Irrigation for frost protection
Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
A farmer seeking to avoid crop loss from last week's freezing temperatures reported that in some fields, the irrigated areas received greater injury than plants along the borders where the sprinkler heads could not reach. This seems to contradict what we know about irrigation. Water from our wells is maintained at 50ºF to 55ºF, so any amount of water sprayed onto a field, it would seem, should help keep the air warmer than where no water was applied. In this case, our common sense fails us, because we are forgetting two very basic concepts from physics. What we forget is that: 1) when water freezes, it gives off heat to the surrounding environment, and 2) water that evaporates removes heat from its surrounds.
If a farmer irrigates during freezing weather, but does not apply enough water to maintain continual ice formation, the water will not be giving up a great deal of heat to the air. Furthermore, if the soil's heat causes the water to evaporate, it will behave just like a refrigerator, removing the soils heat energy, resulting in colder air temperatures and more severe frost damage. If, on the other hand, enough water is supplied to keep ice forming, frost damage may be averted.
Irrigation should begin as temperatures reach 33-34ºF and run until the ice melts. Nozzles that produce a mist are better at creating ice without applying as much water and soil saturation as with typical irrigation nozzles.
Of course, there are limits to how much water should be used. Excessive ice accumulation can also injure plants. Those wanting a more thorough treatment of this subject can go to: