University of Florida

Blueberries and
Freeze Protection


Blueberries, like most temperate zone plants, enter a dormant period during the winter, which enables them to survive the cold. Dormancy starts when shorter days and lower temperatures of fall occur; plant growth slows and cold hardiness increases. Fully dormant blueberry plants are cold hardy and seldom suffer serious cold damage in Florida.

Cool temperatures are important for establishing as well as breaking dormancy. This is what’s called a chilling requirement—the amount of cool temperature exposure needed for dormancy and normal growth. Temperatures below 45ºF generally begin this chill accumulation for blueberries, and each cultivar has its own requirement.

Once the chilling requirement has been satisfied, warm temperatures will stimulate bud growth. Early ripening cultivars with lower chilling requirements can break dormancy and bloom during a late winter warm spell. Freezes in February, March, and April will damage fruit blossoms and cause crop losses.

Passive Freeze Protection

Cultivar Selection

Planting cultivars that flower late reduces freeze risk. Unfortunately, late-flowering cultivars also ripen later, usually after prices fall.

Site Selection

Traditionally, blueberry farms have been planted on low, cold land because those soils have high organic matter. During a radiation freeze on a clear night with no wind, there are large temperature differences over elevation; hills may be five to ten degrees (F) warmer than low ground at the same latitude.

Pruning

Pruning immediately after harvesting can delay next season’s flowering by one to two weeks. Flower buds on vigorous shoots that grew all summer and fall will mature later and flower later the next spring.

Overhead Irrigation

Overhead irrigation systems are the most widely used and practical method of protecting blueberries from freezes in Florida, but large volumes of water must be pumped for good protection. Consult an irrigation specialist to determine a system that will meet your needs.

Best use of irrigation requires experience and close attention to the weather. Deciding if and when to turn on an irrigation system depends on the system’s capability, a crop’s development stage, relative humidity, temperature, and wind speed.

Calm Nights

On nights when no wind is predicted, if a decision is made to run the system, it should be turned on when an open air thermometer in the coldest part of the field reads 32ºF. If the dew point is below 25ºF, the system should be turned on at 34ºF.

Windy Nights

Windy freezes complicate the decision to run irrigation. Table 1 below provides guidelines for irrigation levels at various temperature and wind speed combinations. Keep in mind these estimates are for normal relative humidity. If relative humidity is low, greater water amounts may be required to provide protection.

Irrigating Before a Freeze

Irrigating fields the afternoon before a freeze can sometimes reduce damage. There are four situations when this practice may be useful:

  1. Calm afternoons and forecasted minimum temperatures between damaging and safe.
  2. Low dew point and erratic wind speeds during the night OR forecasted temperatures at or below the damaging point with light winds.
  3. A lack of pumping capacity to protect the entire field. Thoroughly wetting the soil in parts that cannot be irrigated during the night may reduce damage.
  4. Severe freezes during late January and early February where no experienced grower would run irrigation for fear of massive damage from evaporative cooling, frozen emitters, broken branches, and uprooted plants. Flower buds may still be dormant and might survive if nothing is done. Wetting the soil before freezing temperatures begin can reduce the fraction of the crop lost.

Table 1. Suggested overhead irrigations application rates for cold protection of blueberries for given wind and temperature conditions.


Minimum Temp. Expected (ºF)

Wind Speed (mph)

0-1

2-4

5-8

10-12

Application Rate (inches/hour)

27

0.10

0.10

0.10

0.10

26

0.10

0.10

0.14

0.20

24

0.10

0.16

0.30

0.40

22

0.12

0.24

0.50

0.60

20

0.16

0.30

0.60

0.80

18

0.20

0.40

0.70

1.00

15

0.26

0.50

0.90

---

Dry air will require higher application rates than indicated for temperature/wind speed combinations

Alternative Protection Methods

Wind Machines and Helicopters

Both methods are based on the fact that strong temperature differentials occur on clear calm nights. Temperatures within six feet of the ground may become much colder than temperatures higher up. By mixing these air layers, wind can raise the near-ground temperature by about 4ºF. On nights with wind, this method is not practical because no temperature inversion develops.

Orchard Heaters

Heaters are an effective, but expensive, method of blueberry freeze protection. Heaters burn a gallon of fuel per hour and costs may be over three hundred dollars per acre for one night.

Experience is the best way to learn how to protect blueberry crops during freezing events. Damage during freezes depends on the interaction of many factors: temperature, wind speed, and dew point; plant tissue hardiness; physical conditions in the field; weather before the freeze; and blueberry variety. Paying attention to the various conditions during freezes and the effect on crop damage will provide better knowledge of how to protect crops during future freezes.

For a more in depth look at blueberries and freeze protection, read the source document Protecting Blueberries from Freezes in Florida. As always, talk with your local Extension agent to determine the best methods for your situation and location.

Adapted and Excerpted From:

blueberries

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