One of the truly fascinating things about hydrangeas, aside from the full size and shape of their flowers, is the fact that the color of the blooms can be manipulated by changing the pH of the soil. If you want to make hydrangeas purple when they bloom, you’ll need to find a balance between the range from blue to pink. Unfortunately, if you’re starting with white flowers, their color is not adjustable, but many other varieties will change successfully over time with the right treatments.
For Established Plants
Make note of the colors on the hydrangeas the season before you plan to treat them. Hydrangeas that are at least two years old can roughly indicate the pH of the soil by their colors, so you'll know how to alter it to get purple flowers. Blue flowers mean you have a low pH, while pink flowers indicate your pH is high.
Raise the pH for the soil of blue flowers by adding a dose of lime, 2 to 4 ounces at a time around the base of the plant. Repeat the dose every two to three weeks until you see purple blooms.
Lower the pH the ph for the soil of pink flowers by mixing 1 tablespoon of aluminum sulfate with 1 gallon of water and pouring it around the base of the hydrangea. Wait two to three weeks before reapplying.
For New Plants
Test the pH of your soil. The pH is measured on a number scale from zero to 14, with neutral being seven. Purple flowers can be grown in soil with a pH range between 5.5 and 6.0.
Increase the pH in one point increments, if needed, for soil results below a pH of 5.5 by adding 2 to 4 ounces of lime around the plant.
Decrease the pH by adding aluminum sulfate for pH measurements reading higher than 5.5. Add 1 tablespoon per 1 gallon of water and pour around the base of your plant.
Repeat applications for either method after waiting two to three weeks and retesting the soil. Be sure to follow any instructions on the package if a longer wait period is recommended for that brand.