Shade hydrangeas prefer moist soil and regular watering during the growing season. For most, in order to obtain the blue flower color, apply 2 tablespoons of aluminum sulfate near the base of the plant. A rusty nail or coffee grounds will also increase the acidity of the soil which makes the flowers turn blue.
Nikko Blue Hydrangea
Hydrangea macrophylla 'Nikko Blue'
Nikko Blue hydrangea has been a long-time favorite mophead hydrangea for good reason. When in full bloom, Nikko looks like a giant bowl of fluffy blue snowballs. It is a fast grower, adding two feet a year under optimum conditions. Receiving up to 6 hours of morning sun and growing in acidic soil keeps this hydrangea looking its absolute best. Nikko Blue sets its blooms for the next year in summer, so prune accordingly. This variety also makes an excellent cut flower if you want to bring the beauty inside.
For best blue flowers make sure your soil remains acidic by top dressing you Nikko with Aluminum Sulfate every February.