Grower Question on Using Sprinkling on Top of Covers (2.17.13)

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Grower question – is there any way a grower could try to save blossoms tonight?

Reply – Under these extremely cold, windy and dry conditions, sprinkling alone is simply too dangerous to attempt, and is not recommended. There may be a way to apply sprinkling on top of row covers tonight to save open blossoms, but this could also be a very risky procedure for reasons explained in this advsiory.

Situation in the state – this is an “arctic clipper”

I am glad that 95% or more of our industry is not interested in trying to save any open blossoms this evening. A lot can go wrong when you attempt sprinkling in winter – that is why ROW COVERS are the safest option for a mid-February cold event. With windborne freezes in the early season, you are almost ways better off using row covers alone when you are mainly concerned about protecting the flower buds, and do not wish to risk more extensive crop losses to evaporative cooling effects if sprinkling is inadequate, or the irrigation system fails during the night.  

If you are committed to trying to save some blossoms tonight, you may wish to first consider “doubling up” on your row covers. If you do not have any extra covers, and you are very serious about trying to save your open blossoms tonight, then you will need to give serious consideration to the sprinking on top of row covers option, but this can still be a very risky procedure to attempt if you fail to start the sprinkling up in time, and keep the sprinklers running all night and into the morning. Never shutdown the sprinklers until you have evidence that your blossoms are above 31-32 F in the morning, and the only way to determine this is with a hand-held digital thermometer. The NCCES has an excellent video on digital thermometers and their use: (see –

For the “die-hards” who are interested in saving open blossoms

If you are still thinking about trying to sprinkle on top of covers tonight, you want to be doubly sure that:

  • Your sprinkling system is in tip-top shape, and that you have plenty of labor on hand tonight for knocking ice off of sprinklers when they “freeze up” due to conditions tonight that favor evaporative cooling problems (low dew points and possibly high winds after sunset).
  • You have an accurate digital thermometer with thermocouple wires that can be inserted into the blossoms directly. There are no tables that will tell you the air temperature, dew point, wet bulb and winds beneath the covers! The only way to decide when to start and stop sprinkling is to have a digital thermometer! ONLY blossom temperatures beneath the row covers late today and after sunset can be your “guide” as to what action is needed and when.
  • In the absence of more experience and experimental data, the best I can do is tell you to use a higher blossom temperature of 33-34 F (blossom temperature as read with digital thermometer) to guide your decision as to when to start up the sprinkling on the row covers in February winter conditions. Normally, we don’t start to sprinkle on row covers at Clayton for cold events during bloom season (March) until the blossom temperature reaches 30-31 (as read by digital thermometer). However, under the very dry atmospheric conditions today and tonight, I think we need an additional “fudge” factor to avoid the dreaded “cold jolt” phenomenon at start up.

digital thermometer

Fig. 1 This is just one example of a hand held digital thermometer. This particular grower was checking blossom temperatures underneath a fresh snow – the reading was 41 F (well above the critical temperature of 28 F for an open blossom) 

Full text of the grower’s question:

At 7:30 am this morning I recevied this note from a farmer north of Raleigh who had beeing thinking about “pulling out all the stops” tonight to protect his open blossoms beneath his row covers. He knows that the row covers may not be “enough” for these extra cold temperatures coming tonight.

“The wet bulb reaches it’s high point of 29 at 4pm this afternoon . Winds up still at 17 mph at 4 . Although I was planning to irrigate tonight on top of covers … I don’t think I will ever be able to get a starting point (31 wet bulb) without having a “cold  jolt ” ! Do you see my situation that way too ?

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

31 F Wet Bulb Rule (avoiding the dreaded cold jolt)

What caused this grower to “freeze” on the whole idea of running water on the covers tonight was the rather bizarre circumstance that the wet bulb temperture will never even reach 31 F today during the day!

Under conditions of high winds and low dewpoints, all strawberry growers adhere to the rule of not turning sprinklers on at wet bulb temperatures below 31 F. This is because of a phenomenon known as the “cold jolt” that occurs in the first 10-15 minutes of sprinkling.

To understand the ‘cold jolt,’ you need to know that the wet bulb temperature is the temperature air cools to when water is added. As soon as you start irrigating, the blossoms actually drop in temperature to the wet bulb temperature. Knowing this, you can appreciate the danger involved with starting at a wet bulb air temperature of even 29 F at 4 pm today, as this temperature is 1 degree lower than the critical temperature (30 F for the open blossom). And, even if we “admit” that open blossoms can often survive down to 28 F, if this grower were to start sprinkling on open blossoms (without row cover protection)  at sunset tonight (5:56 pm), the wet bulb will already be down to 26 F at this hour, and he would basically “jolt” most of the open blossoms soon after irrigation is started.

You can avoid the ‘cold jolt’ entirely by starting before the wet bulb temperature of 31 F is reached.  But, the wet bulb will never reach 31 F today!

However, the presence of a row cover over the top of the strawberry canopy alters the plant’s microclimate in a very important way – air temperatures beneath the covers are nicely elevated, and there is also a different environment beneath the covers compared to the “outside world.” Wind speeds are different, as are dewpoint temperatures beneath the covers, but I am not aware of specific data sets that show the relationship bewteeen “outside” the cover wet bulbs and dew points vs. what it’s like underneath the cover at the same time. We know the 1 – 1.2 oz row covers can provide a nice 4-5 degree “boost” in dry bulb temperatures under the covers, and this is probably enough extra warmth to keep most emerged flower buds out of trouble tonight. Tight flower buds may only need an extra “boost” in temperature of just several degrees to stay above their critical minimum, which can vary from 20-25 (see below).

It is diffcult to pinpoint critical temperatures

It can be very difficult to pinpoint critical temperatures for strawberry reproductive parts such as the “tight buds.” The critical temperature is the temperature at which cold injury (freezing strain) is likely to occur, but we can find exceptions to these average critical temperatures, such as what occurred in Easley, SC, 2/12/12.

But, a 1-1.2 oz cover won’t be enough to keep the more tender “open blossoms” out of trouble if we get minimums in the very low 20s, or teens! Even a nicely super-cooled open blossom will not likely make it much lower than about 26.7 F based on some field research we did in the late 1980s. If the minimum temperature tonight is 20 F, then a medium weight row covers (1 and 1.2 oz) will not be enough! These covers can protect down to about 22 F at the very best.

Row cover propterties

The quandary we’re in (we don’t know the DP beneath the cover)

Sprinkling on top of the covers would be a way of adding “energy” (heat) to the system, and if the grower is able to start sprinkling at a high enough blossom temperature to safely avoid a “cold jolt” right at the beginning of the sprinkling procedure, then I think he/she has a real shot at making this work. But, this is also where we are in some trouble in terms of really having enough good information to guide growers who are trying to sprinkle on top of covers in very difficult conditions, such as we are faced with tonight. Not only will temperatures be dropping into the low 20s and teens in many areas, but did you know the temperature will drop a whole 6 F in just 2 hours time tonight from 5 pm to 7 pm (sunset is around 6 pm). This is a very rapid drop in temperature, and is an indication of just how little moisture is in the air right now. If we knew what the DP was was beneath the cover, that would help our decision-making a lot as far as knowing what the “evaporative cooling issue” is beneath the cover? If we knew this, we could better assess the “cold jolt” risk to blossoms beneath the covers when we start sprinkling on top of the covers. I know from a bad experience last year that when we started sprinkling on a row cover at a blossom temperature of 30-31 F on a windy, low dewpoint night, we had evaporative cooling losses, and the grower was not happy with my recommendation of starting the sprinkling at a blossom temperature of 30-31. The best I can do under these very dry atmospheric conditions today and early this evening, is simply suggest a higher start-up blossom temperature than 30-31. It is my “belief” that a start-up temperature of 33-34 F would be adequate to buffer the 3-4 F cold jolt (immediate drop in plant tissue temperature) at start-up. In other words, if there is a 3-4 degree drop at start-up, then the blossoms would cool to around 29-30 F. If they do not go below 29-30 F at start-up, then I believe you will be successful with this procedure! Assuming that 28 F is critical.

Let me emphasize again that trying to save blossoms tonight is at best an experimental procedure, and I would only recommend trying this by “advanced” strawberry growers. I have seen this back-fire.

It is very important to consider a more conservative blossom temperature for start-up than 30-31, and I am suggesting 33-34 F (as read with digital thermometer). It will also be important to have a tip-top irrigation system, enough labor on hand to keep the sprinklers turning tonight if they should freeze up, and an accurate digitial thermometer.

Winds and low dew points are the enemy, as both “drive” evaporative cooling problems. But, one thing that may be in our favor tonight is the fact that winds could really taper off after about 7 pm. And, what was technically a windborne freeze at 6 pm (sub-freezing temperatures and winds > 10 mph) could be a “black frost” at 7 pm. A BLACK FROST has sub-freezing temperatures and winds below 5 mph. It is actually a radiation frost event without the “white stuff.” With a black frost the air is too dry to make “white frost,” but temperatures will surely get below the blossom critical temperature of 28 F tonight and open blossoms will be killed if it gets into the low 20s (which looks very likely), even though there will be no visible frost on the ground tomorrow morning. Bottom-line: a black frost is a very protectable cold event with row covers and sprinkling — if you know what you are doing and have a little luck!

So, let me conclude by wishing those of you who may attempt this, lots of luck!

Barclay Poling

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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
Updated on Feb 17, 2013
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