Not the Time for an Experiment! Use Wet Bulb of 33 F for Shutting Down Irrigation This Morning! (6:35am, Saturday,3/4/17)
In managing a freeze event with sprinkler irrigation, it is critical to know when it is “safe” to shut down the system in the morning. It just so happens that we are experiencing right now (6:35 am, Sat) a freeze event in NC and across much of the region that is not any different from the Easter freeze of 2007 from the standpoint of very low dew points this morning. That particular event was technically a black frost:
Black frost. Many growers are unfamiliar with black frosts. This may be partially due to the fact that they occur with considerably less frequency in strawberry plantings than hoar frosts. Another reason may be due to their “invisibility.” Few or no ice crystals form on plant surfaces in a black frost because the lower atmosphere is essentially too dry. Thus, the grower who depends on seeing evidence of “frost” (ice crystals) before starting countermeasures (for frost protection), could potentially suffer catastrophic losses if the dew point temperature (frost point) is below the critical temperature of the open blossom.
Given that we are experiencing a black frost event presently, it is imperative that growers using sprinkling realize that you can forfeit an entire night of cold protection by shutting the irrigation off too early in the morning. Take it from me, and my former Research Technician, John Earp, we can tell you all about the dangers of shutting down too early on a morning just like this one. Just a minute ago I sent a grower in Faison area a text that said, “Keeping running irrigation until wet bulb of 33 is reached this morning.” Since this particular farm is relatively close to Kenansville, I noted at about 5:30 am that the
AWIS Hourly Forecast For Kenansville-Dup, NC
shows a wet bulb of 33 F being reached at 11 am (just click on Kenansville-Dup below). IN THE INTEREST OF TIME, I don’t think it is appropriate at this time to enter into a longer explanation of why it is important not to shut down before a wet bulb of 33 F is reached, unless you are interested in conducting an experiment with your crop this morning?
So, please click on the AWIS Hourly Forecast that is closest to your farm to determine the hour when a wet bulb of 33 will be reached for safe irrigation shut down. Alternatively, you can use a digital thermometer with thermocouple inserted right in the blossom (this had to have been done yesterday) to figure our when it will be safe to cut off. In working with Rudd Farm in Greensboro over the years on cold protection issues, we have learned that when this instrument reads 32 F, you can safely shut down. Of course, if you are irrigating on top of row covers, then having a digital thermometer to guide your decision on safe shutdown is the very best approach! No need to consult wet bulb temps when you have one of these instruments. It is also valuable to note that SkyBit includes a table at the bottom of their forecast to give you easy access to hourly wet bulb temperature information (I have included a SkyBit from 6:25 am for Clayton and another for a VA location)
Figure 1. Note the Hourly Forecast for Clayton shows a wet bulb of 33 will be reached at 11 am. At this location they are likely using row covers, so this information is not needed. Wet bulb information is critical to any grower using sprinkler irrigation!
Virginia – consult wet bulb temps for location nearest your farm
Figure 2. The wet bulb for this location in VA will not be 33 F until NOON. Actually, I know this grower very well, and because he has row covers that he has been irrigating on top of, AND because he has a digital thermometer to tell him when the blossoms are reaching 32 F, my guess is that he can be shutting down well before noon. BUT, this is where it is critical to own a digital thermometer with thermocouple inserted in blossom to make this determination.
A digital thermometer with thermocouple will help improve your decision-making by:
1) Providing the actual temperature of the blossom, which can differ from the air temperature around the blossom.
2) Telling you when to start irrigation.
3) Telling you when to stop irrigation in the morning.
Even if you use row covers, rather than sprinkling, as your primary method of frost, the digital thermometer remains an indispensable tool for extra chilly nights when you may need supplemental heating, using irrigation on top of the row cover.
When to start irrigation in a freeze?
If you use only sprinkler irrigation for cold protection, be sure to start irrigating as soon as the digital thermometer indicates the blossom temperature is 31/32 F. The air temperature may still be as high as 38 F if the air is dry.
If you combine irrigation and row covers, we suggest starting irrigation on top of the covers as soon as blossom temperatures beneath the covers fall to 28 F.
Monitoring system performance during irrigation
The digital thermometer is an excellent tool for monitoring the success of your protection. If the blossom dips below 31 F, you need to step up your irrigation rate!
When to shut down?
The digital thermometer helps to eliminate guessing when to stop irrigation in the morning. On very cold mornings with wind, you may need to keep running well past sunrise. When blossoms provide a reading of 32 F or higher on the thermometer, you can safely stop irrigating. Again, the blossom temperature differs greatly from the air temperature, and you want to keep irrigating until the blossom temperature reaches at least 32 F.
South Carolina – consult wet bulb temps for location nearest your farm
Georgia – consult wet bulb temps for location nearest your farm