Row Covers Are Recommended Everywhere This Week in Mid-South (2pm, 3/24/14)

— Written By Barclay Poling

Dear Growers, Agents, Agronomists and Others,

I have been doing electronic strawberry weather alerts since 2000, and I have learned that it is always a good practice to build into my advisories a “correction factor” for when the forecast is really “off”. Personally, I don’t think we’ll ever see a time when forecasters consistently get it right. Over the years I have begged these folks for better wind forecasts, especially at ground level. And, lately I have been very puzzled by a phenomenon that I just started noticing last week in the AWIS 5-day detailed forecast (for specific locations), and what I am noting is they are calling for F (frost) AT THE SAME TIME IT IS OVERCAST (overcast).

Obviously, I need to do some updating in my understanding of how you get a radiation frost. I had always understood it that you need a CLR (clear) sky? Along with a high enough dew point (usually 27 – 31 range), and light to no winds. So, take a look at this AWIS 5-day product for Kenansville, NC, that I just reviewed with a grower on the phone about an hour ago (Table 1).

Table 1. Kenansville 5-day forecast

The obvious thing to note is that there will be F (frost) in this area of the state on:

  • Tuesday morning
  • NOT on Wed morning
  • Thursday morning will be the coldest day, and FROST from 11 p.m. (Wed) through 8 a.m. Thur
  • Friday – should be frost free

This area sort of runs between Goldsboro and Kinston,  in weather conditions. Note how they did have frost this morning, too! Not far away, growers found that it was a waste of time to try to frost protect this morning. I just spoke to grower who got up and started running at 3 a.m., and never made ice. Another grower in Kenly did make ice.

The next thing to note is that it will be OVC (overcast) early tomorrow morning (Tue). Take a look a the table and see how OVC runs through each hour of the early morning and day. Then, note how there is an “F” just below (for frost). This is unexpected! In fact, if you look at a sister product by AWIS, their FROST/FREEZE forecast for NC (for tonight), you see over and over the statement:

**** FREEZE COLD LOCATIONS **** **** LITTLE/NO FROST DUE TO DRY AIR **** Lowest Temperatures: 29 – 36 Louisburg Min 29 Durations at/below 32: 0 – 6 Range Dewpoint Temps: 11 – 24 Range Wetbulb Temps: 28 – 32 AVG Wind Direction/Speed: SE 4 Long Periods of Calm AVG Sky Condition: Mostly Cloudy 

So, it is becoming clearer to me, as time goes on, that we cannot assume an overcast condition will preclude a white frost from forming on your strawberry plants! It is tough enough having to be so skeptical of unreliable wind forecasts, but now it seems that I better not trust OVC any longer! At least, that’s my interpretation for now.

I have already formed ideas about Dewpoints and Frost, and you need to be very concerned even about DPs down to 25 F, and possibilty of frost – which you can see happening in the Kenansville situation above. At 11 p.m. tonight when DP is 24 F and sky is SCT (scattered clouds), there will be FROST. But, what is really remarkable is to see a forecast of F (frost) all the way from midnight tonight until 9 a.m. when it is OVERCAST?

My wife asked me yesterday, “Do growers really want to know all this stuff you pay attention to?”   And, I said, “Heck no!”. They are too busy to read a lot of stuff. They want a short blanket recommendation.

And, so that is what I am giving everyone now, a blanket recommendation. PUT YOUR ROW COVER BLANKET ON TODAY AND LEAVE IT ON.

If you get bored this week, and wish to do a little more reading, then check these advisories for more detailed information. But, the one thing I love about a row cover is that it truly is a “correction factor” for a lot of things that can go wrong in weather forecast. With sprinkling, you can’t be off by very much WITHOUT MAKING COSTLY MISTAKES, but with row cover protection, there is this wonderful fudge factor.

2nd NIGHT (Tue/Wed): (will there be frost or not?…who really knows!!)

Now, to see why I really like the row cover blanket recommendation this week, just look  in Table 1 at the hourly informaion for Tue night/Wed morning. Now, AWIS is saying NO FROST on this 2nd night. But, notice how cold it might be? Bascially, it will be as cold as tonight. The reason there is NO FROST though for Wed morning is due to those winds! A grower said today to me how worried he is about high winds on Tue night/Wed morning. So, while I had him on the phone, I said let’s talk about those winds you are so worried about (that is when he started telling me about the wet cover worry and freezer burn as well). But, I told him that I have been following AWIS for nearly 25 years, and I do not get excited about winds of 9 mph. When they indicated winds of maybe 14-15 mph, they’ve got my attention. But, I cannot tell how many times these kinds of forecasts on winds CHANGE. In fact, the Easter FREEZE of 2007 was not a windborne freeze, but a black frost for all of eastern NC. Yes, the were calling for winds in teens, and they ended up being less than 5 mph. But, the real problem with winds and having to irrigate is that you have this devilish quandary of having to start sprinkling at a time in the early evening when winds really are a problem for sprinkler irrigation alone. The beauty of row covers and sprinkling is that the wind effect is minimized, and then you can nicely focus on monitoring blossom temps beneath the covers with the digitial thermometer that your wife got you for Christmas, and how thankful you are to own that instrument today. By closely monitoring the open blossom temperature underneath the cover you know exactly when there’s trouble. And, there’s trouble when that blossom starts dropping below 31/30. You need to run water on top of the cover when this starts happening, and once you start irrigating you don’t stop until the next morning when the blossom temperature stays above 32 F. But, what do you do if you don’t know blossom temperatures beneath the cover. Answer:  it is a precarious situation to be in. I would buy one now – even if you don’t get it delivered until a week from now. You will always have use for this instrument – even for evaporative cooling later on.

So, let’s return to Tue night/Wed morning in Kenansville again, and realizing that winds of 9 mph may be more like 3-5 mph on Wed morning, then you are right back into having a FROST. And, now the skies are clear. The temperature is cold enough, and I would say those dew points in mid-20s are high enough to support frost (i.e. air is not too dry). Supposing it does frost on Wed morning, it sure would be nice to have a row cover on!

3rd Night (Wed night/Thur morning):

Then, looking at Wed night/Thur morning, this is supposedly the BIG ONE for Kenansville with a low of 24 F (Table 1). But, in looking at the whole event, I also see CLEAR skies, winds dropping off nicely after 7 p.m. (1900) to 5 mph, and then down to 2 mph, and also dew points in 23-24 F range. Not exactly dry air, as you expect with dewpoints in teens or very low 20s, but not really that mosit either. So, in handling a cold event like this, it is really challenging to know just when to start sprinkling. Do you play safe and start at 6 p.m. (1800) when wet bulb is 31, or chance waiting a while longer? But, if you wait to see “ice crystals”, would that be too late. Its almost an event requiring a new label, like brown frost. But, why go there at all if you apply a row cover, you don’t have to be concerned about any of this. Except, you do want to monitor bloom temperatures closely, which requries a _________  ______________________ (digital thermometer) with ___________________  ___________ (thermocouple wire) inserted in the bloom. If your area is going to be colder than 22 F, then you can consider a double row cover for extra protection (8 F protection  is about the limit for 1.25 oz), and that can go on today. Or, if you detect blossom temps slipping below 31 F with your digital thermometer and thermocouple, turn on the sprinkler irrigation.

Do you see now why row covers make so much sense this week!

So, if your covers are already on, then leave them on for THIS WHOLE CRAZY WEEK where we may even experience a “brown frost“. 


clyde 2

Fig. 1. Photo from South Carolina farm yesterday where crop is at its most vulnerable point in the season. The unexpected frost this morning forced this producer to use sprinkler irrigation, but that also made the row covers WET. And, now they will have to be pulled wet back over the crop because of extreme conditions expected tonight. The grower has decided he has too much at risk, and will sprinkle on top of covers tonight and tomorrow morning.

Another SC grower that I have been in close contact with this weekend was originally not going to pull his covers over the crop this weekend, but when he saw the forecast showing potential for frost this morning in his area, he pulled yesterday morning while the covers were dry. He went with “his gut” and I discussed his decision in a weekend advisory, and said that I saw no real problem with pulling the covers when he did. The reason he was hoping to hold off with the covers was related to another crop requirement — sunshine, winds and bees for good pollination and fruit set! Even though he pulled covers for this morning’s frost, he was ready to pull them back today. But, we talked and decided it’s really “too darn cold”  for much bee activity today, and something tells us both that with some very high winds in forecast this early week, there will be some pretty darn good wind movement even under the covers soon!

The real trick may be keeping them on!

For the growers who did have covers off this weekend, the good news is that THEY REALLY DID GET excellent bee activity!

If only we could string 2 or 3 nice days of weather/week? I know, I am dreaming. But, the problem with row covers being “ON” too long (MORE THAN 3 DAYS IS TOO LONG IN BLOOM PERIOD), is that you definitely get poorer pollination conditions with the covers being on during peak bloom. And, if pollination is poor, you get “really ugly fruit”.

So, as soon as we get through this week’s exciting (and possibly historic) arctic clipper, please plan on GETTING THOSE COVERS OFF on Friday! I doubt any sooner for most areas, as we have got nearly a full week of sub-freezing conditions coming our way.

We can’t afford to have any more of our spring hijacked by winter. In fact, this spring is going so poorly that a grower near Ellerbe, NC, quipped this morning:

“In the 52 years I have been here spring has always come, but it seems to be hanging in the balance just now.”

How do you know when your row cover is not up to snuff?

blooms dead

Fig. 2. Grower wrote: Three Sweet Charlie blooms under a 1.2 oz Typar cover last night with a low temp of 24. Did the one on the right make it??

Reply – no it did not. I will need to check and see if other growers are experiencing such losses with 1.2 oz Typar? It should have provided enough protection for a 24 F low. I will follow up when time and see what was meant by 24 F low? Was that an weather  shelter temp, canopy air temp, or blossom temp?

How cold was it last night? Due to extreme time limitations this morning, I am going to skip the usual grower reports, but I do think that this report from Eric Hunter in Easley pretty much captures what is happening with our fast “deteriorating” weather conditions.

Here is Eric’s report from last night:

Hey Dr. Poling,
Here is our latest information.
Our lows last night at both locations were 26 and 29. Forecasts were NOAA 33, Accu 36, SkyBit 29, and AWIS (Table Rock Dam) 28.

Editor’s aside:   Eric  has figured out a more distant location, Table Rock Dam, gives him “better” data from AWIS than a nearer one (this is a really ‘smart’ thing to figure out over time – I have seen a grower in Seven Springs do the same thing – he uses RDU as a good indicator of how cold it may get at his farm near Kinston. It is always a good idea to “peg” a location, even if it is not near you, that actully matches up pretty darn close to your farm’s actual temps! SkyBit provides another reference point as well, and it showed a low in canopy of 29 – very similar to Table Rock. And, in just speaking to the NC grower who has learned to gauge his farm by RDU, they have also installed an on-farm weather station this year, to provide further on-site information that is critically important in a week like this – in the end, you will find that forecasts are just forecasts, and will predictably flip-flop around! The grower has to be careful not to place too much stock in any one forecast service, though my experience has been that some are much more reliable for certain weather products than others. And, once you understand the “strengths” of each service, you begin to build greater confidence in what might really occur. The one thing that still leaves me speechless is how many NC growers still do not have a digital thermometer with thermocouple wire that can be inserted in the blossom to guide all on site decision making once the cold event arrives!


1. Weather conditions will predictably keep changing this week, and what plays out tomorrow could be a lot different from the forecast today. Forecasts are inherently poor on wind speed products, and these should never be fully trusted. Recently, there have been frost occurrences with OVC skies. There is even question on one night this week whether one of the several cold events even fits into any existing cold event category? So, with this much variability to deal with in the week ahead, it is best to make a row cover “blanket recommendation” for our whole region. A  row cover of 1.25 oz/sq yd is a good overall choice. The cover will potentially be sufficient for what’s ahead in your location, and nothing else needed. The cover saves water, and can help to minimize issues with botrytis and need to keep re-applying sprays (a big issue with sprinkling).

2. The typical problem with row covers during bloom is related pollination problems that occur from leaving them on for several days. But, this not so much an issue this week. Temperatures will be quite cold all week and not conducive to good insect activity for pollination.

3. In areas prone to temperatures that exceed the limits of the row cover, a second row cover can be the answer (if you have the extra cover)

4. If the protection level of the cover is going to be inadequate, you can use spinkler irrigation on top of the cover, and once you start do not quit until the blossom temps are safely above freezing.

5. If the cover becomes  wet (from rain) and freezes before nightfall, then plan to monitor blossom temperatures closely (with digital thermometer with thermocouple inserted in blossom) and start sprinkling on top of covers when blossom temps begin to dip below 31/32, and don’t stop until next morning when blossom temps are 32 and higher.

A follow-up advisory of pointers for sprinkler irrigation use in these conditions is being prepared presently, but under this weeks conditions, there could be some real challenges with sprinkling alone, and it is strongly recommended that you apply a row cover – if you can find one?

p.s. are you possibly looking for a digital thermometer?
Here is one that I ordered last year:
dig therm
Important note on thermocouple wire from a grower:A different wire
Figure 1. At 9:30 a.m. (EST) a strawberry grower in Ruther Glenn, VA (north of Richmond) recorded a blossom temperature of 92.8 F on a Chandler plant.

Digital thermometer and thermocouple wire that inserts right into blossom for exact temperature measurement  – useful for cold protection at this time of year (March) as well as evaporative cooling later on (heat protection of blossoms).

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
Updated on Mar 24, 2014
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