Hydrangeas – Keeping their true color
Q: I bought 5 Monrovia ‘Red and Pretty’ hydrangeas this spring. They are doing well in my garden, but the blooms are hot pink, not red! Is there anything I can add to the soil to change the color to a true red?
A: Sprinkle a cup of lime around each plant (no closer than 1 foot from the stem) each spring.
That should keep the pH high and make them as red as possible.