## Using sprinklers to protect blueberries from spring freezes

Irrigation sprinklers can be used to protect plants from freezing when the expected lows are just below freezing. But, it's all about temperature. Irrigation will result in severe damage when the low is just below freezing.

Many Michigan fruit growers, especially blueberry growers, use sprinkler systems to protect their crops during spring freezes. Sprinklers are very effective under certain circumstances but can increase injury if used at the wrong time. Sprinklers used for irrigation do not protect below 23-24 degrees Fahrenheit. If the system fails due to cold or wind, the blueberry blossoms will get much colder than in areas where you are not sprinkling.

When you use sprinklers to prevent freezing injury, you are using the energy that water releases when it freezes to keep the temperature in the ice right at the freezing point –32 F. When water goes from a liquid to a solid, it is an exothermic reaction which means that it releases heat. If you keep the ice wet, the ice temperature will stay at 32 F. If the ice dries out, and water starts to evaporate from the ice, the ice will get colder than the air temperature as it evaporates.

### Protection with sprinklers

When using sprinklers to help protect from frost injury, it’s important to understand that you need to keep the ice wet, and the point when your system won’t be able to keep the ice wet anymore in order to properly prevent freeze injury. The freeze protection from sprinkler systems is limited by the irrigation rate. Most sprinkler systems in Michigan are designed to provide about 0.12 to 0.15 inches of water per hour. This volume protects plants to about 22 F with no wind or 24 to 25 F with a light wind. More water is needed to protect at lower temperatures and higher wind speeds (see Table 1).

Table 1. Irrigation rate (inches/hour) needed to protect fruit buds under different wind and temperature conditions (U of Florida Ext. Circ. 287)

Temp (F)

Wind speed (mph)

0-1

2-4

5-8

27

0.10

0.10

0.10

26

0.10

0.10

0.14

24

0.10

0.16

0.30

22

0.12

0.24

0.50

20

0.16

0.30

0.60

18

0.20

0.40

0.70

15

0.26

0.50

0.90

Most irrigation systems cannot easily be changed to deliver more water and protect to lower temperatures. Just increasing the operating pressure is not effective (you need to increase the pressure four times to double the output). These higher pressures can break lines and reduces the uniformity of application. Larger nozzles can be installed in some systems, but only if the capacity of the system, mainlines, well and pump can handle the added volume. For example, 9/64-inch nozzles that deliver 0.12 inches water per hour require 60 gallons per minute per acre of blueberries. Switching to 5/32-inch nozzles would deliver 0.15 inches per hour but requires 68 gallons per minute per acre. Irrigation systems are not designed to apply enough volume to protect from temperatures in the low 20s and teens.

### Critical temperatures

Growers should only use sprinklers to protect a crop from freezing, when the temperature range for protecting the crop is relatively narrow, from 24 to 32 degrees F. It's important to keep this range in mind because if the temperature gets a little colder than predicted, we could cause more damage than if we had not turned on the system. Once we turn on the system, we need to keep it on until the temperatures are above freezing or you will cause a lot of damage as the temperature of the ice goes down colder than outside the irrigated area.

It is because of this narrow margin of error that it's recommended growers only try to protect when the temperature range that will cause damage is well inside the range that we can protect to with an irrigation system.  Check out this link to critical spring temperatures of blueberries.

Do not use sprinklers if you only think they might work. If it gets colder than you can protect, you will cause more damage, not reduce damage. If the temperature were forecast to fall below 24 F, it is not advised to turn on the sprinklers at all. If windy conditions (more than 10 mph) were forecast, it would also be advised not to turn on the system at all.

### When to turn on the system

Once you decide you are going to turn on the system, you need to decide when to turn it on. At the beginning of the irrigation cycle, the air temperature will fall in the field. This is because the water is evaporating (absorbing heat from the air) and cooling the air. The dryer the air, the greater the temperature fall when you start to irrigate. How dry the air is dictates when you turn the system on. This can be calculated from the dew point, which is measured with a wet bulb thermometer or a sling psychrometer.

Table 2. Starting temperature for overhead sprinkler freeze protection based on the dew point of the air.

Dew point

Start irrigation at:

26 F

34 F

25 to 24

35

23 to 22

36

21 to 20

37

19 to 17

38

16 to 15

39

Once you start the system, it is necessary to keep it running until the ice starts to melt on its own. If your system fails and the ice dries out, evaporation from the ice will be an effective refrigeration system that can significantly reduce your crop. As long as water drips from the ice the system is working. If the ice is clear, this indicates the system is working properly and the water is freezing uniformly.

### When can I stop irrigating?

Generally, you will need to irrigate until after the sun comes up and begins to warm the ground. Stop irrigating when the ice is melting and temperatures are above freezing and rising. Ice breaking free from branches indicates water is forming under the ice and it is likely safe to quit. Normally this is when temperatures are above freezing and rising. Beware of sudden dips in the temperature soon after sunrise.