Answer: The changeable color varieties of hydrangea should be blue in a pH range of about 5.0 to 5.5 and turn pink when the pH is a bit higher, say between 6.0 and 6.5 or so. In your area, applications of lime can change the color of hydrangea flowers. Using an acidic mulch such as pine needles or oak leaves will also help. Lime can take a few years to leach into the soil, so it may take some time for the effects to be visible in the flower color. Note though that some hydrangeas such as "Forever Pink" or "Pia" are pink regardless of the pH and you won't be able to make them turn blue.
Q&A Library Searching Tips