Blueberries are a highly valued crop cultivated across a wide geographic range, but successful production depends on the plant’s thermal environment. Temperature regulation dictates everything from winter dormancy to fruit development and yield potential. Specific tolerances and requirements vary dramatically across the plant’s life cycle. Understanding and managing temperature is the greatest factor in successful blueberry cultivation.
The Role of Variety in Temperature Tolerance
Blueberry varieties possess genetics that define their tolerance for both cold and heat, which determines the regions where they can be successfully grown. The three primary types—Northern Highbush, Southern Highbush, and Rabbiteye—exhibit distinct thermal requirements based on their native habitats and breeding history. Northern Highbush varieties are suited for colder climates, requiring the most winter chilling but demonstrating lower tolerance for high summer heat.
Southern Highbush blueberries are hybrids bred to thrive in warmer regions, possessing a much lower chilling requirement and moderate heat tolerance. This adaptation allows them to be grown in areas with milder winters, such as the southeastern United States. Rabbiteye is the most heat-tolerant type and requires the least winter chilling, making it the preferred choice for the warmest growing zones. While Rabbiteye plants tolerate summer heat and drought well, they are also the most susceptible to deep winter cold, often sustaining damage below 0°F, compared to the hardier Highbush varieties.
Cold Tolerance and Winter Dormancy
Winter survival is governed by dormancy, a physiological process that must be fully satisfied to ensure a productive spring. This requirement is measured in “chilling hours,” defined as the cumulative time the plant spends below 45°F (7.2°C). Northern Highbush varieties typically require 800 to 1,000 chilling hours, while low-chill Southern Highbush types may need as few as 150 to 400 hours.
A plant that receives its full chilling requirement will break dormancy uniformly, leading to synchronized bud break, flowering, and a high-yielding harvest. If the requirement is not met, the plant experiences delayed and erratic bud break, resulting in scattered flowering and reduced fruit yield. While dormant, the plant is extremely cold-hardy; fully hardened Northern Highbush canes can withstand temperatures as low as -20°F to -30°F. This cold tolerance is variety-dependent, as the less hardy Rabbiteye may sustain damage below 0°F.
The Danger of Spring Frosts
Once the plant satisfies its winter chilling requirement and begins to swell its buds, its hardiness quickly declines, creating a period of vulnerability to spring frosts. The temperature that causes damage depends directly on the flower bud’s stage of development. As the buds expand and individual flowers become visible, their tolerance for freezing temperatures drops rapidly.
For most Highbush varieties, temperatures dropping to 28°F or lower can cause significant loss once the flowers are fully open. The most vulnerable stage is immediately following petal drop when the small green fruit is newly exposed; here, even 31°F can cause severe damage in some varieties. A brief cold snap at this stage can destroy the reproductive tissues of the flower, turning the pistil brown or black and leading to total crop loss.
Managing Heat and Drought Stress
High summer temperatures present the next thermal challenge. Prolonged air temperatures above 85°F to 90°F can induce physiological stress, reducing photosynthesis and inhibiting pollen germination, which affects fruit set. Heat damage to the fruit itself occurs when berry surface temperatures exceed 100°F (38°C), causing necrosis, shriveling, and poor coloration. Berry surfaces can be 7°C to 11°C warmer than the ambient air on a sunny day.
The shallow, fibrous root system is sensitive to warm soil, making root zone temperature management important. Applying a thick layer of organic mulch, such as aged pine bark or sawdust, acts as an insulating barrier, shielding the soil and reducing heat absorption. Consistent soil moisture through irrigation is also effective, as water stabilizes the root zone temperature and allows the plant to use transpiration as a natural cooling mechanism. These practices mitigate stress that causes smaller fruit size and reduced antioxidant levels, symptoms often seen above 82°F (28°C).