Blackberry primocane growth management in the Southeastern US relies on tipping to manage vegetative growth. Tipping can increase lateral branching, bearing surface, and subsequent yield. However, the practice is labor intensive and expensive (~$600/acre). Furthermore, tipping wounds are sites for cane blight infection (Leptosphaeria coniothyrium) resulting in cane dieback, decline and death. Alternative strategies to tipping could reduce labor and fungicide inputs in a given season. Prohexadione calcium (P-Ca) is commonly used to control terminal shoot growth in other Rosaceous crops. We evaluated the relationship between P-Ca rate on blackberry vegetative and reproductive development. The experiment was conducted on a mature planting of ‘Osage’ floricane-fruiting blackberry at Clemson University’s Musser Fruit Research Center in Seneca, SC. The experiment had a completely randomized design with four replications. Treatments consisted of an untreated control and P-Ca applied at 62.5, 125, 250, and 500 ppm with a 0.125% (v:v) non-ionic surfactant and 0.39% (v:v) water conditioner. P-Ca treatments were applied 6 times at ~21 day intervals to the same plots across two consecutive years, 2019 and 2020. Regression analysis was conducted to determine the relationships between P-Ca and measured parameters. There was a consistent, negative relationship between P-Ca rate and primocane height across two years, which was linear in 2019 and curvilinear in 2020. A negative curvilinear relationship between P-Ca rate and yield was observed in 2020 beginning 8 weeks after P-Ca treatments were initiated until the final harvest. There were no consistent effects of P-Ca on fruit weight. P-Ca reduced total lateral length, number of lateral branches, and final cane length by 60 - 65% at the 250 ppm and 500 ppm rates. P-Ca is an effective growth retardant of blackberry, but the negative relationship between P-Ca rate and yield merits additional research. Future research should focus on post-harvest applications of P-Ca to minimize dormant pruning costs and applications in controlled environments.
