ASHS 2025 Conference

The blackberry (Rubus spp.) industry in the Pacific Northwest (PNW) has rapidly expanded in recent years due to growers and major berry companies’ capacity to produce high-value fruit for fresh and processing markets. The expansion of the blackberry industry has been further supported by increases in fresh and frozen blackberry consumption. However, revenues and crop expansion in northwest Washington (WA) are limited by freeze events during the winter period that affect fruit production. Freezing temperatures below critical thresholds irreparably damage primary bud meristems. However, little is known about the conditions resulting in freeze damage, as well as the duration of dormancy and time to bud break among key blackberry cultivars. The objective of this study was to evaluate dormancy and cold hardiness of ‘Victoria’ and ‘Celestial’ blackberry floral buds in the PNW. Samples were collected weekly from 15 Oct 2024 to 7 March 2025 from two commercial farms in Skagit County, WA. Sampled laterals were cut into smaller cuttings, each containing one single floral bud, and subjected to either dormancy or cold hardiness evaluations. Samples for the dormancy evaluation were placed in suitable growing conditions (22 C°, 16 h light), and time to bud break was quantified. In parallel, samples for the cold hardiness evaluation were placed into a glycol-bath exposed to 10 freezing temperatures ranging from -4 C° to -20 C°, and a lethal temperature in which 50% of the floral buds were damaged (LT50) was determined through a visual evaluation of damage to the primary meristem. In ‘Victoria’, the maximum bud break rate was observed once canes accumulated over 900 chilling hours (threshold between 0 and 7.2 C°), while ‘Celestial’ required an additional 300 chill units to exhibit similar rates. Regarding cold hardiness, in ‘Victoria’, the LT50 was between -14 and -16 C° through most of the winter season, while the LT50 for ‘Celestial’ fluctuated between -14 and -16 C° during the fall, then between -18 C° and -20 C° during the winter. The higher cold hardiness observed in ‘Celestial’ is attributed to its genetic traits as a semi-erect cultivar rather than environmental factors. The information from this study will be used to develop a decision-support tool for blackberry growers to reduce loss associated with damage from freezing temperatures in the PNW.