Answer: Most hydrangea coloring depends to a large extent on the pH of the soil where they are growing. In acidic soil they are more blue, in neutral to alkaline soil they are usually pinker. A reading of 6.0 to 6.5 should bring out the best pink.
Based on the color change, your soil may be naturally acidic or you may have used a fertilizer for acid loving plants or may be using a mulch material (such as pine bark) that contributes to soil acidity (lower pH.)
You need to run some basic soil tests and check the pH, then adjust it gradually over time (to avoid shocking the plants) until it reaches a more neutral reading. Lime is used to raise pH. It can take a year or two for lime to work its way into the soil and then the effect usually lasts for several years. So this may take some time to achieve and periodic applications to maintain the desired pH once it is reached.
Your local county extension can help with the soil tests and determining how much of change is needed and how to go about it.
I hope this helps.
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