The blueberry plant, a member of the genus Vaccinium, is a highly valued crop prized for its antioxidant-rich fruit. Successful cultivation of this perennial shrub depends heavily on managing temperature. The growth, productivity, and survival of a blueberry bush are determined by its ability to withstand both cold and heat extremes. Temperature is the most important environmental factor influencing the plant’s life cycle, from winter dormancy to summer fruit development.
Understanding Blueberry Variety Requirements
Blueberry plants do not share the same temperature needs, as varieties have evolved to suit distinct regional climates. The primary distinction rests on their requirement for “chill hours,” the cumulative time spent between 32°F and 45°F (0°C and 7.2°C) necessary to break winter dormancy. Without sufficient cold, the plant experiences delayed bud break, poor leaf development, and minimal fruit production. This chilling requirement dictates the geographical regions where specific varieties can be successfully grown.
Northern Highbush varieties, the most common commercial type, require 800 to 1,100 chill hours. Lowbush or Wild Blueberry varieties also require high chilling, often over 1,000 hours. Rabbiteye blueberries, popular in the Southeast, require a moderate 350 to 800 chill hours, suitable for milder climates. Southern Highbush varieties have the lowest requirement, needing only 150 to 500 hours, allowing them to thrive in regions with warm winters.
The Lower Limit: Cold Hardiness and Winter Survival
The blueberry plant’s tolerance for low temperatures varies dramatically depending on its variety and developmental stage. During deep winter dormancy, Northern Highbush stems are resilient, surviving temperatures as low as -20°F to -30°F. Rabbiteye varieties are less cold-tolerant in this state, often suffering damage below 0°F, though they can sometimes survive down to -10°F. This deep dormancy survival involves the plant dehydrating cells to prevent the formation of intracellular ice crystals.
Once the plant emerges from dormancy in the spring, its cold hardiness rapidly decreases. Flower buds are the most vulnerable parts of the plant to late spring frosts, and mild freezing temperatures can cause significant damage. Swollen but closed flower buds can withstand temperatures down to approximately 20°F without severe injury. Tolerance drops further once buds begin to open, with temperatures below 20°F causing irreversible harm to exposed tissues.
When the flower corollas (petals) reach half their length, the damage threshold rises to between 25°F and 26°F. The most sensitive stage is when flowers are fully open or young fruit has just set, where temperatures dropping to 27°F or 28°F can result in complete crop loss. Frost damage manifests as browning or blackening of the pistil and ovules, the reproductive structures necessary for fruit development. If fruit is developing, brief exposure to 28°F can cause the berry to shrivel, develop a dark, water-soaked appearance, and prematurely drop.
The Upper Limit: Heat Stress and Growing Season Management
Excessive heat during the active growing season limits productivity and presents physiological challenges. Temperatures exceeding 90°F for multiple consecutive days induce significant heat stress, especially in Northern Highbush varieties. High heat immediately affects the reproductive process, as pollen viability and germination rates decline sharply above 86°F. This reduction in fertilization leads to poor fruit set and reduced harvest yield.
High heat negatively impacts fruit quality and development beyond pollination. Air temperatures above 90°F (32°C) can cause fruit to soften prematurely and lead to sunscald, where intense solar radiation scorches exposed berries. Heat affects the plant’s metabolic rate, increasing respiration that depletes carbohydrate stores needed for fruit growth. Southern Highbush and Rabbiteye varieties are more heat-tolerant than Northern types, showing better performance under high summer temperatures.
High air temperature and intense sunlight often exacerbate drought stress because blueberries have a shallow root system requiring consistent moisture. Elevated temperatures increase evapotranspiration, meaning the plant loses water quickly through its leaves and the surrounding soil. If water loss is not managed, berries suffer dehydration, resulting in small, shriveled fruit with poor quality. Effective management of water and light intensity is necessary to mitigate the damaging effects of high-temperature exposure.
Protecting Plants from Temperature Extremes
Mitigating temperature extremes requires a strategic approach addressing both unexpected frosts and prolonged heat waves. To protect against late spring frosts, overhead irrigation systems spray water over plants when temperatures approach 33°F to 38°F. The constant application of water releases latent heat as it turns to ice, maintaining plant tissue temperature near 32°F and preventing internal freezing. This technique requires continuous operation until the ambient temperature rises above freezing to prevent evaporative cooling damage.
For smaller plantings, covering bushes with floating row covers or frost cloth provides a physical barrier that traps ground heat and offers protection. Managing soil moisture is also a passive form of freeze protection, as wet soil conducts heat upward more effectively than dry soil. During summer, heat stress protection involves installing shade nets (30-50% coverage) to reduce solar radiation intensity and prevent sunscald. Drip irrigation efficiently delivers water directly to the root zone, counteracting increased water demands caused by high temperatures.