This morning, I was perplexed because a beautiful blue Hydrangea that I had received as a gift a couple years ago, was now producing pink blossoms. I thought I was imagining it, until I googled it, and found this post published several years ago by William Johnson.
Hydrangeas Can Change Color
When I selected my own Hydrangeas, I wanted the Blue Purple variety. While I was reading up on them to learn how to care for them, I discovered that you can actually control the color. Allow me to add, this is true only sometimes, because where they are planted can have a strong affect on the color as well. You can better control the color when you raise the Hydrangeas in a large garden pot.
Using Aluminum Sulfate in the soil of a potted Hydrangea will tend to make the flowers more blue or purple. Using a fertilizer high in phosphate will tend to make the flowers more pink.
Take those same plants and plant them in the ground near a sidewalk and you will notice that even if you start out with blue colored plants, they will soon turn pick. The lime from the concrete leaches into the surrounding soil and it has a higher Ph and that changes the blue flowers to pink.
Garden centers sell fertilizer for both generating the blue color and for generating more pink tones. If you happen to be blessed with a white variety, don't bother trying to change the color because white stays white to a pale greenish cast forever. That is what Mother Nature intended and she doesn't take too well to being messed with.
I recently bought a new Hydrangea for a large pot and the first thing I did was to remove the flowering heads ( deadheading) and then started to add some used coffee grounds to the soil. That tends to lower the Ph which helps offset the high calcium content of our water. That was 4 weeks ago and the new growth is just amazing. The plant, when it finishes growing out with the new growth will be about twice the original size. Before Fall sets in, I expect to have some large beautiful Blue/Purple blooms. I will be sure to show them here when the blooms come out.