Q: I know you have covered how to make pink hydrangea flowers turn blue and vice versa. But how do people get purple blooms?
A: You make a hydrangea have purple blooms by manipulating the pH, just as you do with the pink and blue flowers. The easiest way is to start with a hydrangea that likes to be pink, like ‘Cardinal Red’, ‘Ami Pasquier’, ‘Alpenglow’ or ‘Pia’.
Dissolve two tablespoons of aluminum sulfate in a gallon of water and drench the plant each spring for two years.
Also scatter 1/4 cup of powdered sulfur widely under the plant for two years.
The flowers will gradually turn purple in a couple of years. If not, continue adding sulfur but do not add more aluminum sulfate to the soil.