Deciding on Your Best Cold Protection Strategy for This Thur / Fri (Issued 12:06pm Wed., 3/20)

— Written By
fulks-sunset low res

Sprinkling on top of row covers is a technique that strawberry growers in NC and VA used with great success during the Easter freeze of 2007 to protect open blossoms. The cold event heading our way this Thursday night (3/21/13) and Friday morning (3/22/13) is going to be best managed with using row covers – the thicker the better! Some growers are adding a 2nd row cover this Wednesday (3/20) before winds become too high to apply covers. Most growers have 1.2 oz covers. If you have a 1.2 oz cover, and add another 1.0 oz cover, that should provide plenty of “insurance” in case it gets even colder than what is being forecasted. Right now, we have many areas that will have low 20s in the canopy on Friday morning, and the crop is highly susceptible to injury at this point in the season with so many farms reporting several new blossoms and popcorns per plant in the last week. Popcorns and open blossoms will be killed at temperatures below 28 F. The larger tight buds can be killed at 27 F. If the grower has only a single cover of say 1.0 oz or 1.2 oz; minimums that will be in the very low 20s (e.g. 21-22), and lots of vulnerable blooms, popcorns and tight buds, then sprinkling on top could be a very good idea! But, to do sprinkling on top of a row cover, you definitely need a digital thermometer with thermocouple that can be inserted into the bloom (to monitor bloom temps closely – start sprinkling before bloom temperatures get below 33-34 to avoid a cold jolt). Good luck! ebp 1 pm, 3/20/13 (1st day of spring).

fri morning

Figure 2. Friday morning minimums across the state of NC. “Additional weather details, including custom weather forecasts, are available from AWIS at www.awis.com.”

Dear Growers, Agents, Agronomists & Others,

A potentially dangerous freeze is coming our way on Thur night/Friday morning! Some areas will also have a FROST tonight! In an early morning analysis of the AWIS State FROST/FREEZE product I noted that Elizabethtown will have a low of 31 F tomorrow morning, and FROST will likely occur in that southeastern NC location tomorrow morning at around 6-7 am. So, let’s take a real fast look at this particular AWIS product before we tackle a much tougher assignment, which is the cold event coming Thursday night and Friday morning.

AWIS Weather Services, Inc.
North Carolina Frost/Freeze Forecast
Produced at  507 AM CDT on Wed Mar 20 2013

… Forecast for Tonight …

Northeast NC Coast
Currituck/Camden/Pasquotank/Tyrrell Counties

**** NEAR FREEZING COLD LOCATIONS  ****
**** LITTLE IF ANY FROST DUE TO CLOUDS ****

Lowest Temperatures:      34 – 40
Elizabeth_City Min          35
Range Dewpoint Temps:     19 – 32
Range Wetbulb  Temps:     32 – 39
AVG Wind Direction/Speed:  SW  6     Long Periods of Calm
AVG Sky Condition:         Cloudy

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

03/22/13  03/23/13  03/24/13  03/25/13  03/26/13  03/27/13
——–  ——–  ——–  ——–  ——–  ——–
27 – 33   32 – 41   38 – 46   42 – 48   36 – 44   35 – 43
——–

East Central/SE NC Coast
Carteret/Pamlico Counties

**** FREEZE COLD LOCATIONS  ****
**** LITTLE IF ANY FROST ****

Lowest Temperatures:      32 – 38
Morehead_City Min           38
Durations at/below 32:     0 –  1
Range Dewpoint Temps:     21 – 31
Range Wetbulb  Temps:     33 – 39
AVG Wind Direction/Speed:  SE  5     Long Periods of Calm
AVG Sky Condition:     Mostly Cloudy

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

03/22/13  03/23/13  03/24/13  03/25/13  03/26/13  03/27/13
——–  ——–  ——–  ——–  ——–  ——–
26 – 30   37 – 39   47 – 48   46 – 49   39 – 42   36 – 40
——–

Southeast NC
NWS FORECAST ZONES 87-90,96-101

**** FREEZE COLD LOCATIONS  ****
**** LITTLE/NO FROST DUE TO DRY AIR ****

Lowest Temperatures:      32 – 38
Elizabethtown Min           35
Durations at/below 32:     0 –  1
Range Dewpoint Temps:     17 – 37
Range Wetbulb  Temps:     31 – 40
AVG Wind Direction/Speed:   S  7
AVG Sky Condition:     Partly Cloudy

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

03/22/13  03/23/13  03/24/13  03/25/13  03/26/13  03/27/13
——–  ——–  ——–  ——–  ——–  ——–
25 – 30   36 – 40   45 – 51   45 – 52   36 – 43   35 – 39
——–

North-Central NC
Franklin County Area

**** FREEZING TEMPERATURES ****
**** LITTLE IF ANY FROST ****

Lowest Temperatures:      27 – 34
Louisburg Min               27
Durations at/below 32:     0 –  6
Durations at/below 28:     0 –  1
Range Dewpoint Temps:     14 – 23
Range Wetbulb  Temps:     28 – 37
AVG Wind Direction/Speed:  SW  7
AVG Sky Condition:     Mostly Cloudy

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

03/22/13  03/23/13  03/24/13  03/25/13  03/26/13  03/27/13
——–  ——–  ——–  ——–  ——–  ——–
22 – 30   32 – 40   36 – 44   37 – 45   33 – 40   31 – 37
——–

North/Central NC
Alamance County Area

**** FREEZING TEMPERATURES ****
**** LITTLE IF ANY FROST ****

Lowest Temperatures:      28 – 33
Graham_AG Min               31
Durations at/below 32:     0 –  5
Durations at/below 28:     0 –  1
Range Dewpoint Temps:     15 – 22
Range Wetbulb  Temps:     26 – 37
AVG Wind Direction/Speed:  SW  7
AVG Sky Condition:     Partly Cloudy

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

03/22/13  03/23/13  03/24/13  03/25/13  03/26/13  03/27/13
——–  ——–  ——–  ——–  ——–  ——–
23 – 30   35 – 40   38 – 43   39 – 43   35 – 40   31 – 37
——–

South/Central NC
Richmond County Area

**** FREEZING TEMPERATURES ****
**** LITTLE IF ANY FROST ****

Lowest Temperatures:      28 – 35
Ellerbe Min                 35
Durations at/below 32:     0 –  5
Durations at/below 28:     0 –  0
Range Dewpoint Temps:     17 – 28
Range Wetbulb  Temps:     26 – 38
AVG Wind Direction/Speed:  NW  7
AVG Sky Condition:     Partly Cloudy

Extended Forecast: Range of Lowest Min Temperatures in the Above Zones
Min Temps Valid For Morning of Given Date (May NOT include ALL cold pockets)

03/22/13  03/23/13  03/24/13  03/25/13  03/26/13  03/27/13
——–  ——–  ——–  ——–  ——–  ——–
26 – 29   36 – 42   41 – 47   42 – 47   36 – 40   33 – 37
——–

For more detailed information visit www.awis.com or call 888-798-9955.
Copyright 2013 AWIS Weather Services, Inc. All rights reserved.

Summarizing the forecast for tonight:

1. Northeast NC (e.g. Elizaebth City): there will be long periods of “calm” with minimums from 35-40, but it should also be cloudy, so I don’t see much threat of frost

2. East Central (Coastal):  again, mostly cloudy in this area…and minimums 32-38…some periods of calm. I don’t see much threat of frost

3. Southeast NC (e.g. Elizabethtown):  FROST will likely occur before sunrise…DP is 28, and plenty high for frost formation; they are saying partly cloudy, but if skies clear look out! The minimums are 31-38

4. North Central (Franklin Co):  27-34 lows; low DPs; thus, there are going to be freezing temperatures, but I am not sure they will get quite low enough? I would get covers on NOW (WED) and be prepared just in case there is a frost tomorrow morning. THERE IS DEF A good potential for a BLACK FROST on Friday morning (more on that later).

5. Central NC (e.g. Bulington): same as North Central – get covers on today to be safe. SkyBit is calling for lows the next morning of 21 F

6. South Central (Ellerbee):  I just spoke with a grower in this area and SkyBit is saying a low of 24 in canopy tomorrow (3/21-3/22), but for tonight the air temp mins are around 28-35, partly cloudy, 17-28 DP, winds of 7 mph from NW. I don’t think there will be frost Thur morning, but we definitely anticipate very cold temperatures in this region on Friday morning, and the grower who I spoke with is now putting on double covers today.

What’s ahead for Thur/Fri?

The “minimums” being mentioned today for 3/21-3/22 are down into the low 20s for many areas of the piedmont like Burlington or Gibsonville. I just spoke with Craven Smith in Gibsonville, and he said his local media is calling for the possibility of low 20s, and that has him worried. His SkyBit is saying the temperature outside the canopy will be 26 F and inside 21 F on Friday morning. AccuWeather is also forecasting around 22 F in Gibsonville.

What about his crop stage? Here is what his crop looks like this morning:

craven smith photo 3.20 row cover since mid Jan

Figure 2. This is  not surprising to see how much more advanced AND VULNERABLE the crop is when it has been under cover since mid-Jan (they have only been recently uncoverd). Losses could be EXTREME to a 21-22 F cold event! All flower stages will be killed –  the only stage that can escape (possibly) are the very smallest tight buds. Pretty much everything is on the line if your crop looks like this, and the forecast is for 21 F in the canopy. If the correct protective measures are not taken, then this crop could see a serious reduction in yield this season, and the season will start at least a week later (its already late enough).

So, what did Craven decide is best to do? Well, we talked about how much more susceptible his crop is because the covers have been on since mid-Jan (you can tell from the photo)

What to do:

No. 1 – Craven is getting his labor together NOW (Wed) while the winds aren’t too bad, and they are going to apply DOUBLE COVERS. Why double covers? Well, with a 21 F minimum, a single 1.2 oz cover may not do the job (Fig. 3). He is lucky that they have some unused covers (for vegetables) that they are now applying this late morning – they want to be ready! And, if the winds are fierce tomorrow, they will be glad they took the opportunity today to add the extra covers.

Row cover propterties

Figure 3. Growers would be well advised not to count on protection from a 1.2 oz of more than 24 F. The 1.5 oz can get you down to 22 F, but 20 is a stretch.

CALL FROM NASH COUNTY 11:12 AM

A grower just learned that there are no 1.2 ounce covers to be found anywhere! He asked if given a choice, would I buy the 1.0 or 1.5 oounce. I said 1.5 oz. The 1.5 oz will give enough protection (Fig. 3) so that a grower with minimum of 22 F could sleep pretty well! Honestly, I am begining to really think that there is more place for 1.5 oz cover all the time when you consider the added warmth of this cover – it is just what we need in a week like this!

Summary points:

  • If your crop is now in the open blossom/popcorn + lots of nice “fat” emerged bud stage, the first thing I would do is examine the minimum temperature forecast for your farm using a product that is location specific (like SkyBit), and if you don’t have this service, then please check this AWIS minimum table NOW. AWIS NC Table Min Temp
  • FOR THE NEXT 2 NIGHTS, ROW COVERS ARE THE BEST STRATEGY.  Check the minimums for your area? If you are in the low 20s, and you have only a 1.0 or 1.2 oz cover, you may be well advised to “double up” if you have some extra covers? If you are now on the market for a cover (today), then I would buy a 1.5 oz cover over a 1.0 oz – the 1.5 will give some needed additional protection if you are in one of these areas headed into low 20s. I don’t have that much faith in a 1.2 oz cover below 24. Be sure to get covers on at no later than 3-4 pm to get some good heating underneath the covers before night. It is that heat reservoir that carries you through the coldest hours of the early morning.
  • The cold event we are going to have on Thur/Fri could get real ugly if you don’t know much about frost protection, and all you have is sprinkling. The reason I say this is because we are dealing with potentially very high winds going into Thursday night (one weather service which I do not trust for winds, says the winds will be above 10 mph all night and morning, but a more reliable service that I just checked for Knightdale area is saying they will drop off after about 8 pm and will gradually get down to around 3 mph – this is very manageable for sprinkling. Technically, this is what I call a Black Frost (you won’t see ice crystals forming b/c the air is way too dry).
  • For the irrigation crowd:  You cannot play games with your start up time and it must be before the wet bulb drops below 31 F. In some areas it will hit 31 wet bulb soon after sunset on Thursday. So, it is possible that non stop pumping will be for at least 12-13 hours! Make sure everything is in top top shape with your system before using it. Chances are excellent that winds will be high in early evening on Thur and with such low dewpoints it is pretty predictable that most of you will need to be running soon after sunset! Here is an example of when sprinklers need to go on in Kenansville, NC:
  • Kenansville
    • Thur March 21 wet bulb is 32 at 2100 (9 pm) – start irrigation up (at this same hour air temp is 38 F (wet bulb is 6 F colder). You can calculate the wet bulb if you know the air temp and dewpoint by what many forecasters use to determine the wet-bulb temperature, and its called the “1/3 rule”. The technique is to first find the dewpoint depression (temperature minus dewpoint). Then take this number and divide by 3. Subtract this number from the temperature. You now have an approximation for the wet-bulb temperature. Here is an example: suppose the temperature is 38 degrees Fahrenheit with a dewpoint of 19 degrees Fahrenheit. The dewpoint depres
      sion is 38 – 19 = 19. Now divide 19 by 3 = 6.33. Now subtract 6.33 from the original temperature of 38. 38 – 6.3 = 31.7.
    • Friday March 22 the wet bulb returns to 31 at 9am (see Table) – then, it is safe to shut down, and not before!
  • Finally, if you do not believe the covers alone will give you enough protection, then there is the option of running water on top of covers (assuming you have both technologies). Unfortunately, to do sprinkling on covers you MUST HAVE A DIGITAL THERMOMETER to tell you the temperatures of the blossoms beneath the cover during the night. This tool “guides you” by telling you when blossom temperatures are not likely to stay above a critical temperature of 28 F (for both the open blossom and popcorn), or 25-27 for tight buds. But, you can’t wait until you see blossom temperatures dropping to 28 to start watering on covers! You must start sprinkling when the blossom temperatures are hitting 33-34 because you can get a nasty cold jolt problem by waiting until blossom temperatures are 28 F.
  • In closing, here are the maps AWIS maps for Thur morning and Fri morning and I added the Sat morning map becasue we are not necessarily going to be to drop our guard after Friday morning is over with!
  • thur morn
  • fri morning
  • sat morning
  • Dr. E. Barclay Poling
    Professor Emeritus/Extension Strawberry Specialist
    Department of Horticultural Science
    Campus Box 7609, 162A Kilgore Hall
    NC State University
    Raleigh, NC 27695-7609
    919-418-9687 (Cell)
    919-515-2505 (Fax)
    barclay_poling@ncsu.edu
    https://strawberries.ces.ncsu.edu

Written By

Photo of Barclay Poling, N.C. Cooperative ExtensionDr. Barclay PolingFormer Professor and Extension Specialist, Strawberries and Muscadines (919) 515-5373 (Office) barclay_poling@ncsu.eduHorticultural Science - NC State University
Updated on Mar 21, 2013
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