MSU Extension

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.

Blueberry growers can protect against spring freezes by using sprinklers to reduce damage to blueberry flowers. Sprinklers do not protect blueberries in all circumstances. In 2002, many growers used sprinklers to protect their blueberries when they were at swollen bud and the temperatures were forecast to fall to the upper teens. Sprinklers will not protect crops under these extreme conditions. Sprinklers are used near bloom time when the flowers are visible and the lows are forecast to be in the upper to mid 20s. Unless your system is specifically designed to provide a lot of water in a short period of time, it is doubtful that it will protect below 24 degrees F (view photo).

Know how much protection your sprinkler system can provide

The system's irrigation rate and uniformity determine the freeze protection that sprinkler systems can provide. More water is needed to protect at lower temperatures and higher wind speeds (see accompanying table). Most systems in Michigan are designed to deliver 0.12 to 0.15 inches of water per hour. These systems can protect to 22 degrees F under very still conditions, but only to 26 degrees F if there is a slight breeze (two to four mph). If you do not know your delivery rate, catch water in six  to eight cans placed on the ground throughout the planting and calculate.

When to attempt to frost protect

Blueberry flower buds and flowers become more sensitive to cold as they develop. Swollen but closed buds tolerate 15 and 20 degrees F. At tight cluster or early pink bud (individual flowers are visible but still tight in bud), injury will occur between 18 and 23 degrees F. Once flowers have separated from one another but the corollas (petals) are still closed, 22 to 25 degrees F may be lethal. By the time the corolla is half their full length, they are damaged at 25 to 26 degrees F. Fully open flowers are killed at 27 degrees F. The most sensitive stage is just after the petal fall, when 28 degrees F may cause damage.

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 are open. He reasons that most flowers are tight enough to tolerate 22 to 24 degrees F 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 in 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 24 degrees F.

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). Dr. Mainland suggested studying the weather forecast closely, and hanging colored flagging in the field to indicate wind strength.

How early in the evening should I start irrigating?

When irrigation begins, air temperatures are initially reduced due to evaporative cooling. The amount of cooling depends on the relative humidity. If the air is very dry (dew point 15 to 20 degrees F), start the irrigation when the air temperature drops to 36 degrees F. If the relative humidity is high (dew point above 24 degrees F), start irrigating when air temperature falls to 34 degrees F.

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 capacity of the soil to absorb 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.

Additional Information

MSU Extension’s 2012 Fruit Freeze Resources