Grower Input on Frost Protection: Sprinkler vs. Row Covers / Dew Point and Radiation

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Dear Strawberry community,

This is my second spring in North Carolina and I’m learning every day. The input of experienced and seasoned growers is extremely valuable to me. This is especially true when it comes to cold protection.

Over the last few days I had some very valuable conversations with seasoned growers in response to the last weather advisory. Here I would like to share some of those lessons.

I thank all growers for their continuous input. I can’t thank you enough for supporting my efforts to get out good weather advisories, and for your love and care for the region!

Input (1): Plant Surface, Radiation, Dew Point

I’d like to address some points from your 3/20/19 advisory. On one thing you were wrong. On another point AWIS was wrong. And then I just want to put in my two cents worth on two other points.

  • In your answer to XXX, you said, “Dew always forms if the air temperature reaches the dew point.”  This is wrong and this is what gets growers into trouble! Correct:  Dew always forms if the temperature of the surface in question (such as strawberry flower, grass, car hood, etc.) reaches the dew point. Especially on a clear or even least partially clear night, “surfaces” exchange heat radiantly with outer space, which is very cold.  Clouds block this radiant exchange; more/thicker the clouds = more blockage. On clear nights, surfaces can be colder than the surrounding air. Especially if the surrounding air is warmer than the surface (therefore more moisture is available to form frost than if the air was colder), there can be a BIG frost on clear nights. No matter what the air temperature is, if the surface’s temperature isn’t at or below the dew point, frost won’t form on that surface. For example, the top of a vehicle faces up and exchanges heat (cold) with outer space, cools to the dew point, and frost forms BUT the sides of the same vehicle, which do not face up toward outer space, do not get as cold and frost doesn’t form on them.
  • AWIS doesn’t always get it right! NO weather product does! The AWIS forecast for my Chesapeake farm said no frost last night/this morning but we did have a good frost. We covered our strawberries yesterday because my analysis of AWIS and two other forecast products said there would probably be a frost. The four critical factors are air temp, dew point, cloud cover, and wind speed. The AWIS forecast for this morning said air 37, dew point 28, clear, zero wind. I’ve seen frost at 40 air temp many times under similar dew point, clear sky, and windless conditions, so I knew there could be frost. So…don’t put all your faith in the forecasts; trust your own analysis!
  • Frost often forms when the four critical factors come together right at sunrise but frost can form much earlier, even soon after sunset. Sometime frost will form in the middle of the night but conditions (one or more of the four critical factors) change long before sunrise and the frost disappears before sunrise. Just because you don’t see frost when you get up in the morning doesn’t mean there was no frost.
  • XXX said he didn’t cover up because of spraying. I bet the dollar value of the blossoms lost to frost was more than the value that would have been lost by spraying a day or two later. First things first: protect those flowers!
  • XXX also said he didn’t cover because of REI. I’ll probably be criticized by some for saying this, but I would have sprayed, then I would have started covering as soon as the spray was dry, starting with the first-sprayed areas. When pulling covers, we are not handling plants and we are well-dressed and wearing gloves, so I think any contact with the pesticide(s) would be negligible. REI means without protection but you are protected if wearing gloves, etc.

Okay, I’ll get off my soap box now! 

Input (2): Sprinkler vs. Row Covers

Wet row covers can be WORSE than NOTHING because of evaporative cooling (GUARANTEED FREEZE BURN @ CONTACT TOO)!! If any grower thinks they have that condition, our EXPERIENCE, only 68 years, indicates the best thing to do is protect totally with overhead irrigation, just like the covers were NOT there. Monitoring temperatures under such situations under the covers is akin to conducting a risky research experiment with a total commercial crop at risk. Overhead alone has proven totally effective , in the absence of wind, down to the LOW TEENS. There is no commercially available row cover alone that is even in that ball park. Besides that, the grower turns them on when he needs protection and shuts them off when the temperature is @ a safe level, maximizing the period of necessary OPEN POLLINATION for wind and bees. No dragging wet, cross contaminated covers over the plants every day like dragging a wet DIRTY, CROSS CONTAMINATED mop across a clean floor! Hope this offers some insight from a LOT of experience.