Hi, Joanne! In order affect the color of the blossoms, aluminum sulfate must be applied BEFORE the plant forms buds. What's more the compound must be dissolves in the soil so that it can be taken-up by the hydrangea's root system. Since hydrangeas form buds just about all-season long -- which is why they produce such long lasting blossoms -- aluminum sulfate can be applied throughout the season. Having said that, for best results I recommend application in early spring or in early fall. A fall applications will likely impact the blossoms that appear next spring. Keep growing. Bob Stohler for Marianne Binetti.