ing Jonesville_AP
Langley_AFB Leesburg/Godfre Louisa-Louisa_C Lynchburg
Manassas_Muni(A Marion/Wythev Martinsville Mecklenburg/Bru
Melfa/Accomack Newport_News Norfolk_Hampton_ Norfolk_NAS
Norfolk_VA Oceana Orange_VA Petersburg_(AWO
Quantico Richlands_AP Richlands_VA Richmond
Richmond_Chester Roanoke Shannon_Arpt Stafford_AP_VA
Staunton-Shenan Suffolk-Suffolk Tappahannock_AP Virginia_Tech_A
Wakefield_AP Wallops_Island Warrenton_Fauqui Washington_Dulle
West_Point_VA Williamsburg_VA Winchester_Rgnl Wise/Lonesome_P


Aberdeen Andrews_AFB Annapolis_Naval Baltimore_Sci_Ce
Baltimore/Wash Cambridge/Dorche College_Park_AP Cumberland_AP
Easton_MD Fort_Meade Frederick_MD Gaithersburg_AP
Hagerstown_AP Leonardtown Middle_River Oakland_MD_AP
Ocean_City_AP Patuxent_River Salisbury Stevensville_AP
St_Inigoes/Webst Washington_DC_AP Westminster_AP

As always, we hope that helps,