Depending on the design of the system, damage can be prevented when temperatures drop as low as 20 to 23°F. Varieties that continue to grow late in the season, however, are more susceptible to late-season frost injury than varieties that stop growing earlier in the season. When corollas have reached half of their full length, temperatures below 25 to 26°F will kill the complete flowers. Healthy ovules are plump and white, but become black with cold injury. Although there is almost no wind during a radiation frost an occasional gentle breeze will occur. Early-season frosts can be especially damaging to blueberry bushes: If flowers are partly open on early-flowering cultivars, a spring frost can injure the open blooms and cause a partial to total loss of fruit yield. Thermometers in warm, cold and average locations in the field should be checked in addition to the hygrothermograph to determine differences across the farm. The temperature to begin sprinkling depends on humidity. Blueberry plants are sensitive to frost damage. Most fields should have at least 3 thermometers placed at bush height in high (warm), low (cold) and average locations in the field. In that case, however, the fruit ripens later than usual and is smaller than normal. A&T State University, in all 100 counties and with the Eastern Band of Cherokee These sites satisfy the cultural requirements of blueberries for a constant and uniform moisture supply. Preparing the planting ground Plant blueberries during late fall or early spring. The cultural requirement for a uniform soil moisture makes selecting higher sites that are less subject to radiation frosts much less practical than with other fruit crops. Lowbush varieties (suitable for zones 3 through 7) are more cold-hardy. Protect the blueberry bush from cold winter winds and ice damage by wrapping it in burlap. Ohio State University recommends half-high varieties for extremely cold winter conditions. A sling psychrometer that measures wet bulb and dry bulb temperatures is a less expensive method for determining relative humidity than the hygrothermograph, however, unlike the hygrothermograph, there is no constant measurement or permanent record. Blueberries and frost are not a desirable combination, but understanding blueberry temperature tolerance and how to assess damage from frost can help you protect your blueberry bushes as the days turn colder. The plot of temperature and humidity as the night progresses on the hygrothermograph chart aids in making the best possible management decisions. As flowerbud swell progresses, cold tolerance decreases. As new flowers develop, they become increasingly vulnerable to frost damage. Practices that growers can use to … For highbush without adequate frost protection damage is near. Reliable and accurate thermometers that register current and minimum temperatures are essential if frost control measures will be used and also to help predict the extent of damage if no protection is provided. Most blueberry bushes need some portion of the year for cool-weather dormancy. The extent of frost damage depends on how mature your blueberry plants are. Until the cold requirement is achieved, an extended period of warm weather usually will not cause floral budbreak. a National Cooperative Extension resource, the North Carolina State University Extension, Growing Blueberries: Planting, Growing, Pruning, and Harvesting Blueberries, Blueberry Freeze Damage and Protection Measures, Blueberry Varieties That Require Cross-Pollination. For example, the North Carolina State University Extension recommends balancing early and late-season pruning, to make bushes less vulnerable to frost damage of new growth. Blueberries (Vaccinium corymbosum) are native to eastern North America and related to azaleas and rhododendrons. However, at 40% relative humidity the system should be started at about 37°F to avoid evaporative cooling below 30°F. Wait until your blueberry plant begins growing again, then inspect for frost damage. Causes of frost damage Ground frost occurs when the temperature of the ground falls below freezing point (0ºC/32ºF) and air frost occurs when the temperature of the air falls below freezing point. This publication printed on: Nov. 29, 2020, Blueberry Freeze Damage and Protection Measures, Skip to Practices for Reducing Freeze Damage, NC The more mature the blueberry plant, the more easily the fruit can be damaged by cold. How Long After Planting Blueberries Can You Harvest? Relative humidity is usually 100% on radiation frost nights, however, occasionally lower humidity occurs. The NCSU Extension recommends keeping the soil moist, since moist soil absorbs more heat during the day than dry soil and conducts heat to plants at night. With these measures, and an understanding of blueberry temperature tolerance, you can plant the best blueberry varieties for your region and protect them year-round. If only a few are damaged, fruit development usually continues, but the fruit will be later ripening and of smaller size than berries with a larger number of healthy seeds. Maintain Soil Moisture -- Growers who have hose reel, hose pull, or small portable irrigation systems can benefit from maintaining a moist soil during the period when frosts are possible. Highbush blueberry varieties (suitable for zones 4 through 7, according to the Farmer's Almanac) need some winter chill, but are ideally not exposed to freezing temperatures too early in the season. When these soils are dry, they hold very little heat and a dry surface acts as an excellent insulator to prevent beneficial heat release. As the berry begins to enlarge, susceptibility is similar to the critical temperature of 28°F for open blossoms. Maintaining a moist surface on peat and muck soils is especially important. This publication describes conditions that often occur in commercial blueberry fields during and after bloom when the potential for freeze damage exists. Chances are, however, that blueberries and frost are going to come in contact in your garden, especially if you are growing blueberries where they are native in the north eastern U.S. Most rabbiteye blueberry varieties require 400 chill hours to 600 chill hours (hours below 45 F) to break dormancy. If the humidity is near 100% as it usually is on radiation-frost nights in southeastern North Carolina, 33 to 34°F is satisfactory. According to a National Cooperative Extension resource, blueberry cultivars vary in the amount of cold they can tolerate, and the damage resulting from cold and frost also varies depending on the bush's stage of growth. However, at this stage, blossoms on rabbiteye blueberries may receive corolla damage at temperatures as high as 30°F. Prune off discolored or dead growth with anvil pruners to stimulate new, healthy growth. Commercial blueberries are generally planted in low areas with high organic-matter content. © Copyright 2020 Hearst Communications, Inc. Hygrothermographs that constantly measure temperature and relative humidity placed in a weather shelter at bush height are also very helpful if frost protection measures are available. The system is started when the temperature has dropped into a range of 33 to 38°F and the grower expects the temperature to reach a minimum below 28°F before warming begins after sunrise. Listening to weather reports is not a reliable method of monitoring current temperatures or determining what the low temperature will be in a blueberry field. Cooperative Extension is based at North Carolina's two land-grant institutions, Corolla damage to unopened highbush flowers that prevents pollination is seldom a problem with highbush blueberries. Receive Email Notifications for New Publications. commitment to diversity. Avoid Cultivation -- Cultivation in late winter and early spring tends to increase freeze damage. Indians. The temperature at which freeze injury begins to occur depends on the stage of development from dormant flower buds through young fruit. Also, the increased soil air will cause the surface layer to be a better insulator which will decrease the amount of heat released from deeper in the soil. The ideal climate for growing blueberries is one that receives no frost, rare occurrences of frost or only short periods of frost. Therefore, avoid planting in low-lying areas. Following temperatures well below the critical level, the complete flower or small fruit will develop a water-soaked appearance, shrivel and drop. If a large number of ovules or young seeds are black, the flower or fruit will probably drop. Temperatures must drop below 28°F for economic losses to occur on highbush blueberry (Vaccinium corymbosum L.). You can take measures to protect blueberries from frost.