I am trying to change the color of my potted Hydrangea blooms from Pink to Blue/purple with Aluminum Sulfate...but I know I started too late. It already had a few tiny buds and one fairly large buds when I applied it...

For those that have used it when can I reapply again? I used it 2 weeks ago and as you can see made no difference on the one set of buds that was already well on its way. I have several baby ones coming up and was hoping to get those to change color?

Thanks!