The Nikko Blue hydrangea is the most commonly grown hydrangea, hardy in USDA zones 5 through 9. A fast-growing, deciduous shrub, this plant depends upon the acidity level of the soil in which it is grown to maintain the lovely blue shade of its flowers. A soil that is too alkaline will produce a softer shade of blue and sometimes turn the flowers pink. Maintaining the desired bloom color is a balancing act that requires a periodic check of the soil pH and proper fertilization.
Provide your Nikko Blue hydrangea a spot in the garden that receives sunshine in the morning and shady afternoons. The plant also requires a nutrient-rich soil with excellent drainage.
Water the Nikko Blue hydrangea deeply and weekly. You may need to water daily during the hottest months of the year.
Measure the pH of the soil with the soil testing kit, according to directions. If the pH is below 6.5, you will need to add acidity to the soil in order to maintain the blue shade of the flowers. Apply a solution of 1 oz. aluminum sulfate in a gallon of water. For very large plants, double the mixture. Apply the mixture to the soil, at the base of the plant, at the beginning of spring and then again in late July.
Prune your hydrangea in July by cutting off dead branches. Hydrangea bloom on old wood, so be careful when cutting.
Inspect the plant frequently for powdery mildew, a disease caused by a fungus. Researchers at Auburn University suggest using Heritage, a broad-spectrum fungicide, to prevent and control powdery mildew. Follow package instructions for dosage and safety information. Also look for pests such as spider mites, aphids and rose chafers (a type of beetle).