Blackberry floral buds are often injured when temperatures fall below 5°F. The 2021-2022 winter was relatively mild in Missouri, with little or no mid-winter cold injury in mid-Missouri. However, on December 22 and 23, 2022, the overnight temperature dropped to -7.6 °F at the University of Missouri Horticulture and Agroforestry Center (HARC), near New Franklin. In both winter seasons, low-temperature injury to nine erect-growing blackberry cultivars was assessed. Plants at this site were grown on a T-trellis and were not protected with row covers.

Tissue for laboratory freezing tests was collected on January 17, February 8, and November 21, 2022. Sampling dates were selected to assess flower bud hardiness during mid-winter, just before bud swell in late winter, and in the fall as buds were acclimating to low temperatures. For each sampling date, tissue was collected from the middle portion of one-year-old lateral canes at about 3.5 feet above the soil surface.

Immediately after samples were collected, the cuttings were wrapped and then placed in a programmable freezer, and exposed to a range of temperatures (16 to -22°F) likely to produce tissue injury. After removal from the freezing chamber, samples were thawed at room temperature for five days before floral bud evaluation. Low-temperature injury was expressed by oxidative browning of the floral axis when buds were bisected (Figure 1). Green tissue within the buds indicated survival of the floral axis. The mean T₅₀ value (temperature at which 50% of the buds were injured) was calculated for each cultivar for each of the sampling dates and is shown in Table 1.

Figure 1 Left: Injured flower primordia indicated by oxidative browning of tissue of a blackberry bud. Right: Uninjured tissue with viable flower primordia in a bud.

Table 1 Mean T₅₀ values of primary flower buds of nine blackberry cultivars grown at New Franklin, Missouri at selected dates in 2022.

¹ Means represent 5 replications of each cultivar. Means within columns followed by common letters do not differ at the 5% level of significance.

Due to the low temperatures recorded at HARC in December 2023, a different procedure was used to assess low-temperature survival of blackberry flower buds. On January 11, 2023, 150 buds of each of cultivar were sampled as described above and were examined for injury to the floral primordia after 72 hours exposure to room temperature. The percent bud survival was then calculated and are shown below.

Table 2 Percent survival of flower primordia in primary buds of nine blackberry cultivars grown at New Franklin, Mo in January 2023.

¹ Means represent 5 replications of each cultivar. Means followed by common letters do not differ at the 5% level of significance.

Results from these studies showed that at all test dates, 'Ouachita' had consistently low T₅₀ values and 'Natchez' had the highest values. Although T₅₀ values of 'Ponca' were lower than 'Natchez' at all sampling dates, this cultivar was injured at warmer temperatures than most other cultivars. Thus, 'Ponca' plants planted in colder, more northern regions of Missouri than the central part of the state are at risk for substantial bud and crop loss during severe winters. Based on the early results of this study, 'Ouachita' flower buds had superior cold tolerance among the cultivars tested in mid-winter and just before budbreak in late winter.

At this point in the dormant season, it is too late to protect blackberry buds from winter injury. Also, in our previous studies, applications of cryoprotectants to prevent injury to flower buds after they began to grow were ineffective. Even though substantial primary bud injury has already occurred, these blackberry cultivars also have secondary buds at the base of primary buds. Secondary buds are usually more freeze-tolerant than primary ones and have the potential to produce berries.

Figure 2 A large primary bud subtended by a smaller secondary bud on a blackberry cane.