The National Weather Service is forecasting a patchy frost around west central Michigan. Tonight (May 23), temperatures may drop to the low 30s under partially clear sky conditions. This may create some unexpected problems for blueberries and strawberries around west central Michigan.

Currently, blueberries and strawberries are in bloom and as a precautionary measure, Michigan State University Extension advises growers to be alert to weather conditions at their respective sites.

Please consider the following recommendations in the event of a freeze/frost event. The grower’s decision to turn on the overhead irrigation is critical to prevent serious frost damage. A survey conducted in 2006 among blueberry growers indicated that only 59 percent of the growers with overhead irrigation used the irrigation system for frost protection, but frost control timing was a problem. Between 26 and 38 percent of the growers turned on the irrigation system in time (around 37 F). Those that waited until the temperature was close to the freezing point suffered freeze/frost damage.

Before you make the decisions of turning your irrigation system on, make sure of the following.

• You know the critical temperature of the blossoms at the current stages of development.
• The air and blossom temperature monitoring equipment has been set up and checked, e.g., thermometers, sensors calibrated.
• The irrigation system has been set up and checked.
• The application rates that can be provided are known, and an estimate of the rate that will be required on this particular night has been made.
• You know that your system can provide this rate. If it cannot, it is better not to turn the system on at all. Irrigation for frost protection is one method that can do more damage than not doing anything if it is not properly carried out.
• You know the forecast of the minimum air temperature, wind speed and dew point.

How early in the evening should the irrigation be started?

DO NOT turn the irrigation system on when the temperature is already in the low 30s. Depending on the environmental relative humidity, there is an immediate drop of 5 to 6 degrees in the environmental temperature right after turning on the sprinkle irrigation system.

When irrigation begins, air temperatures are initially reduced due to evaporative cooling. The amount of cooling depends on the relative humidity (see table below). For example, if the irrigation system is turned on at 36 F when the relative humidity is 90 percent, the air temperature drops 1 F. However, if the relative humidity is 40 percent, the air temperature will drop 6.8 F. Thus, if the air is very dry, start the irrigation when the air temperature drops to 36 F. If the relative humidity is high, start irrigating when air temperature falls to 34  F.

Approximate decrease in air temperature immediately after turning on the sprinkler irrigation under different percentages of relative humidity (D. Haman, 2003)