Incorporating slow release coated polymer aluminum sulfate into the substrate to produce blue hydrangeas

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. The current method for producing blue hydrangeas is to apply 4 weekly applications of aluminum sulfate (Al₂(SO₄)₃) as a liquid drench. The purpose of this study was to determine if a slow release coated polymer Al₂(SO₄)₃ could be used to produce the desired blue hydrangeas as a standalone alternative to Al₂(SO₄)₃ drench applications. To complete this study 'Early Blue Rose' hydrangeas were grown in 15-cm pots with 0, 5, 10, 20, and 40 g of slow release Al₂(SO₄)₃ (15% Al) incorporated into the substrate. There were 5 replications of each treatment. The substrate pH and EC were monitored every other week by pour-thru extraction, as well as the leaf tissue nutrient concentrations. Plants were visually analyzed for marketable color at the end of the study. The untreated and the plants that received 5 g of Al₂(SO₄)₃ produced bright pink sepals. The plants that received 10 g of Al₂(SO₄)₃ displayed a mix of pink and purple sepals and had optimal plant growth. Plants treated with 20 g of AlSO₄ displayed primarily purple sepals and growth was inhibited. The highest application of 40 g of Al₂(SO₄)₃ inhibited plant growth and produced necrosis on most of the leaves. The use of a slow release coated polymer aluminum sulfate applied at these rates did not produce the desired blue hydrangea. Based on this study the use of the slow release coated polymer aluminum sulfate at these specific concentrations with 'Early Blue Rose' hydrangeas should not be used as the sole method of Al application to produce blue sepals.