Aluminum sulfate applications intensify red hydrangea sepal coloration

Hydrangeas (Hydrangea macrophylla) contain an anthocyanin pigment within the sepal that naturally produces a pink color. In the presence of aluminum (Al) the pigment will bind with the Al, producing a blue or purple color. As the effect of Al on plants is cultivar specific, this study was conducted to determine the effect of Al application doses on the sepal color of two red hydrangea cultivars. In year 1, ‘Hot Red’ plants were treated with 0, 1.2, 1.8, 2.4, and 3.0 g L^‑1 of Al₂(SO₄)₃ (AlS) applied as a constant liquid feed at each irrigation throughout the entire production process. In year 2, ‘Curly Sparkle’ were treated with 0, 9, 12, and 15 g of AlS. The AlS was applied as a constant liquid feed of 10 applications in 150 mL aliquots made over a five-week period (weeks 1-5) when the plants required irrigation. Applying AlS increased the blue level in sepal coloration of red hydrangeas in both “Hot Red’ and ‘Curly Sparkle’. The highest concentration of Al in the leaf and sepal tissue generally corresponded to a greater blue sepal coloration in both cultivars, although the highest AlS dose did not consistently produce the bluest sepal coloration. The desired plant size should also be considered as the application of AlS had a negative impact on plant dry weight for both cultivars. The application of AlS affected the plant dry weight, substrate pH and EC for both cultivars as well as tissue nutrient concentration in the ‘Curly Sparkle’. Therefore, growers should determine the level of blueness desired when deciding the dose of AlS to be applied to red colored hydrangeas.