The Effects of Slow-Release Aluminum Sulfate on Bloom Color of Hydrangea macrophylla

Hydrangea macrophylla is a common greenhouse crop typically produced for the late winter and spring holidays. Most varieties are capable of producing either pink or blue flowers depending on aluminum content and pH of the growing medium. The goal of this experiment was to examine the effects of new controlled-release aluminum sulfate products on bloom color. This new product was applied as a topdress and a pre-plant incorporated mix using two different manufacturer recommended rates. Results were directly compared with those produced using a traditional water-soluble aluminum sulfate drench application. Controlled release aluminum sulfate was found to be at least as effective as traditional drenches while requiring only one application and significantly less labor than traditional drenches. Additionally, this new product was capable of producing blue flowers in the presence of phosphorus, which usually nullifies the effects of aluminum on flower color, leading to pink inflorescences. The results suggest controlled-release aluminum sulfate is a viable alternative to traditional aluminum sulfate drenches that can provide potentially superior results with a fraction of the labor costs.