You won't see many Blue Hydrangeas around Evanston. But it's a plant many gardeners crave, perhaps because of its rarity. One Evanston gardener, Pamela Geyer-Howell (photo), grows some of her hydrangeas in pots, so that she can acidify a limited body of soil. She uses both aluminum sulfate and organic sulfur (second photo) which she gets from Gethsemane Gardens. In the winter she cuts the plants (she also grows a blue lacecap hydrangea) back and keeps them in her unheated garage, putting some snow on them now and then for moisture. Then in both the spring and fall she applies a dose of aluminum sulfate or sulfur according to directions on the package. Pam says the hydrangeas were gifts so she does not know their varieties.
Some Chalet workers said they use aluminum sulfate mixed with water and treat (normally planted) H. 'Endless Summer' in the Spring to make it blue in August. I saw one of their plants and it had many pale blue blossoms and a few that were pink.
It's difficult to get the real blue with our soil but it's worth experimenting for.
It's important not to get any of the solutions on the leaves because it can yellow them. Also the aluminum sulfate can create soil toxicity if you use more than the recommendation. Good luck!
