Mid-Atlantic Gardening: Swap Q&A, 1 by Gitagal

Gitagal wrote:
Jill,

Acid soil makes Hydrangias blue. Alkaline--pink. Often you can see shades of both on the same plant.

To get really blue flowers, water them with Aluminum Sulphate. I don't know when, or how often, or at what time frame in the season, but I would think you could start in early Spring.

Just using something like Holy Tone does not seem to be enough.

Heres picture of my "Endless Summer: Hydrangia in full bloom. I had hoped it would bloom bluer also, but I have not done what I am suggesting to you either. Have to buy some of that Sulphate.

Gita