How many of you remember the part of Walt Disney's Sleeping Beauty where the fairy godmothers fight over whether Sleeping Beauty's ball gown should be pink or blue? I think of that scene every time the old-fashioned mophead hydrangeas (Hydrangea macrophylla) burst into bloom in June. The colors are so rich they are the stuff of magic.
It's not a fairy godmother that determines whether this hydrangea's blooms are pink or blue, it is aluminum. Lots of aluminum makes for blue flowers, lack of aluminum and the blooms will be pink.
The pH of the soil determines how much aluminum H. macrophylla can absorb.
Sweet soils (pH of 6.0 to 6.2) = pink hydrangea blooms.
Neutral soils (pH of 5.5 to 6.5) = purple blooms.
Acidic soils (pH of 5.5 and lower) = blue blooms.
Add lime to the soil for pink blooms; add aluminum sulfate for blue blooms. The amount of lime or aluminum sulfate needed depends on the soil composition.
If you are dead set on your hydrangea being pink or blue, grow it in a container where you can easily control the soil pH.