Use Caution With Irrigation Shut-Down Times This Morning (7:15am, Sunday, 3/5/17)
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Figure 1. Sunrise in China Grove, NC (March 5, 2017). This grower has been using sprinkler irrigation to protect his strawberry crop – last night was the 3rd night in a row of sprinkling! In this location the wet bulb temperature of 33 F will not be reached until 10:30 a.m. (Salisbury-Rowan, NC AWIS Hourly Forecast).
Figure 2. At 8 a.m. it was 22 F outside the row cover and 34 F underneath (Moseley, VA). Using irrigation with row covers was essential at this location with temps in low 20s. A row cover alone would not be sufficient protection! Yes, it is a great idea to double up on covers when temps are heading into low 20s at bloom, but not all growers have that option. Fortunately, this grower and his helpers, will be able to shutdown soon this morning and go to bed — they might want to enjoy some hot soup has well! Technically, you are ok to shut down with blossoms reading 32-33 on digital thermometer, but with winds and dropping dew points, you want to make sure things don’t go in reverse. I am guessing that by maybe 9 a.m. (with the sun shining brightly on top of the ice covers), that the blossom temps beneath will be up around 38-40 F by 9 a.m.? What is really quite interesting is to look at the SkyBit for this location, and note in bottom table that the wet bulb of 33 F will not be reached until 2 p.m., so that’s about 5 more hours of sleep for the grower using row covers in conjunction with sprinkling. The SkyBit for farm is at bottom of advisory. Oh, I just heard now from grower and they are shutting down at 8:45 a.m. One more night of cold protection tonight, and then covers coming off Tuesday or Wed (whenever it is dry enough!).
I have received several reports from growers this morning from different parts of Southeast, including right here in Virginia where I now live (Williamsburg). The thing that has really “struck me” about last night and now this morning are the very low dew points. We had a low of about 25 F here this morning, and never saw any “white frost.” No wonder! At 7 a.m. the dew point was 16 F, and its going down! AWIS shows that Williamsburg will see dew points in single digits by 11 a.m. Also, there will be light winds this early morning to go along with dropping dew points. THIS COMBINATION OF DROPPING DEW POINTS AND WINDS CAN BE LETHAL TO YOUR STRAWBERRY CROP. CONDITIONS ARE IDEAL “EVAPORATIVE COOLING” – THIS PHENOMENON CAN CAUSE THE TEMPERATURE OF A BLOSSOM TO “DROP AFTER SUNRISE” BELOW ITS CRITICAL TEMPERATURE! Believe me, that is something you don’t want to do! Of course, if you have been using row covers (alone), you do not need to be concerned about evaporative cooling, but you do need to leave those covers in place as there will likely be one more night of this!
For growers using sprinkler irrigation, it is important to double-check these tables below to see when the WET BULB temperature will be rising to 33 F today (I normally say morning, but some areas may need to run past noon time – sorry about that). Of course, if you are sprinkling on top of covers, then you will let your digital thermometer and thermocouple inserted in the blossom dictate when it is safe to stop watering. I think those growers will be pleasantly surprised to learn how much sooner they can safely shut down than growers who are using sprinkling alone.
Getting some practice consulting wet bulb temps:
Example: So, if you are not accustomed to consulting hourly forecast tables for wet bulb temperature information, then, for practice, please click on Salisbury-Rowan, NC, and you will see that at 7 a.m. the temperature was about 27 F, but the wet bulb was down around 25 F. The wet bulb needs to reach 33 F for safe shut down, and that should happen between 10 a.m. (Air = 39 WB=32) and 11 a.m. (Air =42 WB=34).
Another example over in Chesapeake VA:
Unfortunately, for my VA grower friends over in Chesapeake this morning (where we had the Field Walk last week), the dew point is dropping this morning (like Williamsburg), and the wet bulb of 33 F will not occur until mid-afternoon! This is indeed a very strange situation! But, if you look at the Hourly Forecast for Chesapeake (click on Chesapeake-Ches, VA), you will see that dew points are dropping today and winds are increasing! Low dew points and winds DRIVE evaporative cooling. Hopefully, most of these growers are using row covers and not relying on sprinkling alone? The low over there this morning was about 26 F (row cover is perfect for this type of situation).
So, please be sure to consult the AWIS Hourly forecast for the city/town closest to you to see when the wet bulb temperature of 33 F will be reached this morning, or afternoon!
Unfortunately, we still have one more night of this to go! (Report coming out next)
SkyBit for Farm shown in FIG 2