Evaporative Cooling on Thursday? Will There Be a Cool Down? (5:30pm, 4/2/14)

— Written By Barclay Poling
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Fig. 1. At Clayton it could reach a high of 89 F in canopy on Thursday (tomorrow), and it “might” be warm enough to cause some “heat damage” to open blossoms of Camarosa. blossoms. The air temperature will only be 82 outside the canopy (at weather shelter), but down at canopy level it will be 7 F warmer according to this SkyBit forecast.

Here is the maximum temperature map from AWIS for NC tomorrow (Friday):


Fig. 2. Most of the state should “not” be in danger of heat damage to open blooms tomorrow, but I would be cautious in areas shown in purple (84-86).

Good evening,

I just returned to my home base from a full day of Strawberry Expo Planning with the NC Strawberry Board of Directors and Advisors, and I noticed on the beltline heading up to N. Raleigh that the air temperature was about 87 F at about 4:15 p.m. Then, a board member, Lee Berry, called me (on his way home) from Southern Pines, and said it was 90 F. To be on the safe side, Lee had one of his workers at his farm in Ellerbe go ahead and do an early morning drip irrigation cylce, just in case! Here is what was posted on this website on April 1st regarding warm/hot temps in bloom:

  • During warm/hot days you can drip irrigate for a few hours in the early morning (around sunrise) to keep plants “fresh” and to minimize the possibility of heat damage to late blossoms and berries that are directly exposed to the sun. On really hot days, you can do daytime drip irrig cycles.
  • During the heavy bloom period (now), you may consider overhead irrigation for evaporative cooling of blossoms. Evaporative cooling with sprinklers is not recommended during harvest, but in the first half of April it can be effective to do sprinkling to “cool things down.”

One thing that is “different now” in early April is the greater amount of exposed black plastic vs. 2 weeks from now when the canopy has filled in more. A full canopy provides very beneficial shading of the black plastic.

There is also a concern that just one week ago this same crop was exposed to temperatures in the low 20s! How well will these same plants handle temperatures 70 degrees higher?

The sprinkling duration for a single evaporative cooling cycle is about 25 minutes. On a day like tomorrow, it may be that only one sprinkling cycle is needed if you see blossom temperatures approaching 90 F – this is the temperature of the blossom, and it can be accurately determined with  a digital thermometer with thermocouple inserted in the blossom like the one shown in this photo:

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.

At 92.8 F in the bloom, this bloom is getting close to being damaged by heat.

sunny day 2

The “thing is”…if it turns out to be a clear afternoon tomorrow (Friday) and only a light scattering of clouds, there is a chance that bloom temps could reach 92-93 F, or higher (with and air temp of 87-88). Over the years, I have observed that a temperature of 92-93 in the bloom can be “related” to an air temp at weather shelter height (~6 1/2 ft) of 87-88 F.

An air temp “trigger” of 87/88

  • The weather shelter air temperature “trigger” for using  sprinkler irrigation for evaporative cooling to protect fresh blossoms from warm temperatures is 87/88°F. So, in Lumberton (below) you can see a high of 86 F. That is just one degree below the trigger of 87.
  • The evaproative cooling procedure works best on “dry air” days – with wet bulbs in low 60s, like tomorrow (look at WETB at 1400 (2 p.m.) of 67 F. This is a good day for doing evaporative cooling.


Step by step instructions (what we do at the research station):

1) we are set up with overhead sprinklers and the precipitation rate of 1/10 inch/hr is fine (low rates are best)
2) we next insert the thermocouple of our Digital Therm in an “open” blossom facing the sky (not shaded by canopy) starting at around 11 a.m. in the morning
3) as soon as we see the blossom reach the 89/90 (not the air, the blossom), we start sprinkling, and we sprinkle for about 25 minutes. This has the effect of reducing the plant and blossom temperatures down into the low 70s, or perhaps even cooler (if the wet bulb is going to be in low 60s, you have an opportunity to really drop the plant temperatures very nicely with this dry air).
4) after the first run, we will check “sky conditions” over the next hour to see if there is any chance of a cloud cover coming in? In the event of some clouds moving in we may will delay or forget about doing another sprinkling for the same duration (25 min).
5) rarely do we ever go a 3rd time, and NEVER past 3:30 p.m.

Here is a possible scenario for this Friday in Lumberton (just one run):

  • 2 p.m. – start up pump to do first sprinkling if air temp reaches 87/88, or you get a blossom temp of 89/90
  • 2:25 p.m. – shutdown (blossoms now should be in upper 60s or low 70s)
  • 3:15 p.m. – blossoms could be moving up to upper 80s and some in low 90s, but I would not do another sprinkling this late in the day
  • We do not like to do sprinkling after 3 p.m. Late afternoon sprinkling can really lead to more disease pressures.

Again, it is hard to imagine that just a week ago we were battling temperatures in the low 20s! Hopefully, we will get some clouds tomorrow, and blossom temps will not approach a critical level.

Be sure to run a good early morning drip cylce.

Good luck,


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

Posted on Apr 2, 2014
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