A Bit of a Quandary…soaked Covers and Possible Frost Tomorrow Morning? (3/7/14)

— Written By

Section 1 – Despite frost in forecast for tonight in some areas (like Central NC), most growers do not have to be concerned if all open blooms are already dead from Arctic Freeze on 3/4/14

I think we can safely say the worst of the bitter arctic air that has been so prevalent ALL WINTER is now behind us, but I am not seeing any evidence of spring in this weekend’s forecast either! In fact, we could have some frost to contend with before sunrise tomorrow? And, that’s not the best news if your row covers are currently rolled up, and completely soaked! If you are in that situation, I recommend using sprinkler irrigation tonight. If you don’t have sprinkler irrigation, and your row covers are OFF, then you need to carefully evaluate whether it is really justified to go to all the trouble of trying to handle a wet cover today? It is not worth it if you’ve already lost the open blossoms and popcorns on 3/4/14 (historic arctic clipper)…I suspect most growers are in this boat today of having lost both open blooms and popcorn stages on 3/4. It will not be cold enough tomorrow morning to injure tight flower buds.


Figure 1. A completely soaked row cover! And the forecast is for more rain  in my immediate region (Raleigh-Clayton-Willow Spring) from 10 a.m. this morning until 6 p.m. tonight. So, if your row cover is now rolled up and wet, it is very difficult to try to unfurl a soaking wet cover (like these poor men are doing). And, is it worth tearing a cover up if it is really not needed? The only growers “at risk” tonight are the ones with live open blossoms and popcorn stage. Its really not going to be cold enough tonight to injure the tight flower buds. So, for most growers (who lost the open blossoms and popcorns earlier this week), just relax! There is very little to be gained today by trying to work with a wet cover to cover a crop that is not needing row cover protection – unless you are going into the mid-20s?

Again, why tear up a good cover when there is no real value to protecting a crop where the open blossoms and popcorns have already been killed (murdered by ‘Titan’ on 3/4).

IN OTHER WORDS THERE IS NO BENEFIT TO PULLING A WET COVER OVER A CROP THAT IS ALREADY HARDY TO MID-20S, AND THE LOWS FOR TONIGHT WILL BE LOW 30s. Let me clarify this even further,  a white frost that may occur in low 30s at canopy level tomorrow morning WILL NOT HURT THE TIGHT BUDS. THUS, IF THAT IS ALL YOU HAVE RIGHT NOW, THIS WOULD BE A GOOD NIGHT TO WATCH BLUE BOODS, and just relax!! But, for growers who do have live open blossoms and popcorns, be sure to read Section 2.

Section 2.  Read this if you have live blossoms (and popcorns)…

For those of you with row covers on right now, and have live blossoms, it would be great if the covers would dry out before midnight? That is about when the front will be moving through and skies could clear and set up a potential radiation frost (the white frost kind). The good news is that there is a lot of wind in the forecast for today and tonight – here is the AWIS hourly information (Raleigh-Durham) and you can see pretty stiff winds from NOW (9:00 a.m. Friday..winds of 18 mph) and then finally tapering off after midnight (see Table below). So, there is a possibility that the covers will be dry by midnight. And, even if the covers do not completely dry out today, I think it is a good thing the covers are already on. Even a moist row cover “should”  provide some level of open blossom and popcorn stage protection. But, you also have the option of running irrigation on top of the moist/wet cover tomorrow.

fulks-sunset low res

Fig. 2. Running irrigation on top of covers is normally reserved for our most severe freeze scenarios (like the Easter freeze of 2007), but with the rather strange scenario tonight of covers already being wet from today’s rain, it could be beneficial to “add heat” to the row cover system with sprinkling on top. The best way to approach the need to do this is to have a digitial thermometer, and thermocouple inserted into some open blossoms beneath the cover.  If you see the blossom temperatures slipping below 31-32, go ahead and start the sprinkling up.

For growers with live open blossoms, and who have already rolled up their covers, just leave the covers rolled up! Use your sprinkler irrigation tonight. Sprinkling is ideal for a situation just like this (where row covers are soaked and basically unmanageable).

When will white frost occur?  What this next table shows is frost (F) is  coming as early as 1 a.m. tomorrow morning for Raleigh-Durham. So, at about 1 a.m. we can expect to see the formation of “ice crystals” on ground level vegetation, wet rolled up row covers, and layflat hoses. If you have sprinkling, that is your cue to start protecting! And, don’t stop until after sunrise and when you see the ice nicely melting away. The wet bulb will reach 33 by 7 a.m. (below), but I would run a little longer to be safe.

AWIS Hourly Forecast For Raleigh-Durham, NC     
Produced at  8:15 a.m. CST on Fri Mar  7 2014
Day-Length = 11:38 / Sunrise at  6:37 a.m. / Sunset at  6:15 p.m.

Local               Forecast For     Friday     March  7, 2014 
Hour  00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
                      MIN= 31                    MAX= 38
TEMP  36 35 35 35 34 34 34 33 32 30 30 31 32 34 34 35 36 38 38 35 34 37 40 39
DEWP  25 25 25 26 26 26 27 29 29 28 28 28 29 30 29 29 29 33 33 26 26 29 33 32
WETB  32 32 32 32 31 31 32 32 31 29 29 30 31 32 32 33 33 36 36 32 31 34 37 36
WNDS  16 15 16 17 18 17 16 15 18 18 17 16 15 14 14 14 13 13 11 10  9  7  5  5
WNDD    NNE      NNE      NNE      NNE      NNE        N        N      NNE   
CLDC    OVC      OVC      OVC      OVC      OVC      OVC      OVC      BKN   
INV    0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
POP6     91       95       95      100      100       66       66        7   
Q1     4  3  3  3  3  4  4  5  4  4  4  7  7  6  7  7  7  3  3  3  0  0  0  0

Local               Forecast For   Saturday     March  8, 2014 
Hour  00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23
                      MIN= 33                    MAX= 65
TEMP  38 37 37 36 33 33 35 35 37 41 45 50 55 60 61 65 65 63 60 57 56 54 52 51
DEWP  31 31 31 30 26 26 29 30 31 34 37 39 41 42 42 38 38 43 44 44 44 43 43 42
WETB  35 35 35 34 31 31 33 33 35 38 42 45 48 51 51 52 52 53 52 50 50 48 48 47
WNDS   3  2  2  1  0  0  0  1  2  3  6  6  7  8  8  8  8  7  5  2  2  2  3  3
WNDD      N        N        W        W      WSW      WSW      WSW       SW   
CLDC    CLR      CLR      FEW      SCT      SCT      SCT      FEW      FEW   
INV    0  0  0  0  2  2  0  0  0  0  0  0  0  0  0  0  0  0  0  0  1  2  2  2
DEWF   D  F  F  F  F  F  F  F  F

Section 3. GROWER REPORTS (unedited)

3.1  VA Beach Report (3/7/14, 8 a.m.) – on Growing Degree Days (a real shortage)

Hi Barclay,

Saturday, March 1:   0 GDD      (low 30.2, high 50.5)
Sun                              1.5            (32.2, 70.7)
Mon                             0               (23.0, 33.3)
Tues                             0               (21.9, 28.6)
Wed                             0               (32.4, 35.3)
Thurs                           0                (34.5, 37.9)
Today’s forecast       0               (high mid-40s)
TOTAL        1.5 GDD   (avg. 0.2 GDD per day!)

Sunday had sun but seldom more than “mostly sunny” and much of the time was only “clouds and sun.”  Yesterday we had two hours of “mostly cloudy” (translation:  a little sun peeked out occasionally), otherwise totally cloudy. Other than Sunday and yesterday, it has been “cloudy” (as in nearly or almost 100%) all week. So very few, if any, additional heat units were gained from the row covers. To add insult to no-heat injury, we have had “clear” skies during part or all of several nights, meaning more radiational cooling.

We had hard rain much of last night and this early morning (~0.8”) with wind mostly 10-15. Since 8 a.m. the wind has increased to 15-20, gusts to 25, plus more rain. As this classic nor’easter passes during the rest of today, wind is forecast to increase to 25-35, gusts 45-55, plus another two-thirds inch of rain. At least soaked, heavy row covers won’t be blowing into the neighbors’ trees!

So March 1-7 has been a total bust so far as our strawberries are concerned (and heat units). Fortunately, we are forecast to have some- to all-sun and warmer Saturday through Tuesday, but then cloudy w/ rain/drizzle next Wednesday. Maybe we can build some heat units!


Tom & Anne Baker and Amanda McCann
Brookdale Farm
2060 Vaughan Rd, Virginia Beach, VA

3.2 Eric Hunter, Easley, SC (March 5th)

Hey Dr. Poling,
We pulled covers off today after putting them on the February 25th (8 days). We hoped to possibly gain some GDD’s during these first couple weeks of March. My intentions were to leave them on until Friday or Saturday, but we have a bit of rain coming in and the temps should warm up nicely this weekend.

Some interesting observations. The covers definitely put us further along into the game. I was quite surprised to see that we have jumped up into the 5% bloom category by my guess. I left a section of plants uncovered, and there is quite a significant difference between the two. The uncovered plants have a number of protruding buds (the exact same place they were when we covered). The covered plants have anywhere from 2-5 fully open blooms. Now this isn’t on every plant of course, but definitely a majority of them. The biggest surprise were with the Chandlers – they have just exploded with growth and have caught up with the Camarosa. The covers definitely gave us some benefit but on the flip side, it also placed me into an earlier mode of having to protect by perhaps a week or so. I traditionally don’t have to worry about frost protection until the 15th or 20th of March.


cam cov hunter mar 5

Fig. 3 Camarosa, covered Feb 25 to Mar 5. Covers are now off.

chanl cov eric

Fig. 4. Chandler covered from Feb 25 to Mar 5.

chan uncov

Fig. 5. Chandler uncovered. The GDD accumulation base 50F  for Easley (from Jan 1 – March 5):  63.5 units (was 94 in 2013)…The readers of Strawberry Weather Alerts know that I posted an earlier advisory saying we expect 2-3 blooms at about 90 GDD (simple method of calcultation with base 50). Clearly, the row cover for 8 days provided the boost in heat units the grower needed to reach 3-5% bloom (Fig. 4). Now the grower has removed his covers, and will not likely re-apply unless needed for frost/freeze. After reaching the 5% bloom stage, it can become pretty “iffy” with leaving row covers on. Row covers will interfere with pollination, and that is not a good thing. So, my vote is to keep them off, unless you are a smaller grower who is willing to recover the crop in late afternoon on days when the nights will be quite cold. Then, the covers come off the next day again. Its a lot of work to do this, but may be worth it for earliness and good fruit shape (this is an idea for small growers to try out).

3.3. Ellerbe, NC (Lee Berry)

Single layer row covers were not effective in saving open blossoms from mid-teen temps that occurred March 3-4…it just got too cold for a 1.25 or 1.5 oz cover. But,  if you had a double row cover (1.0 +1.25), you may have pulled most everything through, like Lee Berry did in Ellerbe:

lee berry success

Fig. 2. Lee Berry sent me this photo on Thursday (3/6/14) and said, “Still alive after the temp dropped to 16 yesterday morning. What I can se is I may have lost 5% of open blossoms…not bad for just row covers.”  This growers GDD accumulation (without covers) is 54 GDD (Jan 1- Mar 5, base 50). I can see 1/2 dozen blooms per plant, so this is now about 10% bloom. Clearly, row covers are responsible for accelerating this crop’s development, as with only 50 GDD you would not see any bloom.

3.4. Willow Spring Grower who Used Sprinkling on Covers (3/3-3/4/14).  I am heading out in the next minute to futher assess the outcome from using sprinkling and row covers both this week for protection of 5% bloom stage blossom against mid-teens and winds. The grower and I spoke last night and here is what we decided:

  •  you should not try to assess the damage from such a serious cold event as this one until 2 days after the event. Reason – on the day after (Wed 5th) some open blossoms looked ok, but by yesterday (Thur) it was clear they did not make it.
  • We think we lost about 25% of the open blossoms, but all the popcorns survived as well as the largest tight buds.
  • Where a single cover was applied (1.25), all open blossoms are dead, and many popcorns were killed.
  • I will be having a look at this planting later this morning to evaluate it further, but with being in the position of having saved 75% of his open blossoms, this grower will def. be irrigating tonight to protect “his gains” from this week
  • My recommendation to him will be to not “mess around” with wet covers (which are now off the beds – rolled up and completely soaked), and to simply run the sprinklers tomorrow morning as soon as he sees the first evidence of “ice crystals” forming on plant vegetation, or on the layflat hoses, at the low end of the field. Sprinkler irrigation is IDEAL for a scenario like this, as his other frost protection tool is rolled up, and soaked!
Dr. E. Barclay Poling
Interim Executive Director, NC Strawberry Association Inc.
& Professor Emeritus (Strawberry Plasticulture Researcher)
Department of Horticultural Science
Campus Box 7609, 162A Kilgore Hall
NC State University
Raleigh, NC 27695-7609
A pessimist sees the difficulty in every opportunity, an optimist sees
the opportunity in every difficulty.
Winston Churchill, Prime Minister of England