Effects of Aluminum Concentration and Application Period on Sepal Bluing and Growth of Hydrangea macrophylla

The floricultural value of Hydrangea macrophylla is significantly influenced by its distinctive blue flower coloration, which results from aluminum uptake and vacuolar complexation. However, commercial cultivation faces challenges in achieving consistent bluing while avoiding Al toxicity. This study investigated the effects of Al concentration (0–971.62 mg/L) and application period on six blue-modifiable cultivars. Optimal results were achieved with an Al2(SO4)3·18H2O concentration of 6 g/L applied from two weeks after pinching until bloom. This regimen successfully induced vibrant sepal bluing without impairing chlorophyll content or plant growth. Furthermore, it enhanced tissue concentration of essential nutrients, including N, K, Mg, Fe, Zn, Mn, Cu, B, and Co. This study proposes a robust technical and theoretical framework for optimizing Al application to ensure consistent sepal color, which provides technical support for commercial production.