Overview

• Gütegemeinschaft Substrate für Pflanzen and the Royal Horticultural Society now recommend using exclusively low-calcium rainwater and aluminium-sulphate fertiliser to maintain the soil acidity needed for vibrant blue hydrangea flowers.
• Gardening expert Bill Hageman advises raising pH levels to around 6.0–6.4 with lime, dolomite or high-phosphorus fertilisers to restrict aluminium uptake and encourage deep pink blooms.
• Manipulating hydrangea colour requires gradual soil amendments and regular pH testing, as visible shifts can take a full growing season or longer to establish.
• Cultivating hydrangeas in containers allows tighter control over soil pH compared with in-ground planting, speeding up adjustments for desired flower hues.
• Gardeners in hard water regions are urged to avoid tap water’s alkalinity by collecting rainwater or using filtered sources, while eco-friendly acidifiers like coffee grounds can supplement pH reduction over time.