Blackberries are adaptable fruit-bearing plants, but their growth and fruit production are closely tied to specific temperature ranges. Understanding how temperature impacts blackberries helps cultivators ensure healthy plants and consistent yields.
Ideal Temperature Range for Blackberries
Blackberries thrive within an optimal temperature range of 65°F to 85°F (18°C to 29°C). Within this range, photosynthesis, nutrient uptake, and cell division occur efficiently, contributing to robust plant vigor. These temperatures promote healthy cane development and the maturation of flavorful, full-sized berries.
When temperatures remain within this ideal zone, blackberry plants allocate energy effectively towards fruit production. This leads to higher yields and improved fruit quality, including better sweetness and firmness. Consistent warmth during the growing season allows for steady development, from bloom to fruit ripening.
Blackberry Cold Tolerance and Winter Considerations
Blackberries enter a natural dormancy cycle to survive cold temperatures. To break dormancy and initiate bud development and fruiting, they require “chilling hours,” defined as hours between 32°F and 45°F (0°C to 7°C). The specific chilling hour requirement varies by variety, from 100-200 hours for low-chill types to 900 hours for others. Inadequate chilling can lead to erratic bud break and reduced fruit production.
Blackberry cold hardiness varies, but most varieties are hardy in USDA Zones 5–10, with some tolerant down to Zone 4. While the USDA system indicates crown survival, it does not guarantee the survival of flower buds, which are essential for fruit production. Canes can suffer damage below 0°F to 10°F, and floral buds are often injured below 5°F, or even 27°F in some cases.
Fluctuating winter temperatures can be damaging. These fluctuations can cause plants to lose cold hardiness prematurely, making them more susceptible to injury. Protecting canes from cold winds and extreme temperatures helps maintain their health and ensures future harvests.
Blackberry Heat Tolerance and Summer Challenges
Blackberry plants face challenges when exposed to high temperatures, particularly above 90°F (32°C). Prolonged heat stress can lead to wilting and leaf scorch. The plant’s ability to perform photosynthesis can also be reduced, impacting overall growth and vigor.
Excessive heat directly affects fruit quality and yield. Temperatures consistently above 90°F can result in smaller berries and reduced flavor. Sunscald is a common issue, where fruit exposed to direct sunlight develops discolored drupelets. This often occurs when air temperatures exceed 90°F, and the fruit itself can become even hotter than the surrounding air.
High temperatures increase the plant’s water demands, requiring more frequent and deep irrigation to prevent dehydration. For primocane-fruiting varieties, many consecutive days above 90°F can interfere with fruit set during blooming and interrupt bud formation, potentially reducing the second crop.
Protecting Blackberries from Temperature Extremes
Protecting blackberry plants from extreme cold and heat ensures consistent yields. To shield plants from severe winter cold, mulching is an effective strategy. A thick layer of organic mulch, such as straw or pine needles, around the base insulates roots and crowns from freezing temperatures. For trailing varieties, canes can be removed from supports, laid on the ground, and covered with mulch or buried under straw. Upright varieties may benefit from windbreaks or burlap wrapping in windy or exposed locations.
During extreme summer heat, proper irrigation is important. Deep and consistent watering, especially at the base, helps meet increased water demands and prevents stress. Avoiding overhead watering also helps prevent disease.
Shade cloth or strategic planting that provides partial afternoon shade can protect fruit from sunscald and reduce plant stress. Ensuring good air circulation also helps mitigate heat buildup. For container-grown blackberries, moving them to a shadier location during heatwaves is an option.
Varietal Differences in Temperature Tolerance
Blackberry varieties differ in their ability to tolerate temperature extremes. Some cultivars are bred for cold hardiness, making them suitable for northern climates. ‘Illini Hardy’, ‘Chester’, and ‘Darrow’ are known for resilience in colder regions; ‘Illini Hardy’ is particularly cold-tolerant, surviving down to about -25°F with protection. These varieties allow gardeners in USDA Zone 4 to successfully grow blackberries with proper winter care.
Conversely, other varieties are developed to thrive in warmer, more humid conditions. Cultivars like ‘Natchez’ and ‘Sweetie Pie’ are noted for heat tolerance and are well-suited for the southern United States. ‘Arapaho’ and ‘Ouachita’ also perform well in warm, humid climates. Primocane-fruiting varieties, while offering a second harvest, can struggle with fruit quality in very high summer temperatures as their fruit ripens during hotter periods. Selecting varieties that align with the local climate’s typical temperature patterns is important for successful cultivation.