Caring for colored hydrangeas begins before the hydrangea is even planted. Test and amend the soil as needed for the desired color, and treat the planting area throughout the year to support ideal nutrient levels.
- Test the soil
Testing the soil in the planting area is one of the best methods for determining the pH (acidity or alkalinity) of the soil and identifying the naturally-occurring minerals in the soil. Blue hydrangeas grow best in aluminum-rich soil with a pH between 5.2 and 5.5. Pink hydrangeas need an aluminum-free environment with a pH of 6.0 to 6.2.
- Prepare the planting site
Work aluminum sulfate and organic matter such as grass clippings or coffee grounds into the soil before planting blue hydrangeas. For pink hydrangeas, add dolomitic lime and a phosphorus-rich fertilizer, which helps reduce the amount of aluminum the plant absorbs.
- Maintain ideal chemical levels
During the growing season, treat blue hydrangeas with a solution of 1 tablespoon of aluminum sulfate dissolved in 1 gallon of water. Likewise, add dolomitic lime to the planting area around pink hydrangeas several times during the year.
- Fertilize the colored hydrangeas
Choose a low-phosphorus, potassium-rich fertilizer such as a 25-5-30 blend to feed blue hydrangeas. Pink hydrangeas do well with a 25-10-10 blend, which has plenty of phosphorus. Follow the manufacturer's directions regarding dosage for ideal results.