HORTICULTURE

Using Sprinklers to Protect Blueberries from Spring Freezes

Mark Longstroth, MSUE District Extension Educator, SW Michigan

 

Many Michigan growers use sprinkler systems to protect blueberry flowers from spring freezes.  Sprinklers are very effective under certain circumstances but can actually increase injury if used at the wrong time.  Sprinklers used for irrigation do not protect below 23-24^oF.  If the system fails due to cold or wind the blueberries 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, and changes from a liquid to a solid, to keep the temperature in the ice right at the freezing point 32^oF.  As long as you keep the ice WET, the ice temperature will stay at 32^oF.  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.  The temperature will fall to the dewpoint (wet bulb temperature).

 

Protection with sprinklers

If you understand that you need to keep the ice wet, and when your system will fail to keep the ice wet, you will understand how to use your sprinklers to prevent freeze injury.  The freeze protection from sprinkler systems is limited by the irrigation rate.  Most sprinkler systems in Michigan blueberries 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.   Since you do not know what the wind speed will be several hours from now I suggest only trying to protect to 25° F.

Most irrigation systems cannot easily be changed to deliver more water and protect to lower temperatures.  Increasing the operating pressure is not advisable because the volume is not increased substantially (You need to increase the pressure 4 times to double the output).  Higher pressure 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.

 

Critical temperatures

Growers should only use sprinklers to protect blueberry from freezing, at around bloom time.  The temperature range where sprinkler can protect the crop is relatively narrow from 24 to 32 F.  This narrow temperature range is also the range that will hurt blueberry open flowers.  When blueberries begin to grow in the Spring the buds can handle very cold temperatures.  Swollen buds can tolerate 15-20° F.  The lower end of the range is where almost all the flowers are killed and the upper end is where damage begins to occur.  At "early pink bud" (individual flowers are visible in bud), injury occurs between 18° and 25° F.  These are temperatures colder than you can protect to with an irrigation system.  In “late pink bud”, when the flowers have separated in the cluster but the flower petals are still closed, 25-28° F is lethal.  This is in the range where we can protect but if there is a wind or 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 I recommend that growers only try to protect at bloom when the temperature range that will cause damage is well inside the range that we can protect to with an irrigation system.  Fully open flowers are killed between 28° and 31° F.  Right after bloom when the petals fall, is the most sensitive, 31° F will damage green fruit. 

Dr. Mike Mainland from North Carolina State University provided a useful rule of thumb during a workshop in 2003. He suggested not even attempting frost control until at least a few flowers in the field are open. He reasons that most flowers are tight enough to tolerate 22-24F until the first flowers open, so protecting before the first bloom is not useful. This rule of thumb is especially useful when there is a wide difference the emergence of buds on a shoot. If most of the flower buds on a shoot are terminal (at the end of the shoot) and are opening at the same time, then you might want to frost protect in late pink bud. But there is no reason to try and protect flower buds at temperatures below 23 or 24F.
Another consideration is wind. Don't attempt to frost protect if the combination of wind and temperature will exceed to capacity of your system to protect (see accompanying table). If the temperature gets colder or if it is windy, we have a safety margin and our system can still protect the blueberries.  If we were operating the system at the edge of its effectiveness it is more likely to fail.  Dr. Mainland suggested studying the weather forecast closely, and hanging colored flagging in the field to indicate wind strength.

 

When to turn on the System

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 have looked at the field and see open flowers and checked the weather and see that the temperature is supposed to get down to 26° F. You need to decide if you are going to turn on the system that night.  I would not turn on the system if the temperature were forecast to fall below 24 F.  If windy conditions (more than 10 MPH) were forecast I would not turn on the system at all.  When you turn the system on and start to irrigate the air temperature will fall in the field. This is because the water is evaporating and cooling the air.   The dryer the air, the greater the temperature falls.  How dry the air is will dictate when you turn the system on.   This can be calculated from the dew point, which is measured with a wet bulb thermometer on a sling psychrometer.

 

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 and begins to evaporate it will change from a blueberry heating system to 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?

Stop irrigating when the ice is melting and temperature is 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.

Soil surface considerations

Some frost avoidance can be gained by keeping the soil surface clean of vegetation, moist and packed. Moist soils have a large capacity to capture and store heat energy during sunny days, and release heat to maintain air temperature during cold nights. Weeds, sod, and plant residues insulate the soil from the sun and reduce heat capture. In addition, tall grass and weeds raise the effective ground level. This is important since cold air is heavier than warm air, and settles along the ground and in the lowest areas of fields. If fields are covered with foot tall grass or weeds, flower buds a foot higher in the canopy may be injured during a frosty night. Mowing fields with tall weeds is worthwhile.

Another consideration is that moist soils have a higher heat capacity than dry soils, and packed soils absorb more heat than recently cultivated soils. It is not worthwhile to cultivate just before a frost. Some growers attempt to irrigate during the day prior to predicted frosts in order to increase the water content of the soil Wet soil will absorb more heat. This may be of some value if water is applied early in the day, and there is ample sun to warm the wet soil. Irrigating late in the day or on cloudy days will not increase soil temperatures and provide more heat at night. The bottom line is that clean, moist, and packed soil surfaces absorb the most radiant energy during the day, and protect from frost by releasing this heat during the night.