Hydrangeas are large bushy plants that produce round pink, blue, white or green blooms at the top of tall stalks. They bloom in the spring and early summer with a few varieties blooming repeatedly until fall. The color of many hydrangea blooms is due to the acidity of the soil. Hydrangeas develop blue flowers when the soil is acidic and pink flowers when it is alkaline. To maintain a blue hydrangea bush you must maintain acidic soil.
Insert a soil pH tester probe into the soil 3 to 4 inches from the base of the blue hydrangea. Take note of the reading, which will be in the acidic range. The ideal pH level to maintain for blue hydrangeas is 5.
Retest the soil at least once per week with the soil pH tester to see if the pH level starts to rise to be more alkaline.
Apply iron sulfate to the soil under the blue hydrangea only if you notice the pH level starting to rise. Use the amount recommended on the packaging.
Rake the top of the soil lightly under the hydrangea with a garden fork to mix the iron sulfate into the soil.
Water the soil under the hydrangea to dissolve the iron sulfate and soak it down near the roots of the hydrangea.
The iron sulfate will not change the color of the existing hydrangea blooms if they have already started to turn pink. However, new blooms will be blue once the buds open.
Iron sulfate is also called aluminum sulfate and is sold in the fertilizer aisle of garden centers.