Blueberry freeze protection and avoidanace
Dr. Paul Lyrene, Professor UF/IFAS
EDIS Publication - http://edis.ifas.ufl.edu/HS216
There are two concerns in blueberry freeze protection: the health and survival of the plants and the current-year's crop.
Blueberry plants are very cold hardy when dormant. In Florida, the plants normally stop growing in October and are ready to withstand subfreezing temperatures without damage by late November. Young plants that were propagated from cuttings the previous summer and fertilized to promote maximum growth during the fall often fail to harden early enough to avoid damage from the first winter freezes. Temperatures below 30F at the level of the plants can kill soft growth and predispose the plants to death at any time during the next 3 years as a result of infection of freeze-damaged stems by the blueberry stem blight pathogen, Botryosphaeria dothidia. Freeze damage to young nursery plants can be reduced or prevented by reducing fertilization in late summer and early fall to induce better dormancy. Alternatively, the plants can be covered with floating row covers or protected with overhead irrigation. If overhead irrigation is used, it is very important that excellent drainage be provided in the nursery area. Standing water or water that washes through the nursery may spread the spores of root rot pathogens such as Phytophthora cinnamomi which will shorten the life of the plantation once the plants are put in the production field. Also, if overhead irrigation is used to protect poorly-hardened plants from freeze damage in the fall, it is important that the quantity of water used is sufficient to overcome the effects of evaporative cooling. Irrigation must be started on time and kept running until the ice has melted or the wet bulb temperature has risen above 32. The water must be distributed evenly and all practices used to protect blueberry flowers and fruit from freezes using overhead irrigation must be adhered to.
Blueberry plants that have been in the field for 1 to 3 years are also subject to cold injury if hard freezes come in the fall before they have hardened or in late winter after they have begun to produce new shoots. Care should be taken to harden the plants in the fall using reduced fertilizer and water. Overhead irrigation to protect flowers and fruit in late winter will also protect the plants.
Blueberry plants produce flower buds during the fall on wood that grew the previous spring and summer. These flower buds remain dormant and cold hardy through the first part of the winter, but become susceptible to freeze damage as they come out of dormancy in late winter. If these flower buds are destroyed by a hard late freeze, the plants will not produce an economically useful crop that spring. In some cases, the plants may re-flower on early spring shoots, but the resulting berries will not ripen early enough to gain a profitable market. Most Florida blueberry growers plant early-ripening southern highbush varieties with the intention of harvesting the berries during April and early May, when the market is best. These varieties were bred to flower between February 1 and February 20. The flowers and fruit will be killed if their tissue temperatures fall below about 26F. Because blueberry flowers and fruit radiate (lose heat) much better than air at night, flower and berry tissue temperature can reach critically low levels when sheltered thermometer temperatures immediately adjacent to the plant are as high as 32F if there is no wind, the dew point is low, and the sky is clear. If it is windy and the wind never goes below 6 mph even briefly, these tender plant tissues can survive shelter temperatures as low as 27 degrees, because the moving air will keep the flowers and berries warmer than they would be with no wind.
As a practical matter, most blueberry growers first determine what are the coldest areas in their fields. Thermometers which have previously been calibrated in ice water to assure their accuracy are hung open to the sky on branches at the level of the flowers and fruit. Irrigation is then turned on when these thermometers recorded 30 degrees. An exception would be if there is a strong and continuous wind and it appears that the temperature will not fall below 28 before sunrise, in which case the water would not be turned on. Because frost pockets within a large field require that the irrigation be run much more frequently than the rest of the field, growers should consider not planting in the frost pockets. When blueberry flower buds are coming out of dormancy, usually in late January or early February, they are progressively becoming less cold hardy. Whereas fully dormant buds may tolerate temperatures to 10F, buds that have expanded to the point where the tips of individual flowers are becoming visible will tolerate temperatures only to about 22F. From that point to the point of open flower, the plants progressively lose hardiness. As stated above, the flowers and berries will survive tissue-temperatures of 26F, but shelter temperatures at the same height alongside the plant may at that moment range from 26 to 32 depending on radiation conditions. Since berries are more massive and better radiators than flowers, extreme care must be used late in the freeze season to make sure that the berries do not freeze. Unsheltered thermometer readings of 32 would require the start of irrigation if radiation conditions are good and the corollas have fallen from the flowers. Aside from passive freeze protection methods such as site selection, variety selection, and maintaining weed-free, packed, moist soil during periods of freeze concern, overhead irrigation is the only method of freeze protection in widespread use in commercial Florida blueberry fields. Blueberry plants are unusually good at bearing the heavy ice load that results from long periods of irrigation during freeze events. Ice loads that would crush citrus and peach trees and strip the branches from the plants cause surprisingly little permanent damage to blueberry plants. The principals of designing and using overhead irrigation systems for blueberry fields will not be discussed here, except to say that sufficient capacity must be designed into the system, diesel rather than electric motors are normally used on the pumps, and knowledge and experience are required in knowing when to turn on the system, when to turn it off, and when not to run it at all.
For more information on blueberries the following publication is available: Protecting Blueberries from Freezes in Florida by P. M. Lyrene and J.G. Williamson