Grow hydrangeas in acidic soil with a pH of 5.2 to 5.5
Test your soil pH before making changes
Add aluminum sulfate to the soil
Use a soil acidifier if needed
Avoid lime or other alkaline amendments
Water with rainwater or low-mineral water
Apply fertilizer low in phosphorus
Mulch with pine needles, pine bark, or oak leaves
Repeat soil pH checks regularly
Start treatment before bloom season for best color change