Endless Summer Hydrangea by geekstogardeners
TheGardenLady received this question from Kathy.
I live in Fenwick, Old Saybrook and have always had a yard full of beautiful, brilliant blue Endless Summer hydrangeas. However, this year is the exception!! I have tried deadheading since the blooms are  overly abundant but the color is still lackluster, pale blue.  I have tried adding hydrangea acid based fertilizer and watering with the hose daily.  Any suggestions?
First of all you should check the pH of the soil where the hydrangea is growing. If your soil is highly alkaline, you will have problems maintaining the blue flower no matter what you do. The pH of the soil should be 5.2 to 5.5. Sometimes if your hydrangeas are planted near cement, like a sidewalk, the alkaline can leach into your soil. And sometimes your water is alkaline. Get test strips to test the water and the soil, or have the soil in your hydrangea garden tested at your state agricultural extension or through your local Master Gardener office.
If you have the proper pH, next Spring, water the plants really well and then add a solution of 1/2oz (1Tbsp) aluminum sulfate to each 1 1/2 gallons of water and feed this carefully to your plants- it can burn the roots. This should be applied in the spring when you see the buds forming- about 6 weeks before the flowers come.
Readers who have new hydrangea plants should NEVER use aluminum sulfate until your plants are about 3 years old. The addition of the aluminum sulfate seems to be the most reliable method of keeping the hydrangea flower blue.
It is good that you used a hydrangea fertilizer that is high in potassium. But most authorities do not think that this will help change the color.
An excellent site that goes into details of how to keep your hydrangea blue or pink is this.