Not the Time for an Experiment! Use Wet Bulb of 33 F for Shutting Down Irrigation This Morning! (6:35am, Saturday,3/4/17)

— Written By

Good morning,

In managing a freeze event with sprinkler irrigation, it is critical to know when it is “safe” to shut down the system in the morning. It just so happens that we are experiencing right now (6:35 am, Sat) a freeze event in NC and across much of the region that is not any different from the Easter freeze of 2007 from the standpoint of very low dew points this morning. That particular event was technically a black frost:

Black frost. Many growers are unfamiliar with black frosts. This may be partially due to the fact that they occur with considerably less frequency in strawberry plantings than hoar frosts. Another reason may be due to their “invisibility.” Few or no ice crystals form on plant surfaces in a black frost because the lower atmosphere is essentially too dry. Thus, the grower who depends on seeing evidence of “frost” (ice crystals) before starting countermeasures (for frost protection), could potentially suffer catastrophic losses if the dew point temperature (frost point) is below the critical temperature of the open blossom.

Given that we are  experiencing a black frost event presently, it is imperative that growers using sprinkling realize that you can forfeit an entire night of cold protection by shutting the irrigation off too early in the morning. Take it from me, and my former Research Technician, John Earp, we can tell you all about the dangers of shutting down too early on a morning just like this one. Just a minute ago I sent a grower in Faison area a text that said, “Keeping running irrigation until wet bulb of 33 is reached this morning.”  Since this particular farm is relatively close to Kenansville, I noted at about 5:30 am that the

AWIS Hourly Forecast For Kenansville-Dup, NC

shows a wet bulb of 33 F being reached at 11 am (just click on Kenansville-Dup below). IN THE INTEREST OF TIME, I don’t think it is appropriate at this time to enter into a longer explanation of why it is important not to shut down before a wet bulb of 33 F is reached, unless you are interested in conducting an experiment with your crop this morning?

So, please click on the AWIS Hourly Forecast that is closest to your farm to determine the hour when a wet bulb of 33 will be reached for safe irrigation shut down. Alternatively, you can use a digital thermometer with thermocouple inserted right in the blossom (this had to have been done yesterday) to figure our when it will be safe to cut off. In working with Rudd Farm in Greensboro over the years on cold protection issues, we have learned that when this instrument reads 32 F, you can safely shut down. Of course, if you are irrigating on top of row covers, then having a digital thermometer to guide your decision on safe shutdown is the very best approach! No need to consult wet bulb temps when you have one of these instruments. It is also valuable to note that SkyBit includes a table at the bottom of their forecast to give you easy access to hourly wet bulb temperature information (I have included a SkyBit from 6:25 am for Clayton and another for a VA location)

10-DAY DETAILED HOURLY WEATHER FORECASTS

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

Figure 1. Note the Hourly Forecast for Clayton shows a wet bulb of 33 will be reached at 11 am. At this location they are likely using row covers, so this information is not needed. Wet bulb information is critical to any grower using sprinkler irrigation!

Virginia – consult wet bulb temps for location nearest your farm

10-DAY DETAILED HOURLY WEATHER FORECASTS

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

Figure 2. The wet bulb for this location in VA will not be 33 F until NOON. Actually, I know this grower very well, and because he has row covers that he has been irrigating on top of, AND because he has a digital thermometer to tell him when the blossoms are reaching 32 F, my guess is that he can be shutting down well before noon. BUT, this is where it is critical to own a digital thermometer with thermocouple inserted in blossom to make this determination.

A digital thermometer with thermocouple will help improve your decision-making by:

1) Providing the actual temperature of the blossom, which can differ from the air temperature around the blossom.

2) Telling you when to start irrigation.

3) Telling you when to stop irrigation in the morning.

Even if you use row covers, rather than sprinkling, as your primary method of frost, the digital thermometer remains an indispensable tool for extra chilly nights when you may need supplemental heating, using irrigation on top of the row cover.

When to start irrigation in a freeze?

If you use only sprinkler irrigation for cold protection, be sure to start irrigating as soon as the digital thermometer indicates the blossom temperature is 31/32 F. The air temperature may still be as high as 38 F if the air is dry.

If you combine irrigation and row covers, we suggest starting irrigation on top of the covers as soon as blossom temperatures beneath the covers fall to 28 F.

Monitoring system performance during irrigation

The digital thermometer is an excellent tool for monitoring the success of your protection. If the blossom dips below 31 F, you need to step up your irrigation rate!

When to shut down?

The digital thermometer helps to eliminate guessing when to stop irrigation in the morning. On very cold mornings with wind, you may need to keep running well past sunrise. When blossoms provide a reading of 32 F or higher on the thermometer, you can safely stop irrigating. Again, the blossom temperature differs greatly from the air temperature, and you want to keep irrigating until the blossom temperature reaches at least 32 F.

South Carolina – consult wet bulb temps for location nearest your farm

10-DAY DETAILED HOURLY WEATHER FORECASTS

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

Georgia – consult wet bulb temps for location nearest your farm

10-DAY DETAILED HOURLY WEATHER FORECASTS

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

Written By

Photo of Dr. Barclay PolingDr. Barclay PolingFormer Professor and Extension Specialist, Strawberries and Muscadines (919) 515-5373 (Office) barclay_poling@ncsu.eduHorticultural Science - NC State University
Posted on Mar 4, 2017
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