Winter Acclimation and Cold Hardiness of Blueberry
Blueberry cold hardiness varies tremendously among types and cultivars. Highbush, half-high, and lowbush blueberries are generally hardy to at least -20 F, although some cultivars are more tender. During recent years, blueberry breeding efforts in the northern United States have produced commercial cultivars which are hardy to between -30 and -40 F if snowfall is sufficient.
Winter injury is not usually a problem in western Oregon and Washington. However, if a severe cold spell occurs early, before plants are fully dormant, winter injury may occur. In Idaho, growers should also be concerned with winter minimum temperatures when selecting sites. Cultivars differ in sus-ceptibility to cold injury. Spring frost injury may also be a problem in blueberry production.
Not all of the tissues of a blueberry plant attain the same degree of cold hardiness. In fully dormant plants, the wood is normally somewhat hardier than the buds, and the roots do not develop any great degree of cold hardiness. Mulching with bark or sawdust can help moderate root zone temperatures and minimize root freezing injuries.
The basal tissue that connects the flower bud to the shoot is the part of the bud that is most easily injured during the dormant period. Following a freeze, florets in a bud may show no injury even though the basal tissue is injured. The amount of growth of a new shoot or flower cluster depends on the extent of injury at the base of the bud. If injury restricts the flow of nutrients and water, growth of the shoot or flower cluster is slow or stunted, or completely inhibited.
Injury to the basal tissue can be determined by slicing longitudinally through a bud from the tip through the bud base with a sharp razor blade. Freeze-injured tissues will have a brown, water-soaked appearance, while healthy tissues will be green or white. For best results, wrap tissues to be tested in a plastic bag and hold at room temperature for several days before slicing and examining for browning.
Winter injury to the vascular cambium (thin layer of tissue beneath the bark) of the cane or roots interferes with the movement of water and nutrients to the buds and, later, shoots. Depending on which tissues have been injured and the degree of injury, symptoms of “delayed winter injury” may not appear until late spring or early summer. Shoots may bloom, leaf out, and even begin setting fruit before suddenly collapsing and dying over a 1- or 2-day period.
Sudden collapse is usually related to the onset of hot weather, which increases the demand for water by the developing shoots and fruit. Injured vascular tissues are unable to supply the needed water and nutrients and the shoot collapses. Often, injury to vascular tissue can be determined by scraping away the bark a healthy vascular cambium is bright green, whereas one injured by cold is brown.
Site selection in cold regions
Selecting cultivars which are adapted to a growing site is the most important step in preventing freezing injury. One method of cultivar selection involves using the USDA Plant Hardiness Zone Map, which separates growing regions into hardiness zones, based upon average minimum temperatures.
Because blueberries are long-lived plants, average minimum temperatures are less of a concern than the probability of a killing freeze. For example, although a particular region may be classified as USDA zone 5a (average minimum temperature -15 to -20 F), occasionally it may experience temperatures of -30 F or less. In such a region, blueberries hardy only to zone 5 would be susceptible to freezing injuries during those occasional severely cold winters.
The best method of selecting blueberry cultivars is to determine how often severely cold temperatures are likely to occur in your area and base your selection upon the life expectancy of the blueberry planting and the probability of a killing freeze. If you do use the hardiness zone concept, select cultivars that are classified at least one zone hardier than the planting site.
The degree to which a blueberry bush hardens off in the fall depends upon many factors, including length of the growing season, alternating day/night temperatures, nutrition, pruning, and fluctuating temperatures during the dormant season.
Actively growing tissues are not cold hardy and are injured by temperatures around 28 F. As the daylength shortens and temperatures decrease in fall, blueberry canes cease active growth and begin a very complex process known as acclimation. Optimum cold hardiness develops when day/night temperatures decrease steadily from mid-summer to late fall, followed by several mild frosts. The degree of cold hardiness varies, according to temperatures, throughout the dormant season. A minimum of 850 to 1,000 chilling hours is needed for shoot growth and flowering to occur the following spring.
Maximum cold hardiness occurs after fully acclimated plants have been exposed continuously to several days of non-lethal, sub-freezing temperatures. Hardiness is lost during periods when temperatures rise above freezing. Most freezing injury occurs when temperatures fluctuate above and below freezing, and is typically associated with sub-freezing temperatures which follow mid-winter thaws. Blueberries in many areas of Oregon and Washington seldom attain maximum cold hardiness due to mild and fluctuating fall and winter temperatures in the coastal areas.
Cultural practices that promote late fall growth can interfere with acclimation and inhibit cold hardiness development. For example, excessive or late fertilization with nitrogen forces late season growth that is susceptible to early fall frosts.
Pruning too early in the fall, before plant dormancy, interferes with cold acclimation by stimulating late, tender growth. Even if no visible growth develops, early pruning can cause cane tissues to de-acclimate. Delay pruning until canes are fully dormant. Pruning during late winter and early spring also allows for identification and removal of injured wood and buds.
Although research indicates that maximum cold hardiness is associated with drought stress in some woody species, blueberry plants should not be allowed to become drought stressed, either during the growing season or after the plants are dormant. In regions with low annual rainfall, irrigate deeply before the ground freezes to provide enough moisture to supply the blueberries during the winter.
Insect damage, disease, other stresses which damage foliage, and overcropping limit the production of food reserves and interfere with acclimation.
When the flower buds begin swelling in early spring, the florets are the most easily injured part of the bud. Once a flower bud opens, it has lost all of its cold hardiness and will be injured at about 28 F. The tip buds on canes and the tip florets within buds are the first to develop and are the most susceptible to early frost.
To reduce spring frost injury, avoid planting in frost pockets and ensure good drainage of cold air by removing cold air dams formed by trees and brush around blueberry fields. In regions where spring frosts are common, select planting sites on gently sloping hillsides.
Overhead sprinkler systems are effective in reducing spring frost injury if enough water is available. Applying about 0.10 to 0.15 inch of water per hour can protect open blossoms down to a temperature of 25 F. Water must be applied continuously until the air temperature warms above 32 F (wait for ice to melt), or frost injury may occur.
This fact sheet is adapted from Oregon State University Extension Publication PNW215, Highbush Blueberry Production. The authors of Highbush Blueberry Production are – Oregon State University: Bernadine Strik, Glenn Fisher, John Hart, Russ Ingham, Diane Kaufman, Ross Penhallegon, Jay Pscheidt and Ray William; Washington State University: Charles Brun, M. Ahmedullah, Art Antonelli, Leonard Askham, Peter Bristow, Dyvon Havens, Bill Scheer, and Carl Shanks; University of Idaho: Dan Barney. PNW215, Highbush Blueberry Production can be purchased from the Department of Extension & Experiment Station Communications, Oregon State University. How to Order