Hydrangeas possess the amazing ability to change bloom color from pink to blue, depending on the pH of the soil. Acidic soils, with a pH between 5.0 and 5.5, cause blooms to appear blue, while alkaline soils, with a pH of 6.0 to 7.0, cause pink blooms. Changing the color of the blooms requires amending the soil to adjust the pH. Although it is a bit easier to raise the pH with lime, lowering the pH to produce blue blooms is also possible.
Plant hydrangeas in full sun or partial shade in rich, well-drained soil. Till the soil to a depth of 12 inches and amend with a 2-to-3-inch layer of compost or well-rotted manure. Work it in well with the existing soil.
Test the pH of the soil with a soil test kit. Follow the directions for gathering the soil and performing the test. Typically, it involves filling a vial with soil and water, adding the contents of the capsule for determining pH and allowing it to sit for a few minutes. Match the color of the water to the enclosed chart to determine the pH of the soil.
Adjust the pH of your soil to 5.0 to 5.5, following the instructions in the kit. This requires an application of aluminum sulfate, ferrous sulfate or sulfur to lower the pH of alkaline soils. Follow the recommended application rate and work the amendments into the soil.
Water hydrangeas deeply once a week during periods of active growth. Saturate the soil to the root level and allow soil to dry slightly. The amount of water required and the frequency of watering depends on the size of the plant, soil conditions and the amount of natural rainfall in your area. Adjust your schedule to meet the needs of your specific plant.
Fertilize hydrangeas in early spring when new growth appears. Apply a balanced formula like 10-10-10 to the soil and work into the top 2-to-3 inches of soil. Follow the application rate on the package, typically 2 lbs. per 100 square feet.
Prune hydrangeas after blooming following the recommended pruning for the specific cultivar you are growing. Pruning procedures vary depending on the growing habits of the cultivar. Typical pruning involves removing old or dead wood, while leaving some mature wood to produce next year's blooms. Regular deadheading of faded blooms improves appearance.