Effect of Gibberellic Acid On Flowering and Growth of Newly Planted Blueberries

Recent studies in northern highbush blueberries (Vaccinium corymbosum L.) showed the negative impact cropping in the first two years after planting can have on root, shoot, and crown growth, and on subsequent yields.   Late summer applications of GA₃ and GA₄₊₇ have been shown to reduce flower bud numbers in potted plants of ‘Duke’ and ‘Bluecrop’.  The present study evaluated use of GA₃ and GA₄₊₇ at 400 mg^.L^-1 in ‘Liberty’ in late summer and fall 2006, of GA₄₊₇ at 400 mg^.L^-1 in ‘Liberty’, ‘Draper’ and ‘Aurora’ in late summer and early fall 2007, and of GA₄₊₇ at 400 mg^.L^-1 in ‘Draper’ and ‘Aurora’ in late summer and early fall 2008.  Treatment timings over the three years ranged from August 7 to November 1, and plants were treated one, two, or three times, either on consecutive weeks or once monthly.  These treatments were compared to two controls, one where plants were allowed to fruit, and one where flower buds were stripped by hand.  Floral and vegetative buds were counted in spring of each year.  In February 2010, ‘Draper’ and ‘Aurora’ plants from the two most effective GA treatments and the two controls were destructively harvested in order to determine the effect on plant growth.  Results indicated that GA₃ and GA₄₊₇ were equally effective, and that treatments applied in late August and early September were the most effective at reducing the number of flower buds, by up to 50%.  Data also suggests that the most effective GA treatments reduced crop load enough to prevent the detrimental effects on plant growth associated with early-cropping.