Blueberries, belonging to the genus Vaccinium, are widely recognized for their adaptability to cold climates. The plant’s overall hardiness is not a fixed characteristic, but rather a spectrum that depends on two primary factors: the blueberry’s specific genetic type, which determines its inherent cold resistance during winter dormancy, and the plant’s current stage of growth. Resistance drops dramatically once spring growth begins. Understanding this variation is necessary for successful cultivation, particularly in regions prone to late-season spring frosts.
Inherent Frost Tolerance by Blueberry Variety
The baseline cold tolerance of a blueberry plant is established by its variety. Lowbush blueberries, often referred to as wild blueberries, display the highest level of cold hardiness, capable of surviving temperatures down to -35°F or even colder. This exceptional tolerance allows them to thrive in harsh northern climates where other types would fail.
Northern Highbush varieties, the most common type cultivated globally, possess a moderate level of dormant cold tolerance. These plants can generally withstand winter temperatures ranging from -20°F to -30°F, depending on the specific cultivar. Their chilling requirement is high, ensuring they remain dormant throughout the winter and reducing the risk of premature bud break.
In contrast, Rabbiteye blueberries, native to the southeastern United States, are the least cold-tolerant of the major types. While they are resilient to summer heat, their dormant flower buds are often damaged when temperatures fall below 0°F to 5°F. This lower hardiness restricts their cultivation primarily to warmer regions with mild winters.
Critical Vulnerability: How Growth Stage Affects Tolerance
A blueberry plant’s ability to survive cold rapidly declines as it transitions from dormancy to active growth, making late spring frosts the greatest threat to a harvest. When buds begin to swell, the critical temperature for damage is around 15°F to 20°F. As the flower cluster progresses to the “pink bud” or tight cluster stage, its vulnerability increases, with damage likely to occur when temperatures fall below 23°F to 27°F.
The open flower stage represents a major vulnerability, with most varieties sustaining damage when temperatures reach 27°F to 28°F for more than a few minutes, as the plant has lost the natural cold protection mechanisms present in the dormant bud. Following petal drop, the newly formed small green fruit becomes the most sensitive stage, with injury occurring at temperatures of 28°F or even slightly higher.
Practical Methods for Frost Protection
Growers can employ several strategies to mitigate the risk of damage during spring cold snaps. Overhead sprinkler irrigation is one of the most effective commercial methods, continuously coating the plants with water when temperatures drop below freezing. The phase change from liquid water to solid ice releases latent heat, keeping the temperature of the plant tissue near 32°F. Water application must be maintained until the air temperature rises above freezing and the ice begins to melt naturally.
For smaller plantings, physical covers offer a simpler solution, with floating row covers or burlap providing several degrees of temperature protection. Double layers of insulating material can increase the protective effect significantly. Passive measures, such as careful site selection, are also beneficial, as cold air is dense and flows downhill, settling in low-lying areas.
Ensuring the soil is moist before a radiation frost event is another helpful practice, as wet soil holds and conducts more heat from the ground than dry soil, helping to warm the air near the plants. However, a heavy layer of mulch can insulate the soil, preventing this heat transfer and potentially resulting in colder air temperatures at the level of the flowers. Strategic use of wind machines can also be employed in commercial settings to mix the warmer air layer above with the colder air near the ground.
Identifying and Recovering from Frost Injury
Assessing the extent of freeze damage requires a careful inspection of the flower buds and emerging tissue several hours after the frost event. Growers should split open the flower buds or small fruit lengthwise to check for internal discoloration. Healthy internal tissue will appear plump and white; any browning or blackening indicates cold injury.
When the pistil, stigma, or ovules are blackened, the flower will likely fail to set fruit or will produce a smaller, later-ripening berry due to the reduced seed count. While a severe frost may cause entire flowers or small fruit to develop a water-soaked appearance and shrivel, a partial injury may only affect the reproductive parts. After a damaging event, it is advisable to delay pruning until new spring growth emerges, allowing for the clear distinction between dead and living tissue, ensuring only the injured wood is removed.