Questions From Growers and Agents (8am, Wed, 4/6/16)
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I received a number of questions late yesterday afternoon, and I regret that I was not able to respond to these in a more timely manner. One thing to keep in mind is that I am unable to read emails when I am on the road (like yesterday and the day before) from my university email address (email@example.com).
Please send your emails to: firstname.lastname@example.org
Second, you can text to this number: 919.418.9687
Questions from last evening
(Western Piedmont, 5 p.m. Tue, 4/5)
1. Growers without overhead irrigation, I told them to water today (TUE) to wet
the ground and cover. If it is supposed to rain on Thursday would they
be better to leave covers on or take off ? With freeze temps again on
Friday. What do you think?
Reply to Que. 1:
I encountered this type of thinking with pre-wetting from another grower over in Durham yesterday who wrote at 7:30 p.m. last night:
I know you are busy on this cold night but wanted to share an idea with you. I spoke with a cousin of mine who raised blueberries. He is concerned about the cold weather so he called the guy he got the plants from to ask about protection. The guy told him to crank his pump right at dark and get everything wet. That would raise the dew point and delay him having to crank his pump for the actual freezing process to take place.
Also, sky bit says to start here at midnight temp at 33 and wet bulb already at 29. Danny was under the belief we are suppose to start with wet bulb is 32, which is at 10 p.m. here. Ive been doing this along time and don’t think I have ever figured it out. Anyway, just a thought about the pre-wetting field. Your thought…. when you have time.
Hope you are well and thanks for everything,
Editor’s reply to all of the above:
1) The technique of pre-wetting the field before sunset can be effective in conditions with very low dew points — like the Easter Freeze of 2007 when we had temps in upper teens and winds in the teens when the crop was just getting ready to pick April 7-8. The dewpoints on that occasion were in the single digits, and even negative. We had nothing like that situation yesterday. I did see some lower dewpoints in the upper teens and low 20s yesterday, but nothing as extreme as what we experienced in Easter Freeze of 2007. Though the grower may gain a delayed start-up time for sprinkling of perhaps an hour or two at most, the central fact remains that for many areas like Smithfield, NC, the forecast was for frost was from midnight until after sunrise this morning! (see Table 1). But, the reasons that I would not have made this recommendation to pre-wet the field yesterday was that you may actually be “helping to make frost” by raising the dewpoint in the canopy. I guess my real hesitation on this practice is related to why do we wish to elevate the dewpoint (moisture in the air) on night with a frost forecast like the one below for Smithfield, NC? Higher dewpoints make frost more easily. All of this makes for some very interesting discussion, but I think we need to keep focused on the actual kind of cold event in the forecast, and what happened last night and this morning was a very simple RADIATION FROST that is easily managed with either row cover or sprinkling without having to do things like sprinkling on top of your plants during the day – this is not a good thing to be doing without full justification for doing it! Honestly, the only benefit is see is a delayed start up time? Is that really so important in the larger scheme of things? Think of how irrigation can spread disease (e.g. anthracnose) and other issues related to applying water to a crop that is now, or soon to be harvested.
Fig. 1. The rule to always follow with sprinkler irrigation for frost protection is to start sprinkling when the wet bulb reaches 32 F (31 F should be ok), and to keep running until the wet bulb rises above 32 F in the morning. On mornings with drier air and winds, like today, you have to be careful to not shutdown immediately after sunrise because of evaporative cooling issues. Wait until wet bulb reaches 32 F.
Let’s continue to answer some of these other questions:
“If it is supposed to rain on Thursday would they
be better to leave covers on or take off ? With freeze temps again on
Friday. What do you think?”
Let’s address this from the perspective of minimum temperatures that a grower will be facing for the balance of this week and into the weekend and even next Monday. Beginning with tomorrow (Thur), you can see that there is minimal threat of frost (immediately below) across the whole state! Perhaps there is some threat on Friday morning, depending on location (second map). Saturday morning may be cold enough for a frost event in western areas of the state, but Sunday morning IS THE REAL DEAL, and Monday morning could be a problem, too (see third and fourth maps). So, if you actually need cold protection on one of these other days, then it is ok to apply the covers sooner. But, the best idea is to take advantage of this rather costly service that is providing you with reliable, detailed hourly forecasts for cities and towns in NC – just below. By examining yourself these very detailed forecasts YOU can make the best decision on when to remove and re-apply covers.
These detailed forecasts for each area of NC can essentially tell you when your area of the state will face its next serious cold event. If your area is not facing any threat until perhaps Sunday morning, then 4/10, I would remove it this morning (Wed), and re-apply on Saturday. This is not the time of year to be leaving a row cover on for 2-3 days on a strawberry crop that is being picked, or about to be picked!
Raleigh-Durham AWIS forecast shows that we may have very strong winds on Thursday. You don’t want a row cover on the crop in these kinds of winds due to injury from the cover to the crop from flapping.
Next, note there is no frost threat in this area on Thur, Fri, or Sat. No need keep covered for those days, and Saturday looks more iffy for this area with a minimum of 37 F, but winds may keep any frost from forming.
Two nights of frost – Sunday and Monday:
Now, let’s examine the potential for frost on Sunday and Monday for this same location, and Note frost on both Sunday and Monday. These could possibly be serious events, but I am not so sure? If there are good winds all night on Sat/Sun and the air minimum is only 30 (need to get to 28 F temp in blossom to kill it), this could be a non-event! I am actually more concerned about Monday morning, as conditions that morning show real potential for a more serious FROST event; one that could damage open blooms. You can play safe by covering on Saturday. But, by covering TODAY for a possible NON_EVENT on Sunday makes little sense to me.