The villagegardener blog has the following advice:
- Feed hydrangeas with Holly-tone® 4-3-4 at planting time, and yearly in the spring. Mulch with pine straw, if possible. This will acidify the soil as it breaks down. The ideal soil ph for blue hydrangeas is 5.2-5.5.
- Your hydrangeas are already planted, and your soil ph is hovering around 7.0. You have a much more difficult task ahead of you, and results can be iffy. Feed your plants with Holly-tone® 4-3-4 once a year in Spring when new growth begins to emerge. Add Aluminum Sulphate to soil a week or two later. Aluminum will make flowers blue, and Sulfur will lower the ph of the soil to make the aluminum available to plants.
- Top dress soil with leaf mold or peat moss, and mulch with pine straw. Pine bark breaks down more slowly, but it is a good alternative mulch.